The Most Compact Bright Radio-loud AGNs. II. VLBA Observations of 10 Sources at 43 and 86 GHz

The radio quasar J0205+3212 is at a redshift of z = 1.466 (Burbidge 1970). It was detected with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) in the 100 MeV–300 GeV energy range (Abdo et al. 2010; Nolan et al. 2012; Acero et al. 2015). In the radio, the source shows a core and an extended component 15 arcsec to the north of the core in the 1.4 GHz Very Large Array (VLA) A-array configuration image. The flux densities of the core and the extended component are 652 mJy and 13 mJy, respectively (Cooper et al. 2007). The extended feature was not detected in the higher-resolution 8.4 GHz VLA image, and the core was detected with a flux density of 765 mJy (Healey et al. 2007).

On mas scales, the source is dominated by a compact core with a curved jet extending to the north and then to the northwest, as was seen previously at 1.4 GHz,¹⁰ 5 GHz (Linford et al. 2012), 2.3 and 8.4 GHz (Beasley et al. 2002), and 15 GHz VLBA images (Lister et al. 2009). The intriguing feature of this source is the large jet bending from the north to the west with a position angle changing nearly 90° (Figures 2(a)–(b)). The jet first points to the north until 7 mas (a projected distance of ∼55 pc north of the core), where it bends with a gradually changing position angle from 0° to −64° at 35 mas (a projected distance of ∼280 pc away from the core).

Our 43 GHz image (Figure 2(c)) reveals the inner jet structure. Two new jet components, J1 and J2, are detected within 1 mas, at position angles of $-4^\circ$ and $-55^\circ$, respectively. The high-resolution 86 GHz image shows a core–jet morphology (Figure 2(d)). The 86 GHz observation was made only 9 days after the 43 GHz observation. We calculated the core spectral index ${\alpha }_{43\mbox{--}86}=-1.4$. It is much steeper than the spectral index ${\alpha }_{8.1\mbox{--}15.4}$ (Hovatta et al. 2014), indicating that the core (jet base) becomes optically thin at 86 GHz. The 86 GHz jet is at a slightly different position angle of $40^\circ$ from that in the 43 GHz image and its position is between the two 43 GHz jet components. Due to the different frequencies and different (u, v) samplings at 43 and 86 GHz, accurate correspondence of the components is difficult. J1 is also detected in the innermost jet in the 15 GHz images at similar distance and position angles (Lister et al. 2009). In combination with the lower-frequency VLBI images, the parsec-scale jet exhibits a wiggling track: the jet direction starts at a position angle of $\sim -55^\circ$ (J2) then bends to P.A.$\,\sim \,-4^\circ$ at J1; the position angle gradually changes from $0^\circ$ at ∼7 mas to $-64^\circ$ until 35 mas. This might result from a projection on the plane of the sky of a helically twisted jet. Helical jets could be triggered by periodic variation in the direction of jet ejection (Linfield 1981; Stirling et al. 2003; Zhao et al. 2011), Kelvin–Helmboltz instabilities in the jet flow (Hardee 2003; Feng et al. 2005; An et al. 2010), or the magnetohydrodynamics of the jet (Camenzind 1986), and can result in quasi-periodic flux density variability (Camenzind & Krockenberger 1992; An et al. 2013; Wang et al. 2014; Mohan & Mangalam 2015). The 15 GHz VLBA monitoring observation inferred superluminal motions of two jet components from 14-epoch data sets between 1995 and 2010, with a proper motion of $148\pm 15\,\mu \mathrm{as}$ yr⁻¹ (10.1 c) at about 7 mas and 36 ± 13 μas yr⁻¹ (2.47 c) at about 1.5 mas from the core (Lister et al. 2013). As the jet kinematics indicate apparent superluminal knot speeds, which result in a very small viewing angle, it could also be the case that small intrinsic jet bends are exaggerated by projection and Doppler-beaming effects.

