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The active galaxy NGC1275 lies at the centre of the Perseus cluster of galaxies, which is the X-ray brightest cluster in the Sky. The nucleus shows large variability over the past few decades. We compile a lightcurve of its X-ray emission covering about 40 years and show that the bright phase around 1980 explains why the inner X-ray bubbles were not seen in the images taken with the Einstein Observatory. The flux had dropped considerably by 1992 when images with the ROSAT HRI led to their discovery. The nucleus is showing a slow X-ray rise since the first Chandra images in 2000. If it brightens back to the pre-1990 level, then X-ray absorption spectroscopy by ASTRO-H can reveal the velocity structure of the shocked gas surrounding the inner bubbles., Comment: 7 pages, 7 figures, accepted for publication in MNRAS

We present new Chandra X-ray observations of the brightest cluster galaxy (BCG) in the cool-core cluster Abell 2597 (z= 0.0821). The data reveal an extensive kpc-scale X-ray cavity network as well as a 15-kpc filament of soft-excess gas exhibiting strong spatial correlation with archival Very Large Array radio data. In addition to several possible scenarios, multiwavelength evidence may suggest that the filament is associated with multiphase (103–107 K) gas that has been entrained and dredged-up by the propagating radio source. Stemming from a full spectral analysis, we also present profiles and 2D spectral maps of modelled X-ray temperature, entropy, pressure and metal abundance. The maps reveal an arc of hot gas which in projection borders the inner edge of a large X-ray cavity. Although limited by strong caveats, we suggest that the hot arc may be (a) due to a compressed rim of cold gas pushed outwards by the radio bubble or (b) morphologically and energetically consistent with cavity-driven active galactic nucleus heating models invoked to quench cooling flows, in which the enthalpy of a buoyant X-ray cavity is locally thermalized as ambient gas rushes to refill its wake. If confirmed, this would be the first observational evidence for this model.

New Chandra X-ray and Herschel Far-Infrared (FIR) observations enable a multiwavelength study of active galactic nucleus (AGN) heating and intracluster medium (ICM) cooling in the brightest cluster galaxy (BCG) of Abell 2597 (z= 0.0821). The new Chandra observations reveal the central ≲30 kpc X-ray cavity network to be more extensive than previously thought, and associated with enough enthalpy to theoretically inhibit the inferred classical cooling flow. Nevertheless, we present new evidence, consistent with previous results, that a moderately strong residual cooling flow is persisting at 4–8 per cent of the classically predicted rates in a spatially structured manner amid the feedback-driven excavation of the X-ray cavity network. New Herschel observations are used to estimate warm and cold dust masses, a lower limit gas-to-dust ratio and a star formation rate consistent with previous measurements. [O i] and CO(2−1) line profiles are used to constrain the kinematics of the ∼109 M⊙ reservoir of cold molecular gas. The cooling time profile of the ambient X-ray atmosphere is used to map the locations of the observational star formation entropy threshold as well as the theoretical thermal instability threshold. Both lie just outside the ≲30-kpc central region permeated by X-ray cavities, and star formation as well as ionized and molecular gas lie interior to both. The young stars are distributed in an elongated region that is aligned with the radio lobes, and their estimated ages are both younger and older than the X-ray cavity network, suggesting both jet-triggered as well as persistent star formation over the current AGN feedback episode. Bright X-ray knots that are coincident with extended Lyα and far-ultraviolet continuum filaments motivate a discussion of structured cooling from the ambient hot atmosphere along a projected axis that is perpendicular to X-ray cavity and radio axis. We conclude that the cooling ICM is the dominant contributor of the cold gas reservoir fuelling star formation and AGN activity in the Abell 2597 BCG.

