A VERY DEEP CHANDRA OBSERVATION OF A2052: BUBBLES, SHOCKS, AND SLOSHING

With its subarcsecond resolution, the Chandra X-Ray Observatory has revealed a wealth of substructure in the X-ray-emitting intracluster medium (ICM) in clusters of galaxies. "Cavities" or "bubbles" are commonly seen in the X-ray gas in cluster centers related to outbursts by the active galactic nucleus (AGN). In addition, surface brightness edges associated with shocks and cold fronts have been observed, as well as spiral features likely related to "sloshing" of the ICM in cluster centers.

The central bubbles are frequently seen in cooling flow (or "cool core") clusters and are often filled with radio emission associated with the AGN. There was some evidence for these features in a very few cases even before Chandra observations (e.g., ROSAT observations of Perseus (Böhringer et al. 1993), A4059 (Huang & Sarazin 1998), and A2052 (Rizza et al. 2000)). Clusters with cool cores have radio bubbles much more often than non-cool core clusters (e.g., Mittal et al. 2009), and some of the most spectacular individual cases observed by Chandra are the Perseus cluster (Fabian et al. 2003, 2006), M87/Virgo (Forman et al. 2005, 2007), Hydra A (McNamara et al. 2000; Wise et al. 2007), MS0735.6+7421 (McNamara et al. 2005), and A2052 (Blanton et al. 2001, 2003, 2009, 2010).

The classic "cooling flow problem" is that sufficient quantities of cool gas (as evidenced by star formation rates, for example) were not found to match the gas cooling rates estimated from earlier X-ray observatories. The X-ray cooling rates have lowered based on Chandra and XMM-Newton observations, and the majority of the gas is seen to cool to only some fraction (typically one-third to one-half) of the cluster average temperature (Peterson et al. 2003). A heating mechanism is required to stop the gas from cooling to even lower temperatures.

The most likely candidate for heating of the central cluster gas is feedback from central AGNs that are fed by the cooling gas (see McNamara & Nulsen 2007 for a review). This heating comes in the form of bubbles inflated by the AGN, described above, that then rise buoyantly to larger radii in cluster atmospheres distributing the heat. The details of this heating, however, are still not completely understood. In addition, there is at least some component of shock heating in many cluster centers, especially early on in the AGN's lifetime. Shocks have been detected in fewer cases than radio bubbles, and observations include those of Perseus (Fabian et al. 2003, 2006; Graham et al. 2008), M87/Virgo (Forman et al. 2005), Hydra A (Nulsen et al. 2005a), Hercules A (Nulsen et al. 2005b), MS0735.6+7421 (McNamara et al. 2005), and the group NGC 5813 (Randall et al. 2011). Typically, the shocks are weak with Mach numbers ranging from approximately 1.2 to 1.7 (McNamara & Nulsen 2007). Temperature rises associated with the shocks are detected in even fewer cases.

Heating of the ICM may also result from gas "sloshing" in the cluster's central potential well (Ascasibar & Markevitch 2006; ZuHone et al. 2010). Cold fronts, regions where the surface brightness changes sharply but the pressure does not, can result from these sloshing motions driven initially by cluster or sub-cluster mergers. The sloshing can produce a spiral distribution of cool cluster gas reaching into the cluster center (e.g., Clarke et al. 2004; Laganá et al. 2010), and is also seen in galaxy groups (Randall et al. 2009b).

Here, we present a very deep Chandra observation of the cool core cluster A2052. The only other cool core clusters observed to similar or greater depth with Chandra are Perseus (Fabian et al. 2006) and M87/Virgo (Million et al. 2010; Werner et al. 2010). A2052 is a moderately rich cluster at a redshift of z = 0.03549 (Oegerle & Hill 2001). Its central radio source, 3C 317, is hosted by the central cD galaxy, UGC 09799. A2052 was previously observed in the X-ray with Einstein (White et al. 1997), ROSAT (Peres et al. 1998; Rizza et al. 2000), ASCA (White 2000), Chandra (Blanton et al. 2001, 2003, 2009, 2010), Suzaku (Tamura et al. 2008), and XMM-Newton (de Plaa et al. 2010). In addition, we present archival radio observations from the Very Large Array (VLA).

We assume H_○ = 70 km s⁻¹ Mpc⁻¹, Ω_M = 0.3, and Ω_Λ = 0.7 (1'' = 0.7059 kpc at z = 0.03549) throughout. Errors are given at the 1σ level unless otherwise stated.