This is a quasar at a redshift of z = 1.16 (Shaw et al. 2012). Fermi/LAT detected the source in the 100 MeV to 300 GeV energy band with a significance of 32.9σ in the third Fermi source catalog (3FGL; Acero et al. 2015). The source 0529+483 has a complex radio spectrum: a steep spectrum with a spectral index of α = −0.95 between 74 MHz and 350 MHz (NED¹¹ ), a slightly inverted spectrum with α = 0.04 between 8.1 and 15.4 GHz (Hovatta et al. 2014), and a flat spectrum with α = −0.1 shown in the WMAP data between 33 and 94 GHz (Gold et al. 2011). The 1.4 GHz VLA image shows a core and a northeastern lobe (Cooper et al. 2007). There is diffuse structure to the southwest of the core in the opposite direction of the northeast lobe. The kiloparsec-scale structure can be described as a triple structure with a core and double lobes. The core flux density from a 5 GHz high-resolution VLA observation is 818 mJy (Laurent-Muehleisen et al. 1997). There is a difference of 229 mJy between the total flux densities measured by the VLA and the 300-foot Green Bank Telescope at 5 GHz (Gregory et al. 1996), suggesting the existence of extended structure is consistent with its steep low-frequency spectrum below 350 MHz.

The VLBA images show a core and a one-sided jet structure extending to the northeast out to ∼30 mas (Figure 3(a)), roughly in alignment with the kiloparsec-scale jet direction (Beasley et al. 2002; Lister et al. 2016). The 15 GHz image shown in Figure 3(b) shows a core and a northeast jet to a distance of 4 mas. The long-term VLBA monitoring observations at 15 GHz from 2002 to 2013 detected superluminal motions of 4 jet components with a maximum speed of $19.8\,c$ (Lister et al. 2016).

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We present for the first time the 43 and 86 GHz total intensity images of this source. The 43 GHz image in Figure 3(c) shows a new jet J1 component appearing ∼0.45 mas to the northeast of the core. The jet direction is consistent with that shown in the 15 GHz image. The 86 GHz image only detected a compact naked core. J1 is resolved at 86 GHz. The VLBA and VLA images indicate that the jet follows a straight trajectory from parsec to kiloparsec scales without significant jet bending. The two-frequency observations have a time separation of 9 days. We calculated the core spectral index ${\alpha }_{43-86}=-0.3$. It suggests a flat spectrum, but is still steeper than the spectral index ${\alpha }_{8.1\mbox{--}15.4}$ derived from the WMAP data (Gold et al. 2011).

The radio quasar 1030+415 is at a redshift of z = 1.1185 (Hewett & Wild 2010) and significant γ-ray emission of 24.7σ was detected by Fermi/LAT (Acero et al. 2015). The VLA in its A-array configuration at 1.4 GHz detected a one-sided jet extending to the east to about 0.5 arcsec (Xu et al. 1995). This jet was marginally detected in the Faint Images of the Radio Sky at Twenty-Centimeters survey with the VLA B-array at 1.4 GHz (White et al. 1997). At 8.4 GHz the VLA observation revealed a point source with a flux density of 384.4 mJy (Healey et al. 2007).

VLBA images at 5 GHz (Helmboldt et al. 2007), 1.7 GHz (Polatidis et al. 1995), 2.3 (Figure 4(a)), and 8.4 GHz (Fey et al. 2004) display a jet extending to the north at a distance of about 10 mas, then bending to the northwest out to about 15 mas. The 15 GHz MOJAVE images (Figure 4(b)) of the source detected a naked core in six epochs during 2010 September and 2013 July, and only showed a marginal hint of a weak northeastern jet in the latest epoch (Lister et al. 2016).

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Figures 4(c)–(d) present the 43 and 86 GHz images of 1030+415 with a high resolution of ∼0.18 mas (corresponding to 1.5 pc). The core that was unresolved in 15 GHz images is resolved into a core and inner jet structure. Two new jet components are detected within 0.5 mas in the 43 GHz image: J2 at 0.19 mas away from the core in a position angle of $-110^\circ$, and J1 at 0.41 mas in ${\rm{P}}.{\rm{A}}.\,=\,-54^\circ$. J2 is also detected at 86 GHz at 0.14 mas, in P.A. = −105°. The overall jet morphology shows an oscillatory trajectory with multiple bends within 20 mas: bending from southwest to northwest at ∼0.2 mas; and from northwest to north (and slightly northeastern) until 10 mas, where the jet bends to northwest until ∼20 mas. One possibility is that this source contains a helical jet. Further discussion of the physical driving mechanisms of helical jets is presented in Section 3.4.1. We found that the source is not substantially variable at 15 GHz (Richards et al. 2011). The flux densities at 15 GHz between the two epochs differ by less than 10%.