We present a new 400-ks Chandra X-ray observation of the merging galaxy cluster Abell 2146. This deep observation reveals detailed structure associated with the major merger event including the Mach number M= 2.3 ± 0.2 bow shock ahead of the dense, ram pressure stripped subcluster core and the first known example of an upstream shock in the intracluster medium (ICM) (M= 1.6 ± 0.1). By measuring the electron temperature profile behind each shock front, we determine the time-scale for the electron population to thermally equilibrate with the shock-heated ions. We find that the temperature profile behind the bow shock is consistent with the time-scale for Coulomb collisional equilibration and the post-shock temperature is lower than expected for instant shock heating of the electrons. Although like the Bullet cluster the electron temperatures behind the upstream shock front are hotter than expected, favouring the instant heating model, the uncertainty on the temperature values is greater here and there is significant substructure complicating the interpretation. We also measured the width of each shock front and the contact discontinuity on the leading edge of the subcluster core to investigate the suppression of transport processes in the ICM. The upstream shock is ∼440 kpc in length but appears remarkably narrow over this distance with a best-fitting width of only 6+5−3 kpc compared with the mean free path of 23 ± 5 kpc. The leading edge of the subcluster core is also narrow with an upper limit on the width of only 2 kpc separating the cool, multiphase gas at 0.5–2 keV from the shock-heated surrounding ICM at ∼6 keV. The strong suppression of diffusion and conduction across this edge suggests a magnetic draping layer may have formed around the subcluster core. The deep Chandra observation has also revealed a cool, dense plume of material extending ∼170 kpc perpendicular to the merger axis, which is likely to be the disrupted remnant of the primary cluster core. This asymmetry in the cluster morphology indicates the merger has a non-zero impact parameter. We suggest that this also explains why the south-western edge of the subcluster core is narrow and stable over ∼150 kpc in length, but the north-eastern edge is broad and being stripped of material.

We present new Chandra X-ray observations of the brightest cluster galaxy (BCG) in the cool core cluster Abell 2597. The data reveal an extensive kpc-scale X-ray cavity network as well as a 15 kpc filament of soft-excess gas exhibiting strong spatial correlation with archival VLA radio data. In addition to several possible scenarios, multiwavelength evidence may suggest that the filament is associated with multiphase (10^3 - 10^7 K) gas that has been entrained and dredged-up by the propagating radio source. Stemming from a full spectral analysis, we also present profiles and 2D spectral maps of modeled X-ray temperature, entropy, pressure, and metal abundance. The maps reveal an arc of hot gas which in projection borders the inner edge of a large X-ray cavity. Although limited by strong caveats, we suggest that the hot arc may be (a) due to a compressed rim of cold gas pushed outward by the radio bubble or (b) morphologically and energetically consistent with cavity-driven active galactic nucleus (AGN) heating models invoked to quench cooling flows, in which the enthalpy of a buoyant X-ray cavity is locally thermalized as ambient gas rushes to refill its wake. If confirmed, this would be the first observational evidence for this model., Comment: 16 pages, 11 figures; Accepted for publication in MNRAS

We present a new 400 ks Chandra X-ray observation of the merging galaxy cluster Abell 2146. This deep observation reveals detailed structure associated with the major merger event including the Mach M=2.3+/-0.2 bow shock ahead of the dense, ram pressure stripped subcluster core and the first known example of an upstream shock in the ICM (M=1.6+/-0.1). By measuring the electron temperature profile behind each shock front, we determine the timescale for the electron population to thermally equilibrate with the shock-heated ions. We find that the temperature profile behind the bow shock is consistent with the timescale for Coulomb collisional equilibration and the postshock temperature is lower than expected for instant shock-heating of the electrons. Although like the Bullet cluster the electron temperatures behind the upstream shock front are hotter than expected, favouring the instant heating model, the uncertainty on the temperature values is greater here and there is significant substructure complicating the interpretation. We also measured the width of each shock front and the contact discontinuity on the leading edge of the subcluster core to investigate the suppression of transport processes in the ICM. The upstream shock is ~440 kpc in length but appears remarkably narrow over this distance with a best-fit width of only 6^{+5}_{-3} kpc compared with the mean free path of 23+/-5 kpc. The leading edge of the subcluster core is also narrow with an upper limit on the width of only 2 kpc separating the cool, multiphase gas at 0.5-2 keV from the shock-heated surrounding ICM at ~6 keV. The strong suppression of diffusion and conduction across this edge suggests a magnetic draping layer may have formed around the subcluster core.[abridged], Comment: 20 pages, 17 figures, accepted by MNRAS