A2052 was observed with Chandra for a total of 662 ks in Cycles 1, 6, and 10 from 2000–2009. A summary of the observations is given in Table 1. All observations were performed with the ACIS-S as the primary instrument. The ACIS-S was chosen for its greater response at low energies than the ACIS-I, yielding a higher count rate for this relatively cool, kT ≈ 3 keV cluster. The nominal roll angles used for the observations varied from 99° to 265° providing coverage at large radii at a range of azimuthal angles around the cluster. The events were telemetered in Very Faint mode for all but the Cycle 1 observation, where the events were telemetered in Faint mode. The data were processed in the standard manner, using CIAO 4.2 and CALDB 4.2.2. After cleaning, and filtering for background flares (using the 2.5–7.0 keV range for the backside-illuminated (BI) chips and the 0.3–12.0 keV range for the frontside-illuminated (FI) chips), the total exposure remaining for the 11 data sets was 657 ks. Background corrections were made using the blank sky background fields, including the new "period E" background files for the more recent data. For each target events file, a corresponding background events file was created, normalizing by the ratio of counts in the 10–12 keV energy range for the source and background files to set the scaling.

We have used the NRAO data archive to extract observations of 3C 317 at 4.8 and 1.4 GHz. A summary of the data sets is presented in Table 2. The archival radio data were calibrated and reduced with the NRAO Astronomical Image Processing System (AIPS). Images were produced through the standard Fourier transform deconvolution method for each frequency and configuration. Several loops of imaging and self-calibration were undertaken for each data set to reduce the effects of phase and amplitude errors in the data. The final radio image at 4.8 GHz was obtained through combining the three VLA configurations and two observing frequencies. We have also produced a combined configuration image at 1.4 GHz but do not show it as the structure is remarkably similar to the 4.8 GHz image.

We have made a spectral index map between 1.4 GHz and 4.8 GHz to compare the spectral features more directly with the X-ray structure. This map was created from the combined configuration data at each frequency. The u–v coverage of both data sets was matched and both frequencies were imaged with a 43 circular beam. We have blanked all pixels in the spectral index map that were lower than the 5σ level on either of the input maps.

Merged X-ray images of the source were created. The images were corrected using merged background images and exposure maps. An image showing the combined data from all CCD chips used in the analysis (ACIS S1, S2, S3, I2, and I3) in the 0.3–10.0 keV band is shown in Figure 1. This figure illustrates the different roll angles that were used when the observations were performed. The extended cluster emission is visible, as well as numerous point sources which appear more extended while increasingly off-axis due to the larger point spread function in these regions. The image has been smoothed with a 3'' Gaussian.

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An unsmoothed image in the 0.3–2.0 keV band of the central region of the cluster is shown in Figure 2. The image reveals exquisite detail related to the interaction of the AGN with the ICM. Point sources, including the central AGN, are visible. Cavities or bubbles to the N and S of the AGN are seen, as well as outer cavities to the NW and SE. The inner cavities are bounded by bright, dense, rims, and the NW cavity is surrounded by a very narrow, filamentary loop. A filament extends into the N bubble. A shock is seen exterior to the bubbles and rims, and a probable second shock is visible to the NE.

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A three-color image of the central 656 × 656 region of A2052 is displayed in Figure 3. The image has been slightly smoothed, using a 15 Gaussian, and the scaling for each of the three colors is logarithmic. Red represents the soft (0.3–1.0 keV) band, green is medium (1.0–3.0 keV), and blue represents the hard band (3.0–10.0 keV). The bright rims surrounding the inner bubbles appear cool. The first shock exterior to the bubble rims is slightly elliptical with the major axis in the north–south direction (Blanton et al. 2009). It is visible as a discontinuity in the surface brightness, and the bluish color is consistent with a temperature rise in this region. Outside of the inner shock, a second surface brightness discontinuity is seen as greenish emission in this figure. The discontinuity is sharper to the NE and more extended to the SW. This feature may represent a shock or a cold front. These features will be explored in further detail in Section 6.

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A composite X-ray/optical/radio image is shown in Figure 4. The 0.3–2.0 keV Chandra image is shown in red, radio emission at 4.8 GHz from the VLA is displayed as blue, and optical r-band emission from the Sloan Digital Sky Survey (SDSS; Abazajian et al. 2009) is shown as green. The AGN is visible in the X-ray, radio, and optical, and the radio lobes fill the cavities in the X-ray emission. This includes the inner cavities as well as an outer cavity bounded by a narrow loop to the NW and the outer cavity to the S/SE. In addition, the radio emission is breaking through the northern bubble rim to the north. The X-ray filament extending from the northern bubble rim toward the AGN was found to be associated with Hα emission in Blanton et al. (2001). In Figure 5, we show SDSS r-band contours superposed on a Chandra image in the 0.3–10.0 keV range that has been smoothed with a 15 radius Gaussian. The central cD galaxy is oriented in the NE–SW direction in the optical. The inner bubbles seen in the X-ray emission are within the cD galaxy.