1128+385 (B2 1128+38) is a low-spectral peaked source at a redshift of z = 1.733 (Schneider et al. 2007) and an optically violent quasar (Véron-Cetty & Véron 2010). It was not detected by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory satellite, but was detected by Fermi/LAT at a level of 5.5σ (Acero et al. 2015). The 1.6 GHz VLA image in the combined A+B-array configuration shows a compact core and a hint of a weak feature at 15'' (∼130 kpc) southeast of the core (Murphy et al. 1993).

The snapshot VLBI images derived from the VLBI calibrator survey¹² only show a naked core at 2.3 and 8.4 GHz. In a 5 GHz VLBI observation (Figure 5(a)), Xu et al. (1995) detected a weak extended jet to the west of the compact core. The higher-resolution 15 GHz VLBA images (Figure 5(b)) show that the jet points toward the southwest to a distance of 1.5 mas (corresponding to a projected distance ∼12.8 pc). The maximum jet speed is 3.29 ± 0.28 c (Lister et al. 2016).

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The 43 GHz image derived from our observations is shown in Figure 5(c) and resolves the source into a core and a southwestern jet component J1. The jet shows an extension to the west from J1, in good agreement with the outer jet that is seen in the 15 GHz image (Lister et al. 2016). The 86 GHz image from Lee et al. (2008) shows a core and a weak jet ($\sim 6\sigma$) to the southwest at 0.21 mas. However, our new 86 GHz VLBA image in Figure 5(d) only shows a compact core with a flux density of 170 mJy, although it has a much higher sensitivity compared to the previous image. We note that the total flux densities between the two 86 GHz data sets differ by 370 mJy; the non-detection of the jet component at about 0.2 mas is probably due to variability. The southwestern jet J1 seen in the 43 GHz image was resolved in our 86 GHz image.

1418+546 (OQ 530) is a BL Lac source at a redshift of z = 0.153 (Shaw et al. 2012). It was not detected by the EGRET, but detected by Fermi/LAT (3FGL J1419.9+5425) with 20.1σ (Acero et al. 2015). The 1.6 GHz VLA map displays a one-sided kiloparsec-scale halo with a component west of the compact core at a separation of about 32'' (Murphy et al. 1993). The diffuse halo was also detected in the 1.4 GHz VLA D-array image (Cassaro et al. 1999). The western lobe was confirmed by the 1.4 GHz observations made by the VLA in its B-array configuration and the Westerbork Synthesis Radio Telescope (Cassaro et al. 1999). However, the VLBI images revealed a single-sided jet to the southeast, in an opposite direction of the kiloparsec-scale lobe: the 5 GHz VLBA image (Figure 6(a)) shows the jet extending to the southeast to approximately 25 mas (Helmboldt et al. 2007); the 15 GHz VLBA image (Figure 6(b), Lister et al. 2016) shows a consistent morphology with the 5 GHz image. The VLBA structure does not show rapid structural change or superluminal motion. A maximum jet speed of 0.93 ± 0.27 c was determined based on the 15 GHz data over a time baseline of 18 years from 1995 to 2013 (Lister et al. 2016).