There is a strong spatial correlation between brightest cluster galaxies (BCGs) and the peak density and cooling rate of the intra-cluster medium (ICM). In this paper we combine integral field spectroscopy, CO observations and X-ray data to study three exceptional clusters (Abell 1991, Abell 3444 and Ophiuchus) where there is a physical and dynamical offset between the BCG and the cooling peak to investigate the connection between the cooling of the intracluster medium, the cold gas being deposited and the central galaxy. We find the majority of the optical line emission is spatially coincident with the peak in the soft X-rays. In the case of A1991 we make separate detections of CO(2-1) emission on the BCG and on the peak of the soft X-ray emission suggesting that cooling continues to occur in the core despite being offset from the BCG. We conclude that there is a causal link between the lowest temperature (< 2 keV) ICM gas and the molecular gas(~ 30K). This link is only apparent in systems where a transitory event has decoupled the BCG from the soft X-ray peak. We discuss the prospects for identifying more examples of this rare configuration., 11 pages, 7 figures, 2 tables, Resubmitted to MN after addressing referees comments

Using WHT OASIS integral field unit observations, we report the discovery of a thin plume of ionised gas extending from the brightest cluster galaxy in Abell 2146 to the sub-cluster X-ray cool core which is offset from the BCG by ~37 kpc. The plume is greater than 15 kpc long and less than 3 kpc wide. This plume is unique in that the cluster it is situated in is currently undergoing a major galaxy cluster merger. The brightest cluster galaxy is unusually located behind the X-ray shock front and in the wake of the ram pressure stripped X-ray cool core and evidence for recent disruption to the BCG is observed. We examine the gas and stellar morphology, the gas kinematics of the BCG and their relation to the X-ray gas. We propose that a causal link between the ionised gas plume and the offset X-ray cool core provides the simplest explanation for the formation of the plume. An interaction or merger between the BCG and another cluster galaxy is probably the cause of the offset., Comment: 14 pages, 18 figures, accepted for publication in MNRAS

We present a new Chandra observation of the galaxy cluster Abell 2146 which has revealed a complex merging system with a gas structure that is remarkably similar to the Bullet cluster (eg. Markevitch et al. 2002). The X-ray image and temperature map show a cool 2-3 keV subcluster with a ram pressure stripped tail of gas just exiting the disrupted 6-7 keV primary cluster. From the sharp jump in the temperature and density of the gas, we determine that the subcluster is preceded by a bow shock with a Mach number M=2.2+/-0.8, corresponding to a velocity v=2200^{+1000}_{-900} km/s relative to the main cluster. We estimate that the subcluster passed through the primary core only 0.1-0.3 Gyr ago. In addition, we observe a slower upstream shock propagating through the outer region of the primary cluster and calculate a Mach number M=1.7+/-0.3. Based on the measured shock Mach numbers M~2 and the strength of the upstream shock, we argue that the mass ratio between the two merging clusters is between 3 and 4 to one. By comparing the Chandra observation with an archival HST observation, we find that a group of galaxies is located in front of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak., Comment: 13 pages, 13 figures, accepted by MNRAS

