MEGAPARSEC RELATIVISTIC JETS LAUNCHED FROM AN ACCRETING SUPERMASSIVE BLACK HOLE IN AN EXTREME SPIRAL GALAXY

It is now widely recognized that supermassive black holes (SMBHs) of masses ∼10⁶–10¹⁰ M_☉ lurk in the nuclei of almost all massive galaxies (e.g., Kormendy & Ho 2013; Magorrian et al. 1998). A symbiotic relationship between the growth of the central black holes and the bulges of galaxies is suggested by a remarkably tight correlation between the masses of the black holes and the galactic bulges (Magorrian et al. 1998; Gebhardt et al. 2000; Haring & Rix 2004; Marconi & Hunt 2003). The most spectacular manifestations of massive black holes in active galactic nuclei (AGNs) are the powerful bipolar relativistic jets, which form twin-lobed giant radio galaxies on 10²–10³ kpc scales, and the extremely luminous quasars. The gravitational accretion of matter onto the SMBH is believed to be the "central engine" that powers such sources (Lynden-Bell 1969; Soltan 1982) and their growth governed by accretion processes (Soltan 1982; Begelman et al. 1984). Understanding the physics of black hole formation and relativistic jets is a major focus of modern astrophysics. Powerful radio jets on ≳100 kpc scales are nearly always launched from the nuclei of elliptical galaxies and not spirals, and the typical radio luminosity of spiral galaxies with AGNs is about 10³–10⁴ times feebler than ellipticals, making them comparatively radio-quiet. Moreover, while both ellipticals and spirals may host radio-quiet quasars, radio-loud quasars are found in ellipticals only—never in spiral galaxies—and about 10% of quasars are radio-loud at a given epoch, for reasons that are still unknown (e.g., Dunlop et al. 2003).

The physical origin of the radio-loud and radio-quiet dichotomy and the mechanisms by which relativistic jets are launched from accretion disks around black holes (the disk–jet coupling) have long been the subject of intense investigations, yet the issue still remains unresolved despite a wealth of observations. It is plausible that the prime difference between radio-loud and radio-quiet AGNs is connected to the fundamental black hole properties, namely its mass, spin, and accretion rate. Any theoretical model must explain why it is so difficult for massive black holes in disk galaxies to eject collimated radio jets extending to 10²–10³ kpc distances. The question of whether the black hole mass or spin and the radio loudness of an AGN host are interconnected remains unanswered and is still under close scrutiny. It was suggested by Laor (2000) that only AGNs with black hole masses ≳ 3 × 10⁸ M_☉ in elliptical galaxies produce large-scale jets, in contrast to smaller mass black holes found in spirals that obviously lack such jets. According to the "spin paradigm," powerful large-scale jets originate near rapidly spinning accreting SMBHs (Wilson & Colbert 1995; Sikora et al. 2007; Tchekhovskoy et al. 2010; Dotti et al. 2013) found in bulge-dominated systems and are launched at relativistic speeds via the magnetohydrodynamic (MHD) Blandford–Znajek (BZ) mechanism (Blandford & Znajek 1977; MacDonald & Thorne 1982; Penna et al. 2013), although observational evidence for this conjecture is sparse (Wang et al. 2006; Doeleman et al. 2012). Alternatively, in the Blandford–Payne (BP) mechanism (Blandford & Payne 1982), jet power is extracted from the rotation of the accretion disk itself, via the magnetic field threading it, without invoking a rapidly spinning black hole. However, in both the processes the intensity and geometry of the magnetic field near the black hole horizon strongly influence the Poynting flux of the emergent jet (Beckwith & Hawley 2008). This has given rise to the "magnetic flux paradigm," which posits (Sikora & Begelman 2013) that jet launching and collimation require strong magnetic flux anchored to an ion-supported torus of optically thin, geometrically thick, extremely hot gas with poor radiative efficiency (Rees et al. 1982). Such hot, magnetic field saturated ion tori may arise naturally around spinning black holes via advection-dominated accretion flows (ADAFs) at low Eddington rates (Eddington luminosity for a black hole of mass M is L_Edd = 1.26 × 10³⁸ M/M_☉ erg s⁻¹), particularly in elliptical galaxies showing large-scale jets (Narayan & Yi 1994; Narayan & Yi 1995). On the other hand, for AGNs in spirals (narrow-line Seyfert 1s, optically selected quasars), the mass accretion rates are plausibly near Eddington, forming optically thick, geometrically thin accretion disks that radiate efficiently but fail to create large-scale collimated radio jets.

The physical mechanism linking the SMBH mass to the galactic bulge properties is little understood, while it is believed that an energetic AGN feedback, either via the mechanical power of the radio jets or through a pressure-driven wind, on the surrounding medium plays a major role in regulating the growth of the black hole and even the surrounding host galaxy. Important details of how SMBHs grow, and their energetic feedback processes, are still missing, but it is gradually becoming clear that SMBHs are essential ingredients shaping the lives of galaxies across cosmic time (for review see Alexander & Hickox 2012; Kormendy & Ho 2013). Thus, a detailed study of active galaxies hosting massive black holes assumes great importance; in particular, finding any clear counterexamples of jet launching under extremely unusual circumstances, such as the case of radio galaxy J2345−0449 presented in this work, may play a key role in informing us what factors determine the relativistic jet formation and its role in galaxy evolution.

Our paper is organized as follows. In Section 2 we introduce the radio source J2345−0449 and highlight its main radio and optical properties. In Section 3 we present the radio, optical, and mid-IR observations of the source, describe the data analysis procedure in detail, and obtain the main results. In Section 4 we focus on the scientific content of the results and discuss their wider astrophysical implications. Finally, in Section 5 we present our main discussion and conclusions in light of the results obtained above, and we highlight the possibilities of future multiwavelength observations for an in-depth understanding of this extraordinary galaxy.

We adopt a ΛCDM cosmology model with H₀ = 70.5 km s⁻¹ Mpc⁻¹, Ω_M = 0.27, and $\Omega _\Lambda =0.73$, which results in a scale of 1.43 kpc arcsec⁻¹ for a redshift of z = 0.0755. The radio spectral index α is defined as flux density (S_ν) ∝ frequency (ν)^−α.

Our most important finding is the extraordinary galaxy 2MASX J23453268−0449256 at a redshift of z = 0.0755 (hereafter called J2345−0449), which is an extremely rare and clear example of a megaparsec-scale collimated pair of relativistic plasma jets launched from the nucleus of a massive spiral galaxy. This radio galaxy was first mentioned by Machalski et al. (2007) as a megaparsec-scale object located at redshift z = 0.0757, but they did not provide any further hints about its extraordinary nature. Our Very Large Array (VLA) and Giant Meterwave Radio Telescope (GMRT)⁸ images in Figure 1 show that the synchrotron radio emission arises from an enormous (>megaparsec scale) bipolar structure with two pairs of radio lobes, i.e., forming a giant "double–double" radio galaxy (DDRG) centered on the spiral host. With two nearly aligned pairs of radio lobes sharing a common AGN core, DDRGs are the most compelling, rare examples of recurring jet activity of SMBHs (Schoenmakers 2000; Saikia & Jamrozy 2009). Remarkably, the inner and outer radio lobe pairs extend over ∼387.2 kpc (∼452) and ∼1.63 Mpc (∼191), respectively, making J2345−0449 the largest radio source known to date that is hosted by a spiral galaxy. The VLA 6 cm (4.8 GHz) image shows that inner "active" radio lobes are of edge-brightened (Fanaroff–Riley class II or FR II (Fanaroff & Riley 1974)) morphology, being fed by the collimated jets shot out from the central nucleus, which is typical of DDRGs (Schoenmakers 2000; Saikia & Jamrozy 2009). The GMRT low frequency 330 MHz image shows that the "aged" outer lobes are more diffuse, lacking prominent hot spots and no longer being energized by jets. The diffuse steep-spectrum radio emission near the inner lobes is more visible in the lower frequency image, tracing the back-flowing plasma of outer lobes. The integrated flux density and radio luminosity at 1.4 GHz are S_1.4 = 180.60 ± 20.0 mJy and L_1.4 = 2.5(± 0.3) × 10³¹ erg s⁻¹ Hz⁻¹, respectively. Although moderate, this luminosity is much above the rough divide between the radio-quiet and radio-loud galaxies, which is usually taken at L_1.4 ∼ 10³⁰ erg s⁻¹ Hz⁻¹. However, despite a clear FR II morphology on large scales, the 1.4 GHz radio luminosity of J2345−0449 is close to the luminosity break between the FR I and FR II radio galaxies. The integrated flux density and radio luminosity at 330 MHz are S₃₃₀ = 3.60 ± 0.15 Jy and L₃₃₀ = 5.0(± 0.2) × 10³² erg s⁻¹ Hz⁻¹, respectively, implying a very steep spectral index of ${\alpha ^{1.4}_{330}} \approx 2$ between 1.4 GHz and 330 MHz. Both the outer lobes have a very steep integrated radio spectral index of ${\alpha ^{1.4}_{330}} \sim 2$. We observe a sharp increase in ${\alpha ^{1.4}_{330}}$ between the inner and outer lobes on both sides of the flat spectrum core, as expected because of radiative ageing of radio plasma in the episodic jet activity scenario (Figure 2).