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4.1. Residual Images

In order to better reveal features in the X-ray image, we created residual images using two different techniques. In the first, we used the method of unsharp masking, and in the second, we subtracted a two-dimensional (2D) beta model from the X-ray image. We find that unsharp masking is useful for highlighting the structure in the inner parts of the cluster, while the model subtraction is better at revealing larger-scale features.

4.1.1. Unsharp Masking

We created an unsharp-masked image in the 0.3–10.0 keV energy range. Sources were detected in the image using the wavelet detection tool "wavdetect" in CIAO (Freeman et al. 2002). Several wavelet scales were used, at 1, 2, 4, 8, and 16 pixels, where 1 pixel = 0492. Sources detected using this method were visually examined and several were rejected as being clumpy X-ray gas emission rather than point sources. A source-free image was created by replacing the source pixel values with the average value found in an annulus surrounding each source. We retained the sources in our unsharp-masked image while making corrections with a source-free image. Smoothed images, both with and without sources, were created by smoothing with a 098 radius Gaussian. Another copy of the source-free image was smoothed with a 98 Gaussian. A summed image was made by combining the source-free images smoothed at the two different scales. A difference image was made by subtracting the 98 Gaussian-smoothed source-free image from the 098 Gaussian-smoothed image that contained sources. Finally, the unsharp-masked image was created by dividing the difference image by the summed image. In this way, we retain the sources in the image, smoothed at a scale of 098. This method is similar to that in Fabian et al. (2006), although sources are excluded throughout in their unsharp-masked images.

The unsharp-masked image is displayed in Figure 6 with VLA 4.8 GHz radio contours superposed. The bubbles in the X-ray emission are more easily seen in this image, as are the bright bubble rims, the shock exterior to the bubble rims, and the second shock or cold front feature to the NE. The radio emission fills the inner bubbles and the southern lobe turns to fill the outer southern bubble. The radio lobe to the north appears to be escaping through a gap in the northern bubble rim, and a narrow filament is seen in the radio in this region. The radio emission also extends beyond the bubble rims to the NW to fill a small bubble bounded by a narrow X-ray filament. To the east, an extension in the radio fills a small depression in the X-ray on the scale of approximately 10''.

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4.1.2. Beta-model Subtraction

A 2D beta model was used to fit the surface brightness in both 0.3–2.0 keV and 0.3–10.0 keV images using a circular region with radius 566. The images were source-free, with the surface brightness at the position of sources approximated from the surface brightness in an annulus around each source, as above. Corrections were made for exposure, using a merged exposure map. Similar results were obtained for the fits to the images in both energy bands. Errors were computed using Cash statistics. For the 0.3–2.0 keV image, the center of the large-scale emission was found to be only 12 away from the position of the AGN. The emission was found to be slightly elliptical, with an ellipticity value of 0.18 ± 0.00081 (where ellipticity values range from 0 to 1, with 0 indicating circular emission). The position angle (P.A.) for the semimajor axis of the ellipse is 382 ± 01 measured from north toward east. The core radius using this model is 251 ± 0060 and the beta index is β = 0.46 ± 0.00021.

The residual image after 2D beta model subtraction in the 0.3–2.0 keV band is shown in Figure 7. The image has been smoothed with a 738 radius Gaussian. The smoothing washes out the details in the very center of the image, but the bright bubble rims are clearly visible. A spiral feature is seen, starting in the SW and extending to the NE. Similar spiral structures have been seen in other clusters, with A2029 being a particularly clear example (Clarke et al. 2004).

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Spectral maps were created to examine the distribution of temperature, as well as entropy, pressure, and abundance using the technique described in Randall et al. (2008, 2009a). The temperature maps were created by extracting spectra for the separate Chandra observations and fitting them simultaneously in the 0.6–7.0 keV range with a single-temperature APEC model, with N_H set to the Galactic value of 2.71 × 10²⁰ cm⁻² (Dickey & Lockman 1990) and the abundance allowed to vary. Background spectra were extracted from the blank sky background observations that were reprojected to match each data set. Data from both the FI and BI chips were used, with separate response files and normalizations determined for each ObsID's data set, with the FI and BI normalizations allowed to vary independently for each ObsID. Spectra were extracted with a minimum of either 2000 or 10,000 background-subtracted counts, corresponding to a minimum signal-to-noise ratio of approximately 45 or 100, respectively, depending on our analysis goals. The radius of the circular extraction region was allowed to grow to the size required to extract the minimum counts. Spectral maps of the central region of A2052 were previously presented in Blanton et al. (2003, 2009). Here, with the much deeper Chandra data, we are able to examine a much larger region of the cluster with high precision. We have chosen to include spectral maps created as described above rather than Voronoi tessellation maps, even though some of the map pixels are not independent (some regions used in spectral-fitting overlap), especially in the outer regions of the maps. We find that spectral structures are easier to see on the maps we present, and they are confirmed by extracting spectral profiles in Sections 6 and 7.