We examine deep XMM-Newton Reflection Grating Spectrometer (RGS) spectra from the cores of three X-ray bright cool core galaxy clusters, Abell 262, Abell 3581 and HCG 62. Each of the RGS spectra show Fe XVII emission lines indicating the presence of gas around 0.5 keV. There is no evidence for O VII emission which would imply gas at still cooler temperatures. The range in detected gas temperature in these objects is a factor of 3.7, 5.6 and 2 for Abell 262, Abell 3581 and HCG 62, respectively. The coolest detected gas only has a volume filling fraction of 6 and 3 per cent for Abell 262 and Abell 3581, but is likely to be volume filling in HCG 62. Chandra spatially resolved spectroscopy confirms the low volume filling fractions of the cool gas in Abell 262 and Abell 3581, indicating this cool gas exists as cold blobs. Any volume heating mechanism aiming to prevent cooling would overheat the surroundings of the cool gas by a factor of 4. If the gas is radiatively cooling below 0.5 keV, it is cooling at a rate at least an order of magnitude below that at higher temperatures in Abell 262 and Abell 3581 and two-orders of magnitude lower in HCG 62. The gas may be cooling non-radiatively through mixing in these cool blobs, where the energy released by cooling is emitted in the infrared. We find very good agreement between smooth particle inference modelling of the cluster and conventional spectral fitting. Comparing the temperature distribution from this analysis with that expected in a cooling flow, there appears to be a even larger break below 0.5 keV as compared with previous empirical descriptions of the deviations of cooling flow models., Comment: 19 pages, 22 figures, accepted by MNRAS

We report ALMA Early Science observations of the Abell 1835 brightest cluster galaxy (BCG) in the CO (3-2) and CO (1-0) emission lines. We detect $5\times 10^{10}~\rm M_\odot$ of molecular gas within 10 kpc of the BCG. Its ensemble velocity profile width of $\sim 130 ~\rm km~s^{-1}$ FWHM is too narrow for the molecular cloud sto be supported in the galaxy by dynamic pressure. The gas may instead be supported in a rotating, turbulent disk oriented nearly face-on. Roughly $10^{10}~\rm M_\odot$ of molecular gas is projected $3-10 ~\rm kpc$ to the north-west and to the east of the nucleus with line of sight velocities lying between $-250 ~\rm km~s^{-1}$ to $+480 ~\rm km~s^{-1}$ with respect to the systemic velocity. The high velocity gas may be either inflowing or outflowing. However, the absence of high velocity gas toward the nucleus that would be expected in a steady inflow, and its bipolar distribution on either side of the nucleus, are more naturally explained as outflow. Star formation and radiation from the AGN are both incapable of driving an outflow of this magnitude. If so, the molecular outflow may be associated a hot outflow on larger scales reported by Kirkpatrick and colleagues. The molecular gas flow rate of approximately $200~\rm M_\odot ~yr^{-1}$ is comparable to the star formation rate of $100-180~\rm M_\odot ~yr^{-1}$ in the central disk. How radio bubbles would lift dense molecular gas in their updrafts, how much gas will be lost to the BCG, and how much will return to fuel future star formation and AGN activity are poorly understood. Our results imply that radio-mechanical (radio mode) feedback not only heats hot atmospheres surrounding elliptical galaxies and BCGs, it is able to sweep higher density molecular gas away from their centers., Comment: To appear in ApJ -- expanded discussion of inflow/outflow added to submitted version

We present new, deep observations of the Phoenix cluster from the Chandra X-ray Observatory, the Hubble Space Telescope, and the Karl Jansky Very Large Array. These data provide an order of magnitude improvement in depth and/or angular resolution at X-ray, optical, and radio wavelengths, yielding an unprecedented view of the core of the Phoenix cluster. We find that the one-dimensional temperature and entropy profiles are consistent with expectations for pure-cooling hydrodynamic simulations and analytic descriptions of homogeneous, steady-state cooling flow models. In the inner ~10 kpc, the cooling time is shorter by an order of magnitude than any other known cluster, while the ratio of the cooling time to freefall time approaches unity, signaling that the ICM is unable to resist multiphase condensation on kpc scales. When we consider the thermodynamic profiles in two dimensions, we find that the cooling is highly asymmetric. The bulk of the cooling in the inner ~20 kpc is confined to a low-entropy filament extending northward from the central galaxy. We detect a substantial reservoir of cool (10^4 K) gas (as traced by the [OII] doublet), which is coincident with the low-entropy filament. The bulk of this cool gas is draped around and behind a pair of X-ray cavities, presumably bubbles that have been inflated by radio jets, which are detected for the first time on kpc scales. These data support a picture in which AGN feedback is promoting the formation of a multiphase medium via a combination of ordered buoyant uplift and locally enhanced turbulence. These processes ought to counteract the tendency for buoyancy to suppress condensation, leading to rapid cooling along the jet axis. The recent mechanical outburst has sufficient energy to offset cooling, and appears to be coupling to the ICM via a cocoon shock, raising the entropy in the direction orthogonal to the radio jets., Comment: 19 pages, 17 figures. Submitted to ApJ. Comments welcome!