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The galaxy's favorable inclination to the line of sight (i ≃ 59°) has enabled a direct view of the well-developed bright spiral arms, as seen from the deep high-resolution optical images taken with the MegaCam camera on the Canada–France–Hawaii Telescope (CFHT) and also visible in the Sloan Digital Sky Survey (SDSS) multi-band images (Ahn et al. 2014) shown in Figures 3 and 6. The Fermi Gamma-ray Telescope observations have recently indicated the presence of relativistic jets in a handful of spiral galaxies hosting narrow-line Seyfert 1 (NLSy1) nuclei (Abdo et al. 2009). While those jets are also very rare, they are not known to extend much beyond the galactic scale (≲ 10 kpc) and lack the excellent collimation witnessed in the large-scale FR II jets (Doi 2012; Morganti 2011). It has been pointed out (Foschini 2011) that the BZ mechanism fails to explain the jet power of these jets. Thus, the nature of radio emission in NLSy1s and their failure to produce large-scale radio jets has remained unexplained. Despite decades of extensive observations, only two previous reports exist of a disk galaxy ejecting large-scale (>100 kpc) bipolar radio jets: the radio source J031552−190644 found in the galaxy cluster A428 (Ledlow et al. 1998), and a recently reported megaparsec-scale episodic radio source known as Speca (Hota et al. 2011). In both these exceptionally rare objects, the galactic disk is viewed nearly edge-on, precluding a clear view of the putative spiral arms; thus, the evidence is still indirect. The present giant radio source J2345−0449 is not only the most unambiguous example of this extremely rare class, but its spiral host galaxy also displays a unique combination of several other remarkable properties (as discussed below). Extremely unusual objects like J2345−0449 strongly challenge the conventional ideas of black hole growth and radio jet formation in galactic nuclei; thus, they are of profound interest for models of the central engines of radio galaxies and quasars.

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We used SDSS-III images (Ahn et al. 2014) to explore the large-scale galactic environment in which the spiral galaxy J2345−0449 is located. The SDSS wide-field color image in Figure 3 and mid-IR data (Table 1) reveal that J2345−0449 is an unusually bright (m_r = 14.40, M_r = −23.26 in r band, m₁₂ = 9.00, M₁₂ = −28.66 in 12 μm band), isolated spiral galaxy located in a sparsely populated galactic environment devoid of bright L* galaxies and clearly not at the center of a compact group or a rich galaxy cluster. The next brightest galaxy neighbor, which is located ∼460 kpc to the southeast, is more than 2 mag fainter. The second and third brightest galactic neighbors are ≳2.5 mag fainter and located ∼240 and 480 kpc away, respectively (Figure 3). Interestingly, about 33 arcmin (∼2.8 Mpc) away from J2345−0449 (redshift z = 0.07556) is located a massive galaxy cluster RBS 2042 (redshift z = 0.07860), which is strongly detected in the ROSAT All-sky Bright Survey (Voges et al. 1999; Schwope et al. 2000). An association of J2345−0449 with this cluster is not immediately obvious, but it could be a peripheral member of the massive cluster.

3.1. GMRT Observations

3.2. VLA Observations

3.3. Evidence for Interruption of the AGN Jet Activity

3.4. Optical Spectroscopic Observations and Data Analysis

Optical long-slit spectroscopic data were taken during 2011 November 20–22 with the 2 m telescope at the IUCAA Girawali Observatory (IGO). The spectra were obtained using the IUCAA Faint Object Spectrograph and Camera (IFOSC)¹⁰. IFOSC employs an EEV 2 K × 2 K, thinned, back-illuminated CCD with 13.5 μm pixels. The spatial sampling scale at the detector is 44 μm per arcsecond, giving a field of view of about 10.5 arcmin on the side. We used two IFOSC grisms, grism No. 7 and grism No. 8, in combination with a 1.5 arcsec slit, yielding a wavelength coverage of 3800–6840 Å and 5800–8350 Å and spectral resolutions of 300 km s⁻¹ and 240 km s⁻¹, respectively. The spectra were taken with the slits aligned both along the major and minor axes of the galaxy. In each case, we took 4 exposures (4 × 45 minutes) using the grism No. 8 and 2 exposures (2 × 45 minutes) using grism No. 7, respectively. We carried out the bias and flat-field corrections to all the frames. Dark correction was not required as the CCD was liquid nitrogen cooled and the dark current was negligible for 45 minute exposures. The cosmic ray hits were removed by the IRAF task "crmedian" in the Cosmic Ray Removal Utility Package (CRUTIL). Standard stars were observed during the same nights by placing them at different slit locations. Wavelength calibration was done using the standard helium–neon lamp spectra.

One-dimensional spectra were extracted using the doslit task in the IRAF software. We used the variance-weighted extraction method as compared with the normal one. Air-to-vacuum conversion was applied before combining the spectra, using 1/σ² weighting in each pixel after scaling the overall individual spectrum to a common flux level within a sliding window. The extraction was carried out both with and without cosmic ray removal. Fringing poses a problem for the grism No. 8 (red region) data, and we removed the fringes by subtracting 2 two-dimensional science exposures for which the object was placed at two different locations along the slit. From the absorption and emission features in the spectrum (shown in Figure 4), redshift of the galaxy was determined to be z = 0.0755 ± 0.0005. We compared our spectrum with the 6dF spectrum given by (Jones et al. 2009). The features in both the spectra match closely, and our redshift estimate is very close to the reported 6dF redshift z = 0.075566 ± 0.000150.

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As the galaxy is spatially well resolved, we also performed the two-dimensional spectral extraction around the Hα line detected in the grism No. 8 spectra. For spectral curvature corrections, the spectra of a standard star (Feige 110) were taken at different spatial positions along the slit during the scheduled nights and nearby nights. The same standard star was used for the flux calibration. We created a superimposed frame by joining the different standard star spectra to the target spectra, retaining the original spatial position of each spectra. The data were then corrected for the geometrical distortions by using the IRAF tasks fitcoords and transform. Fitcoords provides the transformation of slit position and wavelength as a function of (x, y) pixel number on the image. Transform applies this transformation to the data to correct the distortions. The resulting frame is used for the spectral extraction.

3.5. Deriving the Galaxy Rotation Curve

We carried out the curvature correction (as above) for each of the four exposures and then added them after proper shifting to improve the signal-to-noise ratio (S/N). We made sure that the addition is correct by looking at the residuals in the difference of the frames. We extracted one-dimensional spectra from sub-apertures of 5 pixel width (spatial direction) placed symmetrically around the center of the galaxy. This has resulted in 15 individual spectra. The width of the sub-aperture corresponds to an angular scale of ∼1.5 arc sec (2.1 kpc), commensurate with typical seeing during the observing nights. In Figure 5 we show a two-dimensional spectrum image around the Hα and [N ii] emission lines with the above sub-apertures marked. In the spectrum both the Hα and [N ii] lines clearly show the signatures of a rapidly rising and then flat rotation curve.

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We simultaneously fitted the Hα λ6562.81 Å and [N ii] λ6585.27 Å lines by using Gaussians and χ² minimization. The second [N ii] λ6549.86 Å line to the blue side of Hα line was faint, well within the noise, and thus we did not attempt to fit it. The rotational velocity V at each sub-aperture position was computed using the equation

$$\begin{equation} V = \frac{z_{\rm ap} - z_{\rm cen}}{(1+z_{\rm cen})}\,\, 3\times 10^{5} \,\, \, {\rm km\,s^{-1}} \end{equation} \tag{ 1 }$$

and plotted against the distance from the nucleus to obtain the rotation curve. Here z_ap is the fitted redshift of the Hα line in a given aperture and z_cen = 0.0755 the redshift at the dynamical center of the galaxy. The two-dimensional extraction is carried out for the minor axis in the same manner as for the major axis data.