In Figure 8, we display a high-resolution (0492 pixels) temperature map of the central region of A2052, where the minimum number of net counts was 2000. Superposed on the temperature map are X-ray surface brightness contours derived from the 15 Gaussian-smoothed 0.3–10.0 keV image. The coolest parts of the cluster are found in the brightest parts of the X-ray rims surrounding the bubbles, including the bright rim to the W and NW, the E–W bar that passes through the cluster center and AGN, and the N filament that extends into the N bubble. An overlay of Hα contours from McDonald et al. (2010) onto the high-resolution temperature map is shown in Figure 9. For comparison, the Hα contours are superposed onto the 0.3–10.0 keV, 15 radius Gaussian smoothed, Chandra image in Figure 10. The correspondence between the Hα emission and both the surface brightness and temperature structure is excellent. Hα emission, representing gas with T ≈ 10⁴ K, is detected in the brightest and coolest structures in the cluster center, including the E–W bar and the filament that extends into the N bubble. While correspondence between Hα and X-ray surface brightness was shown in Blanton et al. (2001), with the deeper Chandra and new Hα data presented here, we see more detail in the association, including the filament extending into the N bubble not reaching the AGN (whereas it appeared to reach the AGN in Hα in the Baum et al. 1988 data shown in Blanton et al. 2001). The association between the Hα and X-ray indicates that at least some gas is cooling from the temperature associated with the X-ray gas in these regions (T ≈ 10⁷ K) to T ≈ 10⁴ K. McDonald et al. (2010, 2011) suggested that this gas represents gas that was previously cooler and then ionized to produce the Hα emission. Given the low UV-to-Hα ratio in this system, they conclude that the ionization source would likely be shocks related to the AGN rather than nearby, luminous, stars, as in some other cluster centers.

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Additionally, significant clumpy substructure in the temperature distribution is seen in the gas throughout the central cluster region. This substructure will be further investigated in an upcoming paper. Errors range from approximately 2% in the very central, brightest regions, to approximately 12% in the outer areas of the map shown in Figure 8.

In Figure 11, we show a projected or "pseudo" pressure map of the central region of A2052. The map was derived using the APEC normalization and temperature from the spectral fits that resulted in the temperature map in Figure 8. The APEC normalization is proportional to n²V, where n is the density and V is the volume projected along the line of sight. We define the projected pressure as kT(A)^1/2, where A is the APEC normalization scaled by area to account for extraction regions that may go off the edge of the chips. Superposed on the pressure map are X-ray surface brightness contours in the 0.3–10.0 keV band. The most obvious feature in the map is the clear ring of high pressure that is coincident with the inner discontinuity seen in surface brightness and the jump in density (Blanton et al. 2009) visible in Figures 2 and 3. The bubbles, including the inner bubbles to the N and S of the AGN as well as the outer, small NW bubble and the outer SE bubble, are visible as lower-pressure regions in this projected pressure map since they are largely devoid of X-ray-emitting gas.

A lower-resolution temperature map covering a larger region of the center is shown in Figure 12. Here, the minimum number of background-subtracted counts is 10,000, and the pixel size is 8''. The errors in temperature range from 1% in the inner regions to 5% in the outskirts of the frame. Superposed are the residual surface brightness contours in the 0.3–2.0 keV band after beta-model subtraction (see Figure 7). The spiral excess traces out a region of temperature lower than its surroundings. In addition, this low-temperature feature continues inward along the direction of the spiral toward the cluster center, beyond the extent of the inner spiral contours.

A higher-resolution temperature map showing the same field of view as in Figure 12 is displayed in Figure 13. The spectra were extracted with a minimum of 2000 background-subtracted counts, and the pixel size is 4''. The map represents more than 80,000 spectral fits. Here, the errors range from 2% in the inner regions to approximately 14% in the outskirts. Two views of a pseudo-pressure map derived from the fits that were used to make the temperature map in Figure 13 are shown is Figures 14 and 15. The pressure jump corresponding to the innermost shock is clearly seen, as well as evidence for pressure structure tracing the second inner shock (seen in the outer X-ray contour). There is no evidence of structure in the pressure map related to the spiral structure.