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We present the first 43 and new 86 GHz VLBA images (Figures 6(c)–(d)) of this source. The direction of the inner 1 mas jet is in excellent agreement with that derived from the 5 and 15 GHz images (Figures 6(a)–(b)). The innermost jet component J3 is at ∼0.2 mas at a P.A. of ∼$120^\circ$, corresponding to a projected linear size of 0.5 pc. Two outer jet components, J1 and J2, were also detected at 15 GHz. The jet components J2 and J3 are also detected in the 86 GHz image. The brightness temperature of the core is $3.8\times {10}^{10}$ K at 86 GHz, showing no evidence of strong Doppler boosting, consistent with the slow jet motion. The OVRO 40 m 15 GHz light curve shows that the flux density does not undergo significant variation from 2008 to mid-2013 (Richards et al. 2011). After experiencing a long "quiet" period, the source flared in early 2014. An even larger flare started in 2015 and peaked at the beginning of 2016. Our observations were carried out before the 2016 flare. Due to its high flux density and compact structure, 1418+546 can be used as a flux density scale calibrator for mm and sub-mm VLBI observations, e.g., the EHT and Atacama Large Millimeter/submillimeter Array observations.

1823+568 (4C +56.27) is a BL Lac object at a redshift z = 0.664 (Lawrence et al. 1986), but Véron-Cetty & Véron (2006) classified this source as a quasar. The source was listed in the Fermi/LAT AGN catalog as 3FGL J1824.2+5649 and was detected at a significance level of 34.5 σ (Acero et al. 2015). The VLA images at 1.6 GHz (Murphy et al. 1993) and 5 GHz (Kollgaard et al. 1992) show a prominent core–jet morphology with the jet extending toward the east up to $\sim 5^{\prime\prime}$. A diffuse halo surrounds the compact core–jet structure. The radio spectrum is flat above 1 GHz, but steep below 1 GHz, indicative of an extended steep-spectrum halo.

The low-frequency VLBA images (Figures 7(a)–(b)), however, show a jet extending to the south to a maximum distance of ∼200 mas (Pearson & Readhead 1988; Gabuzda & Cawthorne 1996; Lister et al. 2016). The total flux density at 15 GHz shows moderate-level variability (Richards et al. 2011). The ratio of $({S}_{\max }-{S}_{\min })/({S}_{\max }+{S}_{\min })$ is only 0.18.

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Our high-sensitivity 43 and 86 GHz images (Figures 7(c)–(d)) reveal rich jet structures on scales of 2 mas (14 pc) and 0.5 mas (3.5 pc), respectively. Four jet components are identified at 43 GHz, and they were also detected at 15 GHz (Lister et al. 2016). These jet components show complex motions, with a mixture of superluminally moving (J1 and J2) and stationary jet knots (J3 and J4). The maximum proper motion $576\pm 48\,\mu \mathrm{as}$ yr⁻¹ (21.9 ± 1.8c) is associated with a jet component at 6.3 mas from the core (Lister et al. 2016). Compared with the 15 GHz image, the 43 GHz image shows a pronounced emission gap between 1.1 mas and 1.8 mas. The jet in 1823+568 likely consists of both fast-moving knots that were detected at all frequencies, and a slowly moving underlying flow that is resolved out in the 43 GHz image. The 86 GHz image obtained by Lee et al. (2008) shows a naked core with a flux density of 485 mJy. The core flux density from our 86 GHz data is 230 mJy, much smaller than that obtained by Lee et al. (2008), indicating variability between the two epochs.

The radio source 1828+487 (3C 380) is at a redshift of z = 0.692 (Lawrence et al. 1996). The Fermi/LAT identified the source as 3FGL J1829.6+4844 with a 24.1σ detection significance in the 100 MeV–300 GeV energy range (Acero et al. 2015). The image made with the VLA in A+B-array at 1.6 GHz shows a large diffuse halo with a size of 6'' surrounding a central compact component (Murphy et al. 1993). The source was mapped with the Multi-Element Radio Linked Interferometer Network, which detected structure up to an overall angular extent of about 6'' (corresponding to a linear size of ∼50 kpc). The radio morphology suggests that the source belongs to the class of compact steep-spectrum (CSS) sources (Wilkinson et al. 1984; Fanti et al. 1985).