In Figure 6, we show the derived rotation curves for the major (red data points) and minor (blue data points) axis data. The major axis data shows a clear rotation signature, while no disk rotation is evident along the minor axis. This is clear evidence that the emission lines originate in a tilted, rotating disk. The rotational curve shown in Figure 5 is fitted with an analytical function (Courteau 1997),

$$\begin{equation} V(R) = V_0+\frac{2}{\pi } V_{\rm c,obs} \times {\rm arctan}^{-1}\left(\frac{R}{R_{\rm turn}} \right), \end{equation} \tag{ 2 }$$

where R is radial distance, V₀ is the systemic velocity, V_{c, obs} is the asymptotic circular velocity, and R_turn is the turnover radius. The χ² minimized fitting led to the following values: V₀ = 0 km s⁻¹, V_{c, obs} = 370.8 ± 25.8 km s⁻¹ and R_turn = 1.4 ± 0.7 kpc. Finally, the inclination corrected circular velocity is derived at V_{c, incl} = 429.3 ± 25.8 km s⁻¹. We have derived the inclination angle (i = 597 ± 2°) by using Hubble's standard formula

$$\begin{equation} \sin (i) = {\left[\frac{(1 - {q^{2}})}{\left(1 - q_{0}^{2}\right)}\right]}^{1/2}, \end{equation} \tag{ 3 }$$

where q = (a/b) is the apparent axial ratio of the galaxy, as obtained from the fitted r-band semiminor and semimajor isophotal diameters of the disk (Table 3), and q₀ is the intrinsic axial ratio, i.e., the ratio of vertical to horizontal scale-heights of the disk. We have adopted a standard value of q₀ = 0.19, although it varies slightly from disk to disk.

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3.6. The Central Velocity Dispersion

3.7. Central AGN, Star Formation Rate, and Metallicity

To decouple the possible AGN lines from the galaxy spectrum, we extracted the nuclear spectrum of only the five central pixels. In the nuclear spectra, a narrow [N ii] line (λ6583 Å) of FWHM ∼400 km s⁻¹ is detected, while the Hα λ6563 Å line is very weak and not reliably detected. Apart from these features, we find no other definitive signature of an AGN in the optical spectrum. The luminosities are 3.02 ± 0.27 × 10⁴⁰ erg s⁻¹ for the [N ii] line and <1.05 × 10⁴⁰ erg s⁻¹ (3σ) for Hα line. The [N ii]/Hα line ratio lies above that for H II regions in the Baldwin, Phillips, and Terlevich diagnostic diagram (Baldwin et al. 1981), and it is a distinct signature of ongoing AGN activity. The small Hα luminosity shows that this AGN is a low-luminosity active galactic nucleus (LLAGN/LINER type), which is defined by having an Hα luminosity smaller than 10⁴⁰ erg s⁻¹ and weak emission lines, further indicating a very small mass accretion rate compared with the Eddington rate (Equation (10)).

For the disk extended emission, from the fitting of two-dimensional spectra, we computed the integrated flux in the Hα and [N ii] lines at different locations of the galaxy along the major and minor axes. The Hα fluxes were used to determine the star formation rates (SFRs), and the Hα-to-[N ii] flux ratios were used for calculating the metallicities. For estimating the SFR, we used the equation

$$\begin{equation} \log ({\rm SFR}) = \epsilon _{{\rm H}{\alpha }}\log L({\rm H}{\alpha }) - \log (\eta _{{\rm H}{\alpha }}), \end{equation} \tag{ 4 }$$

where _Hα and η_Hα values were taken from (Argence & Lamareille 2009), where dust correction is assumed to be negligible. The surface SFRs are obtained by dividing the calculated SFR values by the area over which the extraction is done. For metallicity, we have used the formula (Pettini & Pagel 2004)

$$\begin{equation} 12+\log (\rm O/H) = 8.9+0.57\times \log ([N\,\scriptsize{II}]/H{\alpha }), \end{equation} \tag{ 5 }$$

where O/H is the ratio of oxygen to hydrogen abundance, a proxy for metallicity. The variation of SFR and metallicity along the radial direction from the center is shown in Figure 7. While the surface SFR remains virtually constant, a radial gradient in metallicity across the disk is clearly evident. This indicates J2345−0449 is a spiral galaxy with a large SFR. Also notable is a striking lack of recent star formation activity near the galactic center, which is possibly attributed to energetic feedback from the central AGN, as discussed below.

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3.8. WISE Mid-infrared (MIR) Colors

The spiral galaxy J2345−0449 is strongly detected in the mid-IR bands centered at 3.4, 4.6, 12, and 22 μm (W1, W2, W3, and W4 bands), observed by the Wide-field Infrared Survey Explorer (WISE) satellite (Wright et al. 2010). The S/N ranges from 47.9 in the 3.4 μm band to 10.9 in the 22 μm band. The MIR magnitudes and luminosities and their errors are listed in Table 1. The very high MIR luminosity of the galaxy is unusual and suggests that it is actively forming stars and still building a massive stellar disk. We use the MIR colors to obtain the properties of a possibly dust obscured galactic nucleus and determine the radiative efficiency of the AGN. MIR data are highly suitable for this purpose because the optical–UV radiation from the AGN accretion disk is absorbed by the putative dusty torus and reradiated in MIR bands. It has been shown that WISE MIR colors can effectively distinguish AGNs from star-forming and passive galaxies; moreover, within the AGN subset itself high-excitation radio galaxies (HERGs) and low-excitation radio galaxies (LERG) stand out on the MIR color–color and MIR–radio plots (Stern et al. 2012; Gurkan et al. 2014). The MIR colors for J2345−0449 are

$$\begin{eqnarray} [W1 - W2] &=& 0.064, \nonumber \\ [W2 - W3] &=& 2.817, \, [W2 - W4] = 4.722 . \end{eqnarray} \tag{ 6 }$$

From the blue [W1 − W2] ≪ 0.6 and red [W2 − W3] ∼ 3 colors, we conclude that J2345−0449 is a star-forming spiral galaxy and there is negligible reradiated MIR flux (no MIR "bump") from the accretion disk (Wright et al. 2010; Stern et al. 2012). This is further corroborated by the position of J2345−0449 on the [W3]–L₁₅₁ and [W4]–L₁₅₁ planes. Here L₁₅₁ is 151 MHz radio luminosity, which is estimated from the radio spectrum and by using the spectral index value α = 2 as calculated in Section 2. On these planes, HERGs can be clearly distinguished from LERGs (Gurkan et al. 2014), and J2345−0449 is located in the region of LERGs (plot not shown here). Clearly, J2345−0449 is an LERG hosted by a spiral galaxy, and presently its black hole is possibly residing in a low radiative efficiency (ADAF) state, as also indicated by the weak Hα and the lack of broad emission lines in the optical nuclear spectrum (Figure 4).

3.9. CFHT Imaging and Photometric Analysis: Bulge–Disk Decomposition

Modeling the light distribution in terms of bulge and the disk components of the spiral galaxy J2345−0449 is important for understanding the formation and evolution of this highly unusual galaxy and its central black hole. For this purpose we used the excellent quality archival data taken with the MegaCam wide-field imaging camera on the 3.6 m CFHT (Boulade et al. 2003). These images were processed, stacked, and calibrated using the MegaPipe pipeline (Gwyn 2008) at the Canadian Astronomical Data Centre. We used the g- and r-band images taken in excellent seeing conditions of 0.75 and 0.80 arcsec (FWMH) that have effective exposure times of 960 and 1320 s, respectively. The g-band image was generated by stacking eight images taken during 2008 September 1 to 2009 December 13. Eleven images taken during the same period are used to obtain the stacked r-band image. The individual images were median stacked using the SWarp program (Bertin et al. 2002). The pixel scale of the stacked images was 0.18 arcsec. The MegaPipe also provided the inverse variance weight map for the stacked images. For additional tests and consistency check, we also used the g', r', i', and z' band images from the SDSS.

For obtaining the bulge and disk parameters, we used the PYMORPH toolbox (Vikram et al. 2010), which has a Python-based pipeline for two-dimensional bulge–disk decomposition. PyMorph uses SEXTRACTOR (Bertin & Arnouts 1996) and GALFIT (Peng et al. 2002) for object detection and decomposition. Briefly, we created a cutout of the galaxy based on the half light radius of the object. The cutout size is selected wide enough to include enough sky pixels to model the sky background accurately. This size is set to ten times the half light radius for the current case. We found that the fitting results do not change significantly with the cutout size. After making the cutout image, we generated a mask image that masks all the bad pixels and neighboring objects during the fitting. The galaxy under study does not have any close neighbors (Figure 3); therefore, the contamination from neighbors is minimal. Next, we created a configuration file for GALFIT. The configuration file includes optimal initial values for the parameters of both the bulge and the disk. We derived the initial values using SEXTRACTOR parameters. We set the total bulge and disk magnitude to the MAG_AUTO parameter and the radius to the half light radius of the galaxy. The ellipticity and position angle are derived from the ELONGATION and THETA_IMAGE. A detailed description of the procedure can be found in Vikram et al. (2010).