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A pseudo-entropy map is displayed in Figure 16 with excess surface brightness contours after subtracting a beta model superposed. The map was derived using the spectral fits that resulted in the temperature map in Figure 13. We define pseudo-entropy as kT(A)^−1/3, with A defined as above. In general, the entropy decreases toward the cluster center. There appears to be correspondence between structure in the entropy map and the spiral feature.

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A projected abundance map is shown in Figure 17, corresponding to the temperature map in Figure 12 with 8'' pixels and a minimum of 10,000 background-subtracted counts per pixel. Errors range from 5% in the highest surface brightness regions, to 23% in the outer regions of the map (with the majority of the errors at the 10%–15% level across the map). To the SW, a region of high abundance is coincident with the high surface brightness spiral. This is consistent with higher metallicity gas sloshing away from the cluster center, creating the spiral. A high-metallicity region was found at a similar position to the SW using XMM-Newton data (de Plaa et al. 2010).

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In Blanton et al. (2009), we identified two inner surface brightness jumps that were likely associated with shocks. These jumps are visible in Figures 3, 6, and 15. The first inner jump extends around the cluster center in a slightly elliptical shape in the N–S direction at a radius of approximately 40''. The second jump is elliptical in the NE to SW direction, and is sharper and at a smaller radius from the AGN in the NE direction. To the NE, the second jump is at a radius of approximately 65''.

Similar to Blanton et al. (2009), we have fitted a projected spherical density model to the surface brightness in an NE wedge with P.A. −2° to 98° measured east from north. The projected model characterizes the density using power laws and discontinuous jumps. We use three power laws, and identify two density jumps. See Randall et al. (2008) for further description of this technique.

Our results are generally consistent with, and a refinement of, those presented in Blanton et al. (2009). The density jumps are at radii of 44.2 ± 0.1 arcsec (31.2 ± 0.1 kpc) and 66.3^+0.3_{− 0.06} arcsec (46.8^+0.02_{− 0.04} kpc), respectively, from the AGN. The magnitudes of the jumps are factors of 1.25^+0.016_{− 0.015} and 1.29^+0.010_{− 0.012}, for the first and second jumps, respectively. The slopes of the three power-law components, going from the inner to outer regions, are −0.53^+0.029_{− 0.024}, −1.25^+0.030_{− 0.037}, and −1.06^+0.0067_{− 0.0084}.

The jumps in density correspond to Mach numbers of 1.17^+0.011_{− 0.010} and 1.20^+0.0072_{− 0.0079}, respectively, for the first and second jumps. The temperature is then expected to rise a similar amount inside both shocks (a factor of 1.16 for the first shock and 1.19 for the second shock).

We have calculated a deprojected temperature profile for the NE region, shown in Figure 18. Dashed lines indicate the locations of the two shocks. As with all of our spectral fits, spectra were extracted separately for each data set and fitted simultaneously in XSPEC v12.6. The fits were performed in the 0.6–7.0 keV range. A single APEC model plus Galactic absorption was fitted to the outermost annulus spectra with abundance allowed to vary. The contribution of emission from this shell to the next annulus in was calculated using geometric projection, assuming spherically symmetric shells, and the APEC normalization scaled appropriately to account for the projection of this outer component. The parameters for the contribution from the outer annulus were frozen, and an additional APEC model was added for the annulus of interest. This procedure was continued inward, where the spectral model for each annulus included contributions from all external annuli. See Blanton et al. (2003) for further description.

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In addition, in Figure 18, we present density and pressure profiles for the NE region. The density and pressure were determined from the deprojected spectral fits, since the normalization of the free APEC component for each annulus is proportional to the square of density at that annulus. Note that the radius range extends to approximately 300'' (212 kpc) where the overdensity (the density relative to the critical density) is ≈12,500. For comparison, an overdensity of 500 (r₅₀₀) is at r ≈ 800 kpc (≈1100'').

A clear rise in temperature is seen inside the innermost shock, in addition to the jumps in density and pressure in this region. In Blanton et al. (2009), while the temperatures inside and outside this shock were consistent with the rise expected given the Mach number, the best-fitting temperatures were approximately flat across the shock. Here, with the much deeper data set, the rise is clearly detected. Just outside the shock, the temperature is kT = 2.81^+0.11_{− 0.15} keV, while inside the shock it reaches kT = 3.14^+0.11_{− 0.11} keV, a factor of 1.12^+0.10_{− 0.08} higher, with a significance or 2.1σ. Such temperature rises associated with weak shocks are extremely difficult to measure in cluster centers, with very deep, high-resolution observations required.