In the 1.4 GHz VLBI image (Figure 8(a)), the jet extends to the northwest out to a distance of 70 mas. The jet body is characterized by a chain of bright knots, similar to another archetypal CSS quasar, 3C 48 (Wilkinson et al. 1991). The 5 GHz VLBA data revealed an isolated bright knot at 732 mas (≧20 kpc de-projected) downstream of the jet at a position angle around $-52^\circ$, in good alignment with the inner parsec-scale jet (Koyama et al. 2013). The VLBI Space Observatory Programme (VSOP) 5 GHz image revealed a fine structure along the parsec-scale jet, which is in good agreement with the low angular resolution image (Lister et al. 2001). The 15 GHz images (Figure 8(b)) obtained from the MOJAVE data show that the inner 3 mas jet points to a position angle of $-70^\circ$, slightly different from the outer jet. From about 10 mas, a plume-like diffuse emission appears to the west of the jet body. Proper motions were determined for eight jet components from the MOJAVE program, with the maximum proper motion of 0.33 mas yr⁻¹ (13.06 ± 0.14 c) (Lister et al. 2013).

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Our 43 GHz image (Figure 8(c)) displays rich jet structure to a distance of about 3 mas, and four bright knots are identified. J1 and J2 roughly correspond to components No. 8 and 10 in Lister et al. (2016). But cross-matching of J3 and J4 with their counterparts in the 15 GHz images is difficult due to the different resolutions. The early 86 GHz image detected a compact core and a secondary component (5σ) at about 4 mas to the northwest in a position angle $-31^\circ$ (Lee et al. 2008). This 4 mas jet component is not detected in our 43 and 86 GHz images. Our new 86 GHz image (Figure 8(d)), having a higher sensitivity than previous observations at this frequency, reveals the inner core–jet structure within 0.7 mas. A jet component J5 was identified, but it does not exactly correspond to J4. Comparing the high-frequency images with sub-mas resolution with those at lower resolutions shows a change of the position angles with the distance from the core, suggesting a possible wiggling trajectory. However, the study of Lister et al. (2013) finds that the jet shows moving features along independent trajectories with different vector motion position angles, indicating that the ejection of the knots is occurring at different position angles.

1928+738 (4C +73.18) is a low-spectral peaked AGN at a redshift of z = 0.302 (Lawrence et al. 1996). The source was not detected by EGRET or Fermi/LAT. The VLA image shows a core with jets and diffuse emission. A series of at least four bright components extends 18'' to the south and the jet gradually bends from the south to the southeast (Murphy et al. 1993). To the north of the core, there is one bright hotspot about 1'' from the core and a large patch of diffuse emission beyond the hotspot (Rusk & Rusk 1986; Johnston et al. 1987; Kollgaard et al. 1990).

Lister et al. (2001) presented a 5 GHz VSOP image (Figure 9(b)) showing 5 jet components within 6 mas that seem to indicate a helical trajectory. The kinematics of the jet at 15 GHz (Figure 9(a)) and 43 GHz (Lister 2001) provided a means of constraining the black hole spin (Kun et al. 2014). The maximum jet speed is 8.16 ± 0.21c among 13 moving features (Lister et al. 2013).

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We present our new 43 GHz image (Figure 9(c)), showing the inner 3 mas jet. The 43 GHz image shows 3 jet components, at varying position angles, consistent with the suggested helical jet structure inferred from the 5–15 GHz VLBI images. A published snapshot 86 GHz VLBI image only detected the core feature (Lee et al. 2008). Our new 86 GHz data (Figure 9(c)–(d)) revealed a new jet component J4 at a distance of 0.31 mas and P.A. of $160^\circ$. The 86 GHz flux density of the core is less than 10% of that at 43 GHz, probably the result of the observation at 43 GHz during or just after the flare and the 86 GHz observation during the declining phase or near the quiescent phase, which can be seen from the OVRO 40 m 15 GHz light curve monitoring program (Richards et al. 2011).