We used a composite of the Sérsic and an exponential function to model the galaxy light profile, where the former models the bulge and the latter models the disk. The form of the Sérsic function is

$$\begin{equation} \Sigma (r) = \Sigma _e \exp (-\kappa [(r/r_e)^{1/n} - 1]), \end{equation} \tag{ 7 }$$

where κ is a function of Sérsic index (n). For n = 4, κ = 7.67, which corresponds to the de Vaucouleurs law. Σ_e is the surface brightness at r_e, the scale radius (half light radius). The disk component is modeled using the exponential law, which is given by

$$\begin{equation} \Sigma (r) = \Sigma _0 e^{-\left(r/r_d\right)}, \end{equation} \tag{ 8 }$$

where Σ₀ is the central surface brightness and r_d is the disk scale length. The half light radius (r_e) is related to disk scale length as r_e = 1.68 × r_d and Σ_e = Σ₀/5.36.

The bulge–disk decomposition was done by χ² minimization. In this technique the observed image is compared with a point-spread function (PSF) convolved linear combination of Sérsic and exponential functions. Therefore, it was necessary to model the PSF well in order to reliably estimate the photometric parameters. We used stellar images closer to the galaxy to represent the PSF. For that we selected stars from the CFHT images on the basis of the CLASS_STAR parameter from SEXTRACTOR and created stamp size images. These stellar images were checked manually using the IMEXAMINE program in IRAF. This helped to remove saturated stars and some other stars that are contaminated by bright neighbors. In Tables 2 and 3 we list the fitted values of the bulge and disk parameters estimated using the g- and r-band CFHT images and the SDSS images. We found CFHT results fairly consistent with those obtained using the SDSS images, although the former were obtained under better seeing conditions. Although we used the pipeline, we vary the initial conditions of different parameters to check whether the final results correspond to a global minimum. We found that the results are stable against different initial conditions. Finally, we checked for any possible contribution from a possible point source at the center of the galaxy. We found that any such point source must be too faint and its inclusion does not improve the fit. The magnitudes shown in Tables 2 and 3 have been corrected for foreground extinction using Schlegel et al. (1998). We also applied k-correction using E/S0 spectral energy distribution from Poggianti et al. (Poggianti 1997). Figure 8 shows a comparison of the modeled profiles of bulge and disk components with the observed profile of the galaxy in the CFHT g and r bands. These are generated using the IRAF/ELLIPSE task from the PSF convolved output images generated by GALFIT. It can be seen that the bulge–disk model agrees very well with the observed profile of the galaxy.

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The Sérsic index for the bulge is found to be small at n ≈ 1, which essentially implies a "pseudobulge" with a disklike exponential light profile (n = 1 gives a pure exponential profile, and n = 4 corresponds to the de Vaucouleurs profile). Moreover, the scale length of the bulge is very small compared with the disk (r_d ≈ 7 × r_e), and only about a 14%–18% fraction of the total g-band or r-band luminosity is contributed by the bulge. This again indicates that the J2345−0449 spiral host contains a pseudobulge rather than a classical bulge component. Galaxies with classical bulges generally have a much more centrally peaked light profile, contain a higher fraction of total light, and their Sérsic index is larger (n ≈ 2–6) than pseudobulges, which have n < 2 (Fisher & Drory 2008; Kormendy et al. 2010).

3.10. Additional Checks to Confirm the Pseudobulge Nature of J2345−0449

We have compared several photometric parameters of the (pseudo) bulge in J2345−0449 to some other well-known previous studies in order to further verify its pseudobulge classification. According to Gadotti (2009), pseudobulges have a small Sérsic index and scale radius and occur in galaxies that have a low bulge-to-total light ratio (B/T). Our estimated values of these parameters (Tables 2 and 3) fall very close to those of pseudobulge systems in Gadotti (2009). We further compared our values with those of galaxies studied by Fisher & Drory (2008). In Figure 9 we show that our fitted values for J2345−0449 fall near the pseudobulge galaxies in n_b − M_V, n_b − μ_e, and n_b − r_e planes of Fisher & Drory (2008). Similar trends were found in the μ_e–M_V and μ_e–r_e planes. Here n_b, M_V, r_e, and μ_e are the Sérsic index, absolute V magnitude, scale radius, and average surface brightness within r_e, respectively. It is clear that J2345−0449 is located closer to pseudobulges and far from classical bulge systems. However, the brightness of the bulge and r_e in J2345−0449 is higher than that shown for pseudobulge disk galaxies in Fisher & Drory (2008). Further evidence comes from the location of J2345−0449 in the 〈_b〉/〈_d〉 − B/T plane (Figure 9), where _b and _d are the ellipticities of the bulge and disk. We found that our values for J2345−0449 closely match with those of the pseudobulge population, and the ratio of ellipticities (〈_b〉/〈_d〉) is higher than for classical bulges in galaxies of similar morphological T-type¹¹. All the above checks strongly suggest that the central bulge in J2345−0449 should be classified as a pseudobulge rather than a classical bulge. Consequently, the evolution of the J2345−0449 host galaxy is probably governed by quiet "secular evolution" processes in which disk instabilities drive gas to the center, rather than violent merger events that would have led to a classical bulge (Kormendy et al. 2010). This inference is further bolstered by the location of J2345−0449 in a sparse galactic environment, the absence of any visible tidal debris (like tails, plumes, or shells), and its well-developed spiral arms within a stable, rotationally supported stellar disk.

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Table 1. WISE Mid-infrared Magnitudes and Luminosities for J2345−0449

WISE Band	W1	W2	W3	W4
(Wavelength)	(3.4 μm)	(4.6 μm)	(12 μm)	(22 μm)
Magnitude	11.882 ± 0.023	11.818 ± 0.023	9.001 ± 0.028	7.096 ± 0.100
Luminosity	6.71 × 1043	2.88 × 1043	2.62 × 1043	2.26 × 1043
Luminosity (L☉)	4.04 × 1011	4.41 × 1011	5.69 × 1012	3.35 × 1013
S/N	47.9	47.8	38.7	10.9

Notes. Magnitudes with their errors are given in the first row in the four mid-IR bands. The second row shows the derived luminosities in erg s⁻¹. In the third row the luminosity is shown in solar units, and the fourth row shows the signal-to-noise ratios of detection in four bands.

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Table 2. Results of Two-dimensional Bulge–Disk Decomposition

Telescope	mb	md	mtot
(Filter)	(mag)	(mag)	(mag)
CFHTg	17.14 ± 0.01	15.18 ± 0.01	15.02 ± 0.01
CFHTr	16.36 ± 0.01	14.75 ± 0.01	14.53 ± 0.01
SDSSg	17.52 ± 0.03	15.15 ± 0.01	15.03 ± 0.03
SDSSr	16.56 ± 0.02	14.72 ± 0.01	14.54 ± 0.02
SDSSi	16.22 ± 0.02	14.43 ± 0.01	14.24 ± 0.02
SDSSz	15.72 ± 0.04	14.34 ± 0.01	14.07 ± 0.04

Notes. Here m_b and m_d are the magnitudes of bulge and disk components, and m_tot is the total magnitude. CFHT_x and SDSS_x denote the fits using either CFHT or the SDSS images in filter band x. All magnitudes are k-corrected and also corrected for galactic extinction, as described in the main text.

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Table 3. Results of Two-dimensional Bulge–Disk Decomposition

Telescope	re	n	rd	b/ab	b/ad
(Filter)	(kpc)		(kpc)
CFHTg	1.18 ± 0.01	1.05 ± 0.02	7.51 ± 0.01	0.56 ± 0.01	0.53 ± 0.01
CFHTr	1.25 ± 0.01	1.26 ± 0.02	7.06 ± 0.01	0.56 ± 0.01	0.53 ± 0.01
SDSSg	0.87 ± 0.13	0.26 ± 0.58	5.64 ± 0.03	0.44 ± 0.08	0.54 ± 0.01
SDSSr	1.02 ± 0.07	0.30 ± 0.26	5.38 ± 0.03	0.44 ± 0.04	0.55 ± 0.01
SDSSi	1.12 ± 0.03	0.55 ± 0.15	5.18 ± 0.03	0.49 ± 0.03	0.56 ± 0.01
SDSSz	1.09 ± 0.06	0.95 ± 0.26	5.10 ± 0.08	0.66 ± 0.04	0.54 ± 0.01

Notes. Here n is the Sérsic index, r_e and r_d are the bulge and disk scale lengths, and b/a_b and b/a_d are the axis ratios (semiminor/semimajor) of the bulge and disk components, respectively. CFHT_x and SDSS_x denote the fits using either CFHT or the SDSS images in filter band x. The fitting procedure is described in the main text.