For the second shock, the situation is less obvious. The best-fitting temperatures are approximately flat across the shock: kT = 3.35^+0.20_{− 0.16} keV outside the shock and kT = 3.27^+0.14_{− 0.15} keV inside the shock. Therefore, even within the errors, this is inconsistent with the expected temperature jump of 1.19, with the highest rise permitted giving a factor of 1.07. However, as noted above, temperature rises associated with weak shocks are very difficult to detect, and due to projection effects, measured rises are expected to be lower than might be expected based on shock strengths alone (e.g., Randall et al. 2011). In addition, the gas may cool due to adiabatic expansion (McNamara & Nulsen 2007). Also, we note that the temperature drops precipitously in the next annulus inward from the annulus just inside the shock boundary. This may indicate that the temperature of the gas just inside the shock boundary was also previously much lower before being shocked, and then the rise in temperature in this region may be higher than we have estimated above.

In addition, we fitted a projected density model to the surface brightness profile in a wedge to the SW with P.A. 250°–340° from N out to a radius of 100'' to characterize the inner shocks in this direction. We find similar results to the SW as we did to the NE for the first inner shock. We find a density jump of a factor of 1.26 ± 0.025 at a radius of 48 ± 0.4 arcsec (34 ± 0.3 kpc). However, as can be seen in the surface brightness distribution in the images (i.e., Figure 3), the second inner shock edge seems less sharp and extends to larger radii in the SW than in the NE. We do not find a density jump corresponding to the second inner shock to the SW, only a change in slope at a radius of 107 arcsec (75.5 kpc).

The spiral feature visible in Figure 7 is similar to that seen in simulations of gas sloshing in cluster centers (Ascasibar & Markevitch 2006). The sloshing sets up cold fronts and the distinctive spiral morphology. We therefore would expect the excesses seen in Figure 7 to be visible as excesses in surface brightness profiles containing these regions. Temperature profiles should show cooler gas coincident with the surface brightness enhancements. In addition, the simulations predict that the bright, sloshing, spiral region will contain gas of lower entropy as cluster central gas is displaced to larger radii. As shown in Section 5, the spiral excess is coincident with regions of low temperature and entropy in projected maps.

We have extracted surface brightness profiles from wedges in three directions: SW, NE, and NW. The SW corresponds to the bright, inner part of the spiral, while the NE includes the outer region of the spiral, and the NW is largely free from any spiral excess emission and serves as a comparison region. The sectors used for the profiles are shown superposed on the 0.3–2.0 keV residual image in Figure 19. The surface brightness profiles for the three regions are shown in Figure 20. Error bars are smaller than the symbols. Relative to the NW, non-spiral region, the SW shows excess emission from approximately 60''–195'' (42–138 kpc), corresponding to the inner part of the spiral. To the NE, excess emission is seen beyond approximately 110'' (78 kpc). Note that the enhancement in the NW in the ≈10''–20'' (7–14 kpc) region shows that the bubble rims are brightest in this region, which also corresponds to cooler gas seen in Hα (Figures 9 and 10).

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We have extracted spectra in the three wedges and determined projected temperature profiles. The spectra were extracted separately for each of the observations as well as for the corresponding background files. The spectra for each region were fitted simultaneously using an APEC thermal plasma model. Absorption was fixed at the Galactic value and elemental abundances were allowed to vary. Projected temperature profiles comparing the SW and NW (non-spiral) regions and the NE and NW regions are shown in Figures 21 and 22, respectively. In both cases, significant drops in temperature are seen in the regions corresponding to the surface brightness excesses associated with the spiral described above. In addition, the high surface brightness ≈10''–20'' (7–14 kpc) region in the NW has cooler temperatures than those radii regions to the SW and NE. This is likely due, at least in part, to the larger contribution to the emission measure from the bright, compressed, cool, bubble rims to the NW at these radii as compared to the same radii to the SW and NE, where the bubble rims are not as bright.

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In order to examine the bright region of the spiral to the SW in more detail, we have fitted a projected density model to the surface brightness profile, as described above for the NE sector shock fits. To focus on the cold front/spiral edge, we fit only regions with r > 70''. We find a density jump of a factor of 1.12^+0.022_{− 0.011} at a radius of 154 ± 1.5 arcsec (109 ± 1 kpc) from the cluster center.