The source 1954+513 (OV 591) is a radio-loud quasar with a redshift of z = 1.22 and was detected with a fractional linear optical polarization below 3% (Lawrence et al. 1996; Véron-Cetty & Véron 2010). Neither EGRET nor Fermi/LAT have detected this source. Perley (1982) reported a classical core–double-lobe FR II structure oriented in the north–south direction with a total extent of 16'' (130 kpc). The northern lobe has a trailing jet continuously connected with the core, in which a midway hotspot is seen at about 5'' from the core. The naturally weighted 5 GHz VLA image shows that the southern component is slightly elongated toward the core, but there is no evidence of a counter-jet (Kollgaard et al. 1990). The images acquired by VSOP and the VLBA at 5 GHz (Dodson et al. 2008) showed a compact core and a faint extended component at a position angle $-27^\circ$ separated by 0.4 mas with a flux density about 230 mJy, exceeding 60% of the flux density resolved with space–ground baselines (Lister et al. 2001).

The 2.3 GHz and 15 GHz VLBI images (Figures 10(a)–(b)) revealed a rich jet structure ∼50 mas to the northwest of the core (Kovalev et al. 2005; Pushkarev & Kovalev 2012). Our new 43 GHz image (Figure 10(c)) resolved the inner 2 mas jet into a series of five components that follow a straight trajectory along a P.A. of $-55^\circ$ until 1.3 mas, where the jet bends into a jet component J2 at a position angle of $-68^\circ$. By comparing with the 15 GHz image, we find that the jet turns back to a position angle of $-50^\circ$ after the jet component J1, where polarized emission was detected (Lister et al. 2009). Enhanced polarized emission at the bending location probably implies a local interaction between the jet and a massive cloud in the narrow-line region of the host galaxy. Such jet–cloud interaction may deflect the jet flow and alter its direction.

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The snapshot 86 GHz image detected a naked core with a flux density about 275 mJy (Lee et al. 2008). The core is also unresolved in our new 86 GHz image (Figure 10(d)), with a flux density about 170 mJy. The core flux densities at the two epochs differ by 100 mJy. The peak specific intensity of the inner jet J5 in the 43 GHz image is 26 mJy beam⁻¹. Considering the steep-spectrum nature of the jet, its expected peak specific density at 86 GHz is below the detection threshold, i.e., five times the rms noise of the 86 GHz image ($1\sigma =2.9$ mJy beam⁻¹).

2201+315 (4C +31.63) at a redshift of z = 0.2974 was identified as a low optical polarization quasar (Marziani et al. 1996) and an intermediate-spectral-peaked source (Véron-Cetty & Véron 2010). It was detected by the Fermi/LAT with a 5σ significance (Acero et al. 2015). It is also a strong X-ray AGN observed by Swift with a flux of 3.22 × 10⁻¹² erg cm⁻² s⁻¹ at 0.1−2.4 keV (Maselli et al. 2010). The optical image shows signs of a post-merger with an unusual distribution of dust (Hutchings et al. 1984). In the radio band, this quasar displays a typical FR II morphology with a total size of $\sim 90^{\prime\prime}$ with a bright core and double lobes at opposite sides of the core aligning in the northeast–southwest direction (Gower & Hutchings 1984; Neff & Brown 1984; Cooper et al. 2007). The southwestern lobe is linked to the core with an extended jet. The counter-jet was not detected in the VLA images.

In the 1.4 GHz VLBA image, the southwest jet was observed to extend up to a distance of 90 mas (Figure 11(a)). At higher frequencies of 8.6 and 15 GHz (Figure 11(b)), the jet shows a smooth bending from a position angle of $-150^\circ$ to $-130^\circ$ (Kellermann et al. 2004). Superluminal motions have been detected in seven jet components with a maximum value of 0.45 mas yr⁻¹ ($8.3\pm 0.1c$) (Lister et al. 2013). The published 86 GHz image (Lee et al. 2008) only detected the central core.

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Our 43 and 86 GHz observations (Figures 11(c)–(d)) show the jet structure within 2 mas, in which three jet components are identified. The images at two frequencies are in excellent agreement, revealing the initial section of the jet along a P.A. $\sim -160^\circ$. In combination with the outer mas-scale jet and more extended arcsec-scale jets/lobes, our new images reinforce that the jet has a smooth bending trajectory; the position angle gradually changes from $-166^\circ$ (at ≤1 mas) to $-130^\circ$ (1–90 mas), and ends in a terminal shock at a P.A. = −120° (at $13^{\prime\prime}$).