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The extreme nature of this spiral galaxy is underscored, firstly, by its unusual rotation curve. Rotation curves are the major tools for determining the mass distribution in spiral galaxies, e.g. (Sofue & Rubin 2001). The kinematics of the Balmer Hα line in the long-slit spectrum along the major axis, taken with the IFOSC on the IGO 2 m telescope, shows a rapidly rising rotational velocity in the inner ∼1 kpc, attaining an extremely large value, V_rot ∼ 430 km s⁻¹ (inclination corrected) in the asymptotic flat region at r ≳ 10 kpc from the galactic center (Section 3.5 and Figure 6). No kinematic signature of rotation is detected along the minor axis, which is clear evidence for a planar, rotating galactic disk. Such rapid disk rotation (V_rot ∼ 400–500 km s⁻¹) is rare and has been reported earlier for just a few exceptionally massive spirals (e.g., Giovanelli et al. 1986; Rigopoulou et al. 2002; Akos et al. 2013). Assuming a quasi-spherical halo, the dynamical mass (luminous plus dark matter) enclosed within the radius r is $M_{\rm dyn} = \kappa r {V_{\rm rot}^{2}}/G$, where κ is a correction factor in the range 0.6–1 for a thin disk to a spherical halo mass distribution model (Lequeux 1983), and G is the gravitational constant. We estimate M_dyn = 1.07(± 0.13) × 10¹² M_☉ for κ = 1 and V_rot ∼ 430 km s⁻¹ at r = 25 kpc, i.e., ∼3.5 times the scale radius of the exponential disk. This places J2345−0449 amongst the brightest and most massive spirals known, with ∼7 times the mass of the Milky Way within an equivalent radius, using the Tully–Fisher relation. Furthermore, we obtain its virial mass M₂₀₀ ∼ 1.05 × 10¹³ M_☉ and virial radius R₂₀₀ ∼ 450 kpc by using the baryonic Tully–Fisher relation for ΛCDM cosmology (McGaugh 2005; $M_{\rm 200} \propto V_{\rm rot}^{3.23}$) and scaling from M₂₀₀ and V_rot (448 km s⁻¹) of the fast rotating spiral galaxy NGC 1961 (Akos et al. 2013).

Secondly, photometry and two-dimensional luminosity profile fitting of the CFHT and SDSS images showed that J2345−0449 is an extremely luminous (optical r band: M_r = −23.26, near-IR K band: M_K = −26.15), isolated spiral galaxy (see above) that lacks a spheroidal classical bulge but has a compact disklike "pseudobulge" with a low Sérsic index (n ≈ 1), which contributes only ∼14% to the total g-band luminosity. In contrast, classical bulges generally have a more centrally peaked brightness profile characterized by a larger Sérsic index of n ≈ 2–5 (Fisher & Drory 2008). Recent recognition of a high fraction (∼60%) of nearby spiral galaxies without a kinematically "hot" central bulge challenges the conventional ideas of hierarchical galaxy formation; How have so many bulgeless pure disk galaxies come to exist, despite the galaxy mergers known to occur (Fisher & Drory 2008; Kormendy et al. 2010)? We discuss this further below in the context of mass estimation of the central SMBH. J2345−0449 shows ongoing star-forming activity across the galactic disk, traced by Hα and [N ii] emission lines. It is also strongly detected by the Galaxy Evolution Explorer in the near (2316 Å) and far UV (1539 Å) bands (Martin et al. 2005), indicating a starburst of massive, hot OB stars during the past ∼100 Myr. Interestingly, UV or Hα signatures of recent star formation (∼100 Myr) are absent closer to the nucleus, showing only stellar populations older than ∼1 Gyr (Figure 7). We interpret this as a result of the heating and expulsion of the star-forming medium by energetic, episodic feedback of the central AGN, as revealed by two pairs of radio lobes.

Another unusual property displayed by this disk galaxy is its exceptionally large stellar velocity dispersion of σ_* = 379(± 25) km s⁻¹ measured along its major axis, and σ_* = 351(± 25) km s⁻¹ along the minor axis, averaged within the central 2.35 kpc region (Section 3.6). The slightly higher σ_* for the major axis possibly indicates some contribution from the disk rotation. Hence, we adopt the smaller σ_* of the minor axis as representative of the galaxy's central velocity dispersion. It is noteworthy that this dispersion value is significantly larger than that known for the majority of bulgeless disk galaxies on such a spatial scale (e.g., Hu 2008; Graham et al. 2011; Kormendy et al. 2011; Shankar et al. 2012), and is compares well with that usually found for massive elliptical galaxies (e.g., Haring & Rix 2004; Marconi & Hunt 2003; Gultekin et al. 2009). This strongly hints at a large central concentration of mass, including a putative SMBH. We obtain the dynamical mass of the (pseudo-)bulge M_b = 1.07(± 0.14) × 10¹¹ M_☉, within one scale radius (r_e = 1.25 kpc), and a black hole mass M_BH = 2.54(± 0.48) × 10⁸ M_☉ using the black hole mass versus bulge mass correlation found by (Marconi & Hunt 2003). This value is comparable to another estimate of M_BH = 3.88(± 0.40) × 10⁸ M_☉, obtained from the M_BH–σ_* correlation of (Hu 2008) for the pseudobulges. However if we use the M_BH–σ_* relation of Gultekin et al. (Gultekin et al. 2009), calibrated using a sample of both ellipticals and spirals, we obtain the black hole mass M_BH = 1.43 × 10⁹ M_☉. Similarly, using the K-band M_BH–L_K relation of Graham (Graham 2007), we obtain M_BH = 1.22 × 10⁹ M_☉. Both these latter masses are close to each other yet are significantly larger than the previous two mass estimates. We need to verify these rather large mass values via more direct and robust methods. If confirmed it would imply that J2345−0449 hosts an SMBH as massive as those found in giant elliptical galaxies, which is not expected for normal disk galaxies and is extremely unusual for a pure disk galaxy like J2345−0449.

Despite lacking a classical bulge, this exceptional spiral galaxy is likely to host an unusually massive, accreting (radio-loud) black hole of M_BH > 10⁸ M_☉. However, our present data do not provide strong constraints on M_BH, mainly because J2345−0449 lacks a classical bulge, whereas the black hole mass is more easily obtained from the tight M_BH–σ_* correlation in bulge-dominated systems (Haring & Rix 2004; Marconi & Hunt 2003; Gultekin et al. 2009). For this reason the demographics of black holes in disk galaxies are poorly determined. However, there is growing evidence that a classical bulge is not essential for nurturing a massive black hole and some pseudobulge systems may also host fairly massive black holes of masses 10³–10⁷ M_☉ (Kormendy et al. 2011; Shankar et al. 2012). Therefore, these black holes must somehow attain masses substantially larger than the values implied by their bulge properties, possibly via a disk-driven, non-merger growth route. It appears that J2345−0449 is one such system, being an extreme member of such a population. Very little is known of the growth and AGN activity of SMBHs in bulgeless spiral galaxies. Therefore, it would be a very significant result if more detailed observations show that J2345−0449 harbors an SMBH of >10⁸ M_☉. We note that some previous works have suggested (e.g., Hu 2008; Graham et al. 2011; Shankar et al. 2012) that pseudobulge and barred galaxies may host black holes that are about 3–10 times less massive than pure ellipticals with the same σ_*. It has also been proposed that their black hole masses do not correlate well with the properties of galaxy disks or their pseudobulges (Kormendy et al. 2011), and hence the growth process of massive black holes in flattened galactic disks lacking prominent bulges is still mysterious and a subject of intense debates and investigations (Shankar et al. 2012; Kormendy et al. 2011; Simmons et al. 2013).