Recent observations have revealed massive galactic molecular outflows that may have the physical conditions (high gas densities) required to form stars. Indeed, several recent models predict that such massive outflows may ignite star formation within the outflow itself. This star-formation mode, in which stars form with high radial velocities, could contribute to the morphological evolution of galaxies, to the evolution in size and velocity dispersion of the spheroidal component of galaxies, and would contribute to the population of high-velocity stars, which could even escape the galaxy. Such star formation could provide $\textit{in situ}$ chemical enrichment of the circumgalactic and intergalactic medium (through supernova explosions of young stars on large orbits), and some models also predict it to contribute substantially to the star-formation rate observed in distant galaxies. Although there exists observational evidence for star formation triggered by outflows or jets into their host galaxy, as a consequence of gas compression, evidence for star formation occurring within galactic outflows is still missing. Here we report spectroscopic observations that unambiguously reveal star formation occurring in a galactic outflow at a redshift of 0.0448. The inferred star-formation rate in the outflow is larger than 15 solar masses per year. Star formation may also be occurring in other galactic outflows, but may have been missed by previous observations owing to the lack of adequate diagnostics., R.M., S.Car. and F.B. acknowledge support by the Science and Technology Facilities Council (STFC). R.M. acknowledges ERC Advanced Grant 695671 “QUENCH”. H.R.R. and A.C.F. acknowledge ERC Advanced Grant 340442. S.A., S.Caz., E.B. and L.C. acknowledge support from the Spanish Ministry of Economy, under grants AYA2012-32295 and ESP2015-68964-P.

We present an analysis of deep Chandra X-ray observations of the galaxy cluster MS 0735.6+7421, which hosts the most energetic radio AGN known. Our analysis has revealed two cavities in its hot atmosphere with diameters of 200-240 kpc. The total cavity enthalpy, mean age, and mean jet power are $9\times 10^{61}$ erg, $1.6\times 10^{8}$ yr, and $1.7\times 10^{46}$ erg/s, respectively. The cavities are surrounded by nearly continuous temperature and surface brightness discontinuities associated with an elliptical shock front of Mach number 1.26 (1.17-1.30) and age of $1.1\times 10^{8}$ yr. The shock has injected at least $4\times 10^{61}$ erg into the hot atmosphere at a rate of $1.1\times 10^{46}$ erg/s. A second pair of cavities and possibly a second shock front are located along the radio jets, indicating that the AGN power has declined by a factor of 30 over the past 100 Myr. The multiphase atmosphere surrounding the central galaxy is cooling at a rate of 36 Msun/yr, but does not fuel star formation at an appreciable rate. In addition to heating, entrainment in the radio jet may be depleting the nucleus of fuel and preventing gas from condensing out of the intracluster medium. Finally, we examine the mean time intervals between AGN outbursts in systems with multiple generations of X-ray cavities. We find that, like MS0735, their AGN rejuvenate on a timescale that is approximately 1/3 of their mean central cooling timescales, indicating that jet heating is outpacing cooling in these systems., Comment: 15 pages, 15 figures. Accepted for publication in MNRAS on May 21, 2014