We extracted spectra in larger bins to the SW, and performed a spectral deprojection as described above for the NE. Profiles of temperature, density, and pressure are shown in Figure 23. Dashed lines, going from smaller to larger annuli, indicate the positions of the first inner shock, the second inner shock/edge, and the exterior edge of the cold front/SW spiral. Since the deprojection introduces some scatter, particularly in the region between the inner shocks, we have also plotted the projected temperature profile as well as the pressure profile using the projected temperatures (shown as open circles). A temperature rise is seen associated with the first inner shock, but not the second inner shock. A clear density falloff is seen at the outer edge of the SW spiral. The temperatures within the spiral region are cooler than those outside of it.

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Projected abundance profiles to the SW and NE are shown in Figure 24. The dashed line marks the outer edge of the spiral to the SW (at r = 154'', as determined above). The abundance is higher in the bright, SW, spiral region, than in the corresponding radial region to the NE. Profiles of entropy for the SW and NE regions are shown in Figure 25. The entropy is defined as S = kT/n^2/3_e, and projected temperature values were used. As in Figure 24, the outer edge of the SW spiral region is marked with a dashed line. There is clearly a crossover in the entropy profiles for the SW and NE regions near this radius. From r ≈ 70''–150'', low entropy values are found coincident with the SW spiral excess. In the NE, the spiral excess is at larger radii, and this is seen in the entropy profile, where the values are low in the NE with radii greater than approximately 190''. This is striking confirmation of the predictions from sloshing models, where central, low entropy, cluster gas is displaced to larger radii (i.e., Ascasibar & Markevitch 2006).

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The sum of the evidence, including the surface brightness profiles, as well as temperature, abundance, and entropy distributions, points to the spiral being a cold front/sloshing feature resulting from an off-axis merger earlier in the cluster's history.

Clear cavities (or "bubbles") in the X-ray emission are seen in the Chandra images to the N and S of the AGN. In addition, there is a small bubble bounded by a narrow X-ray filament to the NW and a bubble separated from the S bubble to the SE. A very small depression is also seen to the E. These features are all filled with 4.8 GHz radio emission as seen in Figure 6. As described in Section 3, we created a radio spectral index map using the 1.4 and 4.8 GHz VLA data. The map is shown in Figure 26 with contours of 15 Gaussian-smoothed X-ray emission in the 0.3–10.0 keV band superposed.

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The spectral index is flattest at the position of the radio and X-ray core, and steepens into the radio lobes. The regions we have identified as outer bubbles, to the NW and SE, are associated with regions with distinctly steeper spectral indices. This is consistent with these bubbles resulting from an earlier stage in the current outburst, or from a separate, earlier AGN outburst, since the high-energy electrons age faster than the lower energy electrons, resulting in steepening of the spectrum over time. The radio emission "leaking" out of the N bubble through the bubble rim directly N of the AGN has a spectral index consistent with that filling the N lobe, making it likely that this emission is part of the current AGN outburst.

Obvious additional bubbles are not seen outside of this central region in the images, including the unsharp-masked images. Also, lower frequency radio emission at 330 MHz (Zhao et al. 1993) has a similar extent as the 1.4 and 4.8 GHz emission. To search for more bubbles at larger radii, we have made a pressure-difference map. The map was created by fitting a 2D beta model to a pressure map with 8'' bins, and a minimum of 10,000 background-subtracted counts for each region used for a spectral fit. This corresponds to the temperature map shown in Figure 12. The center was fixed to the position found in the 2D beta model fit to the surface brightness, described in Section 4.1.2. The values for the ellipticity and position angle of the ellipse, 0.17 and 119° from W, respectively, were similar to those found for the surface brightness fit. The pressure-difference map is shown in Figure 27, with 4.8 GHz radio contours superposed. Regions of low pressure are evident to the N and S of the AGN, along the current axis of the radio source. For both the N and S apparently low-pressure regions on the map, we have extracted spectra within circular apertures covering the regions, as well as comparison regions in surrounding circular annuli. We have calculated "projected" pressures, defined as kT(A)^1/2, where A is the APEC normalization, for the apparent bubbles (low-pressure regions) and the comparison annuli. For the N bubble, we find a significance in the lower projected pressure of 2.3σ compared to the comparison annulus, and for the S bubble, the significance is 2.2σ. These regions of lower pressure may represent outer bubbles from an earlier outburst (or multiple earlier outbursts) of the AGN. Future higher dynamic range radio data may give further evidence that these features are related to AGN activity.