Our estimation of black hole mass in J2345−0449 could be a lower limit, constrained by the lack of subarcsecond resolution spectral data. Nevertheless, it is interesting to note that the lower range of our estimated black hole mass (2–4 × 10⁸ M_☉) is close to the minimum required for the production of large-scale relativistic jets in radio galaxies and quasars (Laor 2000; Gopal-Krishna et al. 2008; Sikora et al. 2007; Chiaberge & Marconi 2011). At the present stage one cannot rule out an even more massive black hole of ≳ 10⁹ M_☉ in this galaxy, given its extreme radio properties and exceptionally large mass. Therefore, a more robust and direct estimate of the black hole mass in J2345−0449 is needed, possibly via spatially resolved stellar kinematics or gas dynamics measurements close to its gravitational sphere of influence: r_g ∼ 50 (M_BH/10⁹ M_☉)(σ_*/300 km s⁻¹)⁻² pc ≡ 0035–035 for M_BH ∼ 10⁹–10¹⁰ M_☉. At a distance of ∼340 Mpc ($1^{^{\prime \prime }}\equiv 1.43$ kpc), such a refined measurement is challenging if M_BH ≲ 10⁹ M_☉, requiring very high (∼001) spatial resolution. Nevertheless, this method has recently revealed a 1.7 × 10¹⁰ M_☉ extramassive black hole in the nearby galaxy NGC 1277, another fast-rotating disk galaxy with a pseudobulge (Van den Bosch et al. 2012). Interestingly, for NGC 1277 the stellar velocity dispersion is ∼333 km s⁻¹ at the half light radius (28)—very close to the present case— which rises to >400 km s⁻¹ near the black hole's sphere of influence. Therefore, it is tempting to speculate that J2345−0449 might also harbor an extremely massive black hole, which would help to explain its unusual radio properties, further discussed below.

Because it is not obvious how this galaxy acquired such a range of extraordinary properties, it poses challenges to theoretical models and holds important clues for understanding the coevolution of black holes and disk galaxies, accretion physics, the formation of large-scale relativistic jets in AGNs, and a possible solution to the long-standing puzzle of why disk galaxies are nearly universally devoid of powerful double radio sources. We propose that this spiral galaxy's huge mass, angular momentum, and episodic radio jet ejection are all the results of its rare, unusual evolutionary history that has led to the formation of both a rotationally supported massive, fast-spinning galactic disk (here (V_rot/σ_*)² ∼ 1.5) and a mass-accereting, possibly rapidly spinning, unusually massive central SMBH.

Very little is known about the formation and evolution of such extremely massive spiral galaxies, as they are found rarely and hardly ever known to eject powerful relativistic jets on megaparsec scales like J2345−0449. The cosmic evolution of the galactic disk of J2345−0449 may have been primarily driven by external cold gas accretion and internal "secular" processes instead of recent (t ≲ few× 10⁹ yr) major merger events. This is suggested by its disklike pseudobulge, the absence of close galactic neighbors and tidal debris (e.g., stellar streams, plumes, or shells), and its stable, rotationally supported massive stellar disk showing well-formed spiral arms. Furthermore, a close alignment (within ∼10°) of the inner and outer radio lobe pairs implies that the black hole's spin axis, which in equilibrium orients itself orthogonal to the inner accretion disk because of the Lense–Thirring frame dragging effect (Bardeen & Petterson 1975), has remained stable (nonprecessing), at least between the two last episodes of jet triggering on timescales of t ∼ 10⁸ yr. All this suggests that the galactic disk and its central black hole might have a quiet evolution together, without major mergers. The massive galactic disk is clearly rotationally supported ((V_rot/σ_*)² ∼ 1.5) and should be dynamically stable against gravitational instability.

The formation epoch of J2345−0449 probably also lies in the early universe (redshifts z ≳ 2–3) more than 10 Gyr ago, i.e., during the era of galaxy formation and quasar activity. In that era, most galactic disks (pancakes) were unvirialized, still assembling around isolated halos via minor mergers and mass accretion from cool, gas-rich filaments of the cosmic web and often experiencing high SFRs of ∼10²–10³ M_☉ yr⁻¹. The initial buildup of the present large mass and angular momentum of the galactic disk of J2345−0449 possibly started very early (probably z ≳ 6) during the pancake stage of its parent halo, via gravitational tidal torques and rapid disk feeding through a few dominant, coplanar cold streams ("cold mode" accretion), as recently observed for a star-forming disk galaxy at z = 2.3 (Bouche et al. 2013) and also shown in numerical simulations (Keres et al. 2005; Governato et al. 2007; Dekel et al. 2009; Danovich et al. 2012). For very massive halos of mass ≳ 10¹² M_☉, another "hot mode" growth also takes place via the intergalactic gas accreting quasi-spherically, experiencing an accretion shock and heating to virial temperature as it collides with the hot, hydrostatic halo near the virial radius, and then being accreted onto the galaxy on a radiative cooling timescale (Keres et al. 2005; OcVirk et al. 2008). Simultaneously, the growth of the massive black hole of this bulgeless spiral possibly started from an initial "seed" black hole mass, via an internal disk-driven (secular) accretional route, through a rapid phase of mass and angular momentum (spin) transfer from a gas-rich accretion disk fed at a high Eddington rate (Berti & Volonteri 2008). Its inner accretion zone was coupled to the fast-spinning outer galactic disk via the viscous and magnetic torques. This coherent growth process would spin-up the growing black hole to a high value if copious fuel supply is available for a long enough time, in contrast to the low final spin expected in a stochastic (i.e., chaotic or incoherent) merger-driven growth scenario (Dotti et al. 2013; Dubois et al. 2014). It is well known that a black hole approximately doubling its mass by accreting a constant angular momentum gas from a standard thin disk would acquire a maximum "canonical" spin of a = 0.998 (Thorne 1974) due to a braking torque, where the dimensionless spin parameter is $a = c J_{\rm BH}/G M_{\rm BH}^{2}$ and the black hole's angular momentum is J_BH. Such a massive, accreting–spinning black hole would be a highly efficient "engine" for launching fast, collimated plasma jets via the BZ mechanism (or variants of it), further boosted by a strong buildup of magnetic flux near the black hole horizon by advection of the accretion flow. Below we further comment on the feasibility and implications of such an SMBH growth and jet formation process in J2345−0449.

Obviously, the most striking property that sets J2345−0449 apart from nearly all other spiral galaxies (including Seyferts) is its large mass and unusual radio loudness, resulting from the highly efficient ejection of relativistic jets on megaparsec scales. The extreme rarity of such galaxies implies that whatever physical process that created such huge radio jets in J2345−0449 must be very difficult to realize and maintain for long periods of time in most other spiral/disk galaxies. Thus, an important question is: What could be responsible for the efficient fueling and sustained collimated jet ejection activity in J2345−0449? The answer requires very detailed knowledge of the properties of its central black hole engine, i.e., its mass, spin, and the nature of its accretion flow, namely the mass accretion rate and the magnetic field topology of the inner accretion disk, most of which is clearly lacking at present. Although the exact mechanism of jet launching is not known for J2345−0449, a promising and widely accepted explanation is the BZ process that efficiently extracts the rotational energy of the black hole and the orbital energy of the accretion flow into an intense outflow of a Poynting flux dominated electromagnetic jet. In the BZ process the jet launching requires a massive, spinning Kerr black hole and an accretion disk threaded with strong poloidal magnetic field lines advected near the hole's outer horizon at radius R_H = (GM_BH/c²)[1 + (1 − a²)]^1/2. In the classic BZ process, the electromagnetic luminosity of the jet $\bar{Q}$ scales with black hole properties as

$$\begin{equation} \bar{Q}_{44} \approx 0.2\, M_{8}^{2} \, a^{2} \, B_{4}^{2}, \end{equation} \tag{ 9 }$$

where $\bar{Q}_{44} = \bar{Q}/10^{44}\ {\rm erg\ s^{-1}}$, the black hole mass M₈ = M_BH/10⁸ M_☉, and the poloidal magnetic field B₄ = B/10⁴ G (Blandford & Znajek 1977; MacDonald & Thorne 1982; Tchekhovskoy et al. 2010; Sadowski et al. 2013; Penna et al. 2013). Thus, the jet power depends strongly on the black hole mass, its spin, and the magnetic flux at the horizon. This prediction is confirmed by recent numerical general relativistic magnetohydrodynamic simulations that show the efficiency (η) of the conversion of accretion luminosity to the jet luminosity (including a possible coronal wind) could be as high as $\eta = (\bar{Q}/{\dot{M}} c^{2}) \sim 30\%\hbox{--}140\%$ for spin a = 0.5–0.99, if the magnetized accretion flow manages to accumulate (via advection and turbulent amplification) large magnetic flux near the black hole's horizon (Tchekhovskoy et al. 2010; Tchekhovskoy et al. 2011; Penna et al. 2013). Similar results have been found by Hawley and Krolik (Hawley & Krolik 2006), which demonstrate that the electromagnetic extraction of the spin energy of a black hole is very much feasible.

Observationally, the jet kinetic power ($\bar{Q}$) is a key descriptor of the state of an accreting SMBH system; its mass, its spin, and the magnetic field of the accretion disk. The correlation of low-frequency (ν ∼ 150 MHz) radiative radio power of FR II radio sources against their jet power shows that the radio luminosity of the jet constitutes only a small fraction (<1%) of the total kinetic power output (Punsly & Zhang 2011; Daly et al. 2012). Here we use the low-frequency radio flux, which usually originates in diffuse, optically thin radio lobes moving at low velocities, and thus the relativistic beaming effects are negligible. We estimate the 151 MHz lobe flux density to be 15.75 Jy (excluding the inner jet–lobe pair and the core) using the 0.3 GHz and 1.4 GHz measurements and the observed radio spectral index α = 2. Using the published calibration relations (Punsly & Zhang 2011; Daly et al. 2012), we obtain $\bar{Q} \approx 1.7 \times 10^{44}$ erg s⁻¹ for the (time-averaged) jet kinetic power in J2345−0449. A similar value of $\bar{Q}$ is obtained using the relation for estimating the total jet power from radio power, for an M_BH ∼ 10⁸ M_☉ (Equation (1) in Meier 2001). As we have applied no correction for the (unknown) energy loss in the outer radio lobes, $\bar{Q}$ is likely to be a lower limit. This high value of $\bar{Q}$ in J2345−0449 is typical of high-powered FR II jets found in radio-loud quasars (Punsly & Zhang 2011) and is above the FR I/FR II divide at $\bar{Q} \approx 5 \times 10^{43}$ erg s⁻¹, suggesting a highly efficient supersonic jet ejection (Rawlings & Saunders 1991) that is energetic enough to pierce through the host galaxy's dense environment.

Therefore, assuming M_BH ∼ 2 × 10⁸ M_☉, the lower of the aforementioned black hole mass estimates for J2345−0449, and assuming a rather strong magnetic field of B = 2 × 10⁴ G, Equation (9) above shows that the estimated jet power can be achieved via the classic (MHD-dominated) BZ process for a ≳ 0.73, i.e., needing a rapid yet not maximally spinning black hole. However, even if this black hole is near maximally spinning (a = 0.98), the magnetic field cannot be lower than B ≈ 1.5 × 10⁴ G, highlighting the key role of magnetic field advection in extracting the spin energy of the hole. It is also evident that for the same black hole mass as above, but for a very low spin a ≈ 0.1, the estimated $\bar{Q}$ is not obtained unless the disk magnetic field B ≳ 1.5 × 10⁵ G. However, such an extremely strong magnetic field might never occur normally, as the field strength near the inner accretion disk is not expected to exceed the Eddington value $B_{{\rm Edd}} \approx 6 \times 10^{4} \, M_{8}^{-1/2}$ G for accretion at the Eddington rate. From mid-IR colors, a lack of broad lines, and a very weak Hα flux observed in the AGN's optical spectrum, we inferred above that the accretion state of the black hole in J2345−0449 is possibly sub-Eddington. This is a characteristic of a radiatively inefficient, low luminosity AGNs (LLAGN/LINER). Thus, the above analysis suggests that in the launching of highly efficient jetted outflow in J2345−0449, the spin of the black hole and its accretion state might be playing dominant roles, and additionally, the black hole is required to be unusually massive, despite being hosted by a spiral galaxy. We point out that compared with the pure BZ process, mainly considered here for its simplicity, some more complex hybrid models of jet production that are based on a combination of BP (Blandford & Payne 1982) and BZ mechanisms (Blandford & Znajek 1977) have been proposed (e.g., Meier 2001; Nemmen et al. 2007), which predict higher jet luminosities under similar physical conditions.

Is such a rare combination of an unusually high black hole mass and spin and a low accretion state achievable in a bulgeless disk galaxy? Recent numerical simulations (Dotti et al. 2013) of black hole growth (via accretion and mergers) predict that at low redshifts the most rapidly rotating black holes (a > 0.9) are also the most massive ones (10⁸–10⁹ M_☉) and their spin orientations remain stable over many accretion cycles. However, such a black hole generally ends up in an elliptical galaxy and not in a pure spiral, contrary to the present case. For this to happen in a spiral galaxy like J2345−0449 would require a coherent, high rate of mass accretion, whereby the black hole grows rapidly and is spun-up via matter accreted with a well-defined, almost constant angular momentum direction instead of major mergers. Because of this unusual evolutionary route, the fundamental correlations of the black hole mass and galaxy properties observed in bulge-dominated systems are unlikely to hold in the present case. Future studies might further explore this important aspect in this and similar systems. It is especially important to verify if the black hole in J2345−0449 is indeed more massive than the minimum value (∼10⁸–10⁹ M_☉) inferred in several works for the development of powerful radio jets in AGNs (e.g., Laor 2000; Gopal-Krishna et al. 2008; Sikora et al. 2007; Chiaberge & Marconi 2011). Such a large black hole mass will pose a challenge for the growth models of black holes in disk galaxies. Our ideas can be better tested when in the future more accurate measurements of both the mass and spin of the black hole in J2345−0449 become available.

It seems plausible that the unusual jet activity of J2345−0449 may have been triggered by the AGN accretion state switching from a previous high Eddington rate (λ ≳ 10⁻²–10⁻³) to a sub-Eddington rate, where

$$\begin{equation} \lambda = {\dot{M}} /{{\dot{M}}_{{\rm Edd}}} \end{equation} \tag{ 10 }$$

denotes the dimensionless mass accretion rate in units of the Eddington accretion rate ${{\dot{M}}_{{\rm Edd}}}$. A strong link between jet formation and accretion rate has been discussed in the literature (e.g., Rees et al. 1982; Meier 2001), which is of direct relevance in the context of the giant radio jets found in this unusual spiral galaxy. It is well known that at accretion rates above a critical value λ_crit ∼ 10⁻²–10⁻³, the disk structure is a standard Shakura and Sunyaev, radiatively efficient, geometrically thin disk (Shakura & Sunyaev 1973), whereas at reduced accretion rates (λ ≪ λ_crit), a transition to an ADAF results, fueled by hot gas from a large-scale quasi-spherical halo (Narayan & Yi 1994; Narayan & Yi 1995). In ADAF most of the heat generated by the local, turbulent viscous heating is rapidly advected inside the black hole horizon (and eventually swallowed) or released in a wind or radio jet. Thus, the radiative cooling efficiency of the geometrically thick, optically thin disk is greatly diminished. Therefore, one would find black holes in an ADAF state to be unusually faint (disk bolometric luminosity $L_{{\rm bol}} \ll 0.1 {\dot{M}} c^{2}$), such as the galactic center black hole Sgr A* or the radio galaxy M87/Virgo-A (Falcke et al. 2004). In ADAF the inner region of the accretion disk gets extremely hot, forming a thick ion torus that helps to collimate the relativistic outflow into the "funnel" regions at the poles (Rees et al. 1982). In addition, the thick torus effectively anchors the strong poloidal magnetic field that is required to channel the rotational energy of the hole in the form of electromagnetic fields and particles via the BZ mechanism (Beckwith & Hawley 2008). The magnetic field plays another important role as MHD turbulence in accretion flow is generated by the magnetorotational instability (Balbus & Hawley 1991), which provides the necessary viscous torque for removing angular momentum from the accreting gas, thus driving the inflow and amplifying the large-scale magnetic field.

This is an attractive scenario for J2345−0449 because the AGN in it is presently in the low-excitation state (LERG/LLAGN), showing extremely low radiative efficiency but a very high kinetic luminosity in jetted outflows, strongly reminiscent of an ADAF. This observation provides an important clue as to how the radio jets in J2345−0449 might be launched. We further investigate this rare occurrence of relativistic jets in light of the fundamental paradigm that in spite of a vast difference in involved black hole mass, length, and timescales, almost all relativistic disk–jet coupled phenomena happen in a scale invariant manner in both radio-loud AGNs and the galactic "microquasars" (Mirabel & Rodriguez 1994). In galactic black hole X-ray binaries, a transition to a "low-hard" X-ray state is generally always accompanied by steady radio jet formation, whereas their radio emission is quenched in "high-soft" X-ray state (Fender et al. 2004). This indicates a strong link between the jet formation and accretion state of a black hole. In this context a "fundamental plane" (FP) of accreting black holes that links SMBHs to stellar mass galactic black holes has been proposed, finding a positive correlation between their core radio (L_R) and X-ray (L_X) luminosities, with the black hole mass as a scaling factor, of the form (Merloni et al. 2003; Falcke et al. 2004)

$$\begin{equation} \log L_{\rm R} = 0.60\, \log L_{\rm X} + 0.78\, \log M_{\rm BH} + 7.33, \end{equation} \tag{ 11 }$$