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Approximating both of these possible bubbles as spheres with radii of 34 kpc, located 75 kpc from the cluster center, we calculate their energy input into the ICM as E = 4PV (Churazov et al. 2002). This assumes that the bubbles are filled with relativistic plasma (γ = 4/3). Using the average pressure at the cluster radius of 75 kpc of P ≈ 6 × 10⁻¹¹ dyn cm⁻², we find that each bubble can add 1 × 10⁶⁰ erg to the ICM.

We have presented the first results from a very deep Chandra observation of A2052. Detailed structure is seen in the inner part of the cluster, including bubbles, bright rims, and filaments. Radio emission at 4.8 GHz is seen to fill bubbles to the N and S of the AGN, as well as outer bubbles to the NW and SE, and a small bubble to the E. Hα emission is coincident with the brightest and coolest regions in the cluster center, including the E–W bar and the filament in the N bubble, indicating that at least some gas is cooling to T ≈ 10⁴ K.

An inner shock is clearly seen surrounding the cluster center, as well as a second feature exterior to the first that is also most likely a shock. Both edges in surface brightness can be described as arising from shocks with Mach numbers ≈1.2. For the inner shock, evidence for an associated temperature rise is seen in a three-color image. Also, a temperature profile reveals a significant increase inside this region, consistent with that expected given the shock strength. Such temperature rises associated with weak shocks driven by the AGN in the centers of cool core clusters have only rarely been measured, given the narrow widths of the shocks, the small magnitudes of the temperature rises, the cooling related to adiabatic expansion, and the projection of cluster gas at larger radii.

A spiral feature is seen, and is well described as resulting from gas sloshing related to a cluster–cluster or sub-cluster merger earlier in the lifetime of A2052. There is also evidence of dynamical activity in the cluster from optical spectroscopy: the central cD has a fairly large peculiar velocity (290 ± 90 km s⁻¹) relative to the cluster mean (Oegerle & Hill 2001). The X-ray spiral is brightest to the SW, and continues around the cluster center to the E and NE. The SW portion of the excess emission was shown in Laganá et al. (2010) using a shorter Chandra exposure, and in de Plaa et al. (2010) using a deep XMM-Newton observation. The spiral structure was not seen, however, before this very deep Chandra observation. Laganá et al. presented several residual Chandra images of cool core clusters, and noted that A2052 was an exception in not exhibiting a spiral feature in the available data, and de Plaa et al. described the SW excess as a cold front. Here, in addition to exhibiting an excess in surface brightness on a 2D model-subtracted Chandra image, the brightness distribution is confirmed using surface brightness profiles in different sectors across the image. The spiral is also shown to contain gas cooler than its surroundings in both temperature maps and sector profiles. Regions of low entropy are found coincident with the spiral surface brightness excess. Finally, the abundance is higher in the spiral region than its surroundings. This is consistent with central, low-entropy, high-metallicity gas sloshing out to larger radii. Therefore, sloshing can play a part in redistributing the metals within a cluster (Simionescu et al. 2010; de Plaa et al. 2010). Outbursts from the AGN also play a role in metal redistribution (Kirkpatrick et al. 2009). Detailed measurements of the spectral properties of similar diffuse spiral excesses have only rarely been measured. While Laganá et al. (2010), for example, presented evidence of excess surface brightness spirals in several systems, in many cases temperature declines were not detected associated with the spiral features, and if they were, they were most often only measured for the bright, inner spiral regions.

While the radio emission shows clear correspondence with deficits in the X-ray emission, and the extended radio structure is likely related to typical radio lobes from the AGN, there may be some component of radio mini-halo emission. A correlation has been found between the presence of sloshing features in clusters and mini-halos (ZuHone et al. 2011), and these mini-halos may result from reacceleration of relativistic electrons by turbulence associated with sloshing (Mazzotta & Giacintucci 2008). With our current radio data, however, we do not see obvious correspondence between the radio emission and the spiral sloshing feature.

There is evidence for outer bubbles related to one or more earlier outbursts from the AGN. While not seen in the surface brightness images, deficits are evident in a pressure residual map after subtracting off a model of the average pressure distribution. The deficits are seen to the N and S of the cluster center, in line with the current axis of the radio lobes. Each of these bubbles could inject up to 1 × 10⁶⁰ erg into the ICM.

Support for this work was provided by the National Aeronautics and Space Administration, through Chandra Award Number GO9-0147X. Basic research in radio astronomy at the Naval Research Laboratory is supported by 6.1 Base funding. S.W.R. was supported in part by the Chandra X-ray Center through NASA contract NAS8-03060. We thank Frazer Owen for useful discussions.