where L_R (at 5 GHz) and L_X (in 2–10 KeV band) are in erg s⁻¹ and the black hole mass is in M_☉ units. The FP is a powerful scheme that unifies the physics of accretion and jet formation for black holes across an enormous ∼10¹–10¹⁰ M_☉ mass range, primarily in their sub-Eddington, low-hard state. At present we lack a hard X-ray flux and a robust mass estimate for the black hole in J2345−0449 to place it on the FP. Nevertheless, assuming the central black hole mass is in the range of 10⁸–10⁹ M_☉, the above FP relation predicts a core X-ray luminosity of L_X ∼ 2.4 × 10⁴³ erg s⁻¹ to L_X ∼ 1.2 × 10⁴² erg s⁻¹, respectively, using the VLA 5 GHz core (the jet base) radio luminosity of L_R ∼ 4 × 10³⁹ erg s⁻¹. Therefore, a future measurement of L_X via X-ray observations and an independent estimate of the black hole mass would reveal if the AGN in J2345−0449 (putatively in a low-hard ADAF state) conforms to the FP of accreting black holes or not. Such data will provide a powerful means of testing the disk–jet coupling theories in an extremely unusual circumstance and may also suggest some new, hitherto untested ways of forming relativistic jets. However, the fundamental question remains of why such a transition of the accretion state does not commonly occur in other spiral galaxies, which almost never show extended radio jets and are mostly radio-quiet. As we have discussed above, the answer could be that perhaps in our present galaxy, the efficiency of jet launch is greatly boosted by an unusually massive (and possibly rapidly spinning) SMBH (at least a few × 10⁸ M_☉) in the galaxy's nucleus, formed via a predominantly disk driven accretion route. Moreover, strong, almost saturated magnetic field lines anchoring the accretion disk might have also enhanced the luminosity of the jet, which can take place only via a geometrically thick ADAF (Sikora & Begelman 2013). Moreover, in the ADAF, unlike the thin-disk case, the black hole spin is a crucial factor for powerful jet formation (Meier 2001; Tchekhovskoy et al. 2010; Sadowski et al. 2013).

Such an overmassive, spinning black hole in a pure (spheroidal bulgeless) disk galaxy can result only via a previous rapid growth phase at a high mass accretion rate λ ∼ 0.1–0.01. In this formative phase, now long past, the galaxy J2345−0449 might have shone like a luminous quasar because of the large radiative efficiency of a strongly accreting thin disk. In accretion models, the radiative efficiency and growth rate of the black hole depend only on the dimensionless parameter λ and not on the absolute value of M or ${\dot{M}}$. The e-folding black hole growth time t_BH and the black hole mass at a given time M(t) are given by (Shapiro 2005; Hopkins et al. 2006)

$$\begin{equation} t_{\rm BH} = M/{\dot{M}} = t_{\rm sal}/\lambda \end{equation} \tag{ 12 }$$

$$\begin{equation} M{(t)} = M{\rm (0)} \exp [\, ((1 - \epsilon)/\epsilon) \, (t/t_{\rm sal}) \, \lambda \,], \end{equation} \tag{ 13 }$$

where the Salpeter timescale t_sal = 0.45 Gyr, the initial black hole seed mass M(0) at t = 0, and the radiative efficiency of the accreted mass into energy ≈ 0.06–0.42 (for spin a = 0–1). With gradual mass and angular momentum transfer and a steady increase of the black hole spin a, the radiative efficiency would also rise (for a radiatively efficient disk), slowing down the black hole growth rate. Thus, in the initial phase of black hole growth the mass accretion rate needs to be near Eddington (λ ∼ 1) and the spin-up rate ${\dot{a}}$ slow so that the black hole may grow to an unusually large final mass via gaseous accretion. We have no estimate for the initial seed mass of the black hole in J2345−0449, and the cosmic history of its spin and mass evolution is also unknown. Nevertheless, starting from a Population-III seed black hole of M(0) ∼ 10² M_☉, formed via stellar route at z ≈ 40, and a coherent, average high Eddington rate accretion ($\bar{\lambda } \approx 0.1$, $\bar{\epsilon } =0.1$), it still requires ∼18 e-folding times of t ≈ 18 × t_sal ≈ 8 Gyr (z(t) ≈ 0.6) to grow a black hole to ∼10⁹ M_☉ (Equation (13)). However, this is shortened to t ≈ 1.8 × t_sal = 0.8 Gyr (by z(t) ≈ 6.8) for the near-Eddington rate accretion ($\bar{\lambda } \approx 1$). For significantly lower accretion rates (λ ≪ 1), the growth timescale becomes longer than the age of the universe and the black hole cannot possibly accumulate a significant fraction of its mass. Thus, a relatively short, initial phase of rapid growth of the central black hole at the near-Eddington rate accretion is suggested for J2345−0449, implying that presently we are observing its AGN as a faded remnant of an erstwhile highly luminous QSO phase.

Therefore, in our proposed model, the radio jet launching spiral galaxy J2345−0449 finds itself in an accretion state similar to local radio-loud ellipticals that have a low-excitation state, in contrast to the recently discovered radio-loud, NLSy1 AGNs in spiral galaxies (extreme objects themselves) that are inferred to be accreting at near-Eddington rates (λ ≈ 0.1–1), host smaller mass black holes (M_BH ∼ 10⁶–10⁸ M_☉), and are never observed to eject 0.1–1 Mpc scale relativistic jets (Doi 2012; Foschini 2011; Komossa et al. 2006). Moreover, the existence of this galaxy is at odds with the finding that among the AGN population in the local universe, essentially all AGNs with moderate to large accretion rates (i.e., t_BH < t_Hubble ∼ 14 Gyr) are in spirals, whereas their counterparts in almost every elliptical have a negligible mass accretion rate (t_BH > t_Hubble; Hopkins et al. 2006). In J2345−0449 an accretion rate–dependent state transition to low radiative efficiency, ADAF state might have taken place because of the enormous growth of its accreting black hole itself and its energetic feedback acting on the accretion flow, heating the latter and thus reducing the AGN fueling rate below the critical value λ_crit ∼ 10⁻²–10⁻³. The two sets of radio lobes forming the double–double structure are clearly indicative of an intermittent radio jet activity, possibly resulting from an episodic switching of the accretion modes between a low (ADAF, powerful radio jets, low radiative efficiency) to high (thin disk, no/weak radio jets, high radiative efficiency) accretion states. Thus, finding the black hole's mass and spin and the impact of the highly energetic jet outflow on the surrounding galaxy is clearly very important. Perhaps the most crucial clues for understanding this extraordinary and puzzling radio galaxy would come from its future detailed X-ray, UV, infrared, and radio observations. This will allow the complete spectral energy distribution of the galaxy and central AGN to be determined for theoretical modeling. A sensitive X-ray observation of a possible corona of hot, X-ray emitting gas around the massive spiral host, a remnant of its formation history, and detection of an AGN with a hard X-ray spectrum (peaking at ∼100 keV) showing low bolometric luminosity would strengthen the model outlined above. The spin of the black hole can be determined by the method of continuum fitting the flux and temperature of the X-ray emission from the accretion disk, while the mass of the black hole can be obtained via high-resolution stellar spectroscopy near the black hole's sphere of influence. High-resolution VLBI imaging and polarization mapping of the inner radio jets near the core, in the disk/corona collimation region, would also be very informative for understanding the jet launching process in this highly unusual AGN.

To conclude, the present object J2345−0449 is the most extreme example of powerful jet activity and SMBH growth in a spiral galaxy, probably arising via a nonstandard evolutionary route that mainly involves internal, merger-free processes persisting through most of its formation history. Future observations and numerical simulations should clarify whether its exceptional properties can be understood within the current frameworks of galaxy formation.

In recognition of 50 yr of radio astronomy research at the Tata Institute of Fundamental Research (TIFR) and India, as well as 10 yr of operation of the Giant Metrewave Radio Telescope (GMRT) as an international observatory, we dedicate this work to the vision and pioneering spirit of Professor Govind Swarup, the late Professor Vijay Kapahi, and the many talented Indian scientists, engineers, and technicians who have made it possible. We thank the anonymous referee for providing many constructive comments that improved the paper. We thank IUCAA Girawali Observatory (IGO) staff and Professor Vijay Mohan for their support during observations. Team members of IFOSC instrument on the IUCAA 2 m telescope are thanked for their excellent work. J.J. and B.K.G. acknowledge IUCAA's support under the Visiting Associate program. We used archival data from GMRT, CFHT, and VLA facilities. The GMRT is a national facility operated by the National Centre for Radio Astrophysics (NCRA) of the Tata Institute of Fundamental Research, India. Our results are based on observations obtained with MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope, which is operated by the National Research Council of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. The VLA is a facility of the National Radio Astronomy Observatory (NRAO). The NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. Funding for SDSS has been provided by the Alfred P. Sloan Foundation, the participating institutions, the National Science Foundation, and the U.S. Department of Energys Office of Science.