arXiv subjects

H. R. Russell

Publications and source records attributed to H. R. Russell.

At least 37 records · Page 2Linked to original sources

arXiv2019

Measuring bulk flows of the intracluster medium in the Perseus and Coma galaxy clusters using XMM-Newton

We demonstrate a novel technique for calibrating the energy scale of the XMM EPIC-pn detector, which allows us to measure bulk flows in the intracluster medium (ICM) of the Perseus and Coma clusters. The procedure uses the instrumental lines present in all observations, in particular, Cu-Ka. By studying their spatial and temporal variations, in addition to incorporating calibration observations, we refined the absolute energy scale to better than 150 km/s at the Fe-K line, a large improvement over the nominal accuracy of 550 km/s. We then mapped the bulk motions over much of the central 1200 and 800 kpc of Perseus and Coma, respectively, in spatial regions down to 65 and 140 kpc size. We cross-checked our procedure by comparing our measurements with those found in Perseus by Hitomi for an overlapping region, finding consistent results. For Perseus, there is a LoS velocity increase of 480+-210 km/s (1sigma) 250 kpc east of the nucleus. This region is associated with a cold front, providing direct evidence of the ICM sloshing in the potential well. Assuming the intrinsic distribution of bulk motions is Gaussian, its width is 214+-85 km/s, excluding systematics. Removing the sloshing region, this is reduced to 20-150 km/s, which is similar in magnitude to the Hitomi line width measurements in undisturbed regions. In Coma, the line-of-sight velocity of the ICM varies between the velocities of the two central galaxies. Maps of the gas velocity and metallicity provide clues about the merger history of the Coma, with material to the north and east of the cluster core having a velocity similar to NGC 4874, while that to the south and west has velocities close to NGC 4889. Our results highlight the difference between a merging system, such as Coma, where we observe a ~1000 km/s range in velocity, and a relatively relaxed system, such as Perseus, with much weaker bulk motions. [abridged]

astro-ph.CO↗

arXiv2019

Origins of molecular clouds in early-type galaxies

We analyze $Chandra$ observations of the hot atmospheres of 40 early spiral and elliptical galaxies. Using new temperature, density, cooling time, and mass profiles, we explore relationships between their hot atmospheres and cold molecular gas. Molecular gas mass correlates with atmospheric gas mass and density over four decades from central galaxies in clusters to normal giant ellipticals and early spirals. The mass and density relations follow power laws: $M_{\rm mol} \propto M_{\rm X}^{1.4\pm0.1}$ and $M_{\rm mol} \propto n_{\rm e}^{1.8\pm0.3}$, respectively, at 10 kpc. The ratio of molecular gas to atmospheric gas within a 10 kpc radius lies between $3\%$ and $10\%$ for early-type galaxies and between $3\%$ and $50\%$ for central galaxies in clusters. Early-type galaxies have detectable levels of molecular gas when their atmospheric cooling times falls below $\sim \rm Gyr$ at a radius of 10 kpc. A similar trend is found in central cluster galaxies. We find no relationship between the ratio of the cooling time to free fall time, $t_{\rm c}/t_{\rm ff}$, and the presence or absence of molecular clouds in early-type galaxies. The data are consistent with much of the molecular gas in early-type galaxies having condensed from their hot atmospheres.

astro-ph.GA↗

arXiv2019

Driving massive molecular gas flows in central cluster galaxies with AGN feedback

We present an analysis of new and archival ALMA observations of molecular gas in twelve central cluster galaxies. We examine emerging trends in molecular filament morphology and gas velocities to understand their origins. Molecular gas masses in these systems span $10^9-10^{11}\mathrm{M}_{\odot}$, far more than most gas-rich galaxies. ALMA images reveal a distribution of morphologies from filamentary to disk-dominated structures. Circumnuclear disks on kiloparsec scales appear rare. In most systems, half to nearly all of the molecular gas lies in filamentary structures with masses of a few $\times10^{8-10}\mathrm{M}_{\odot}$ that extend radially several to several tens of kpc. In nearly all cases the molecular gas velocities lie far below stellar velocity dispersions, indicating youth, transience or both. Filament bulk velocities lie far below the galaxy's escape and free-fall speeds indicating they are bound and being decelerated. Most extended molecular filaments surround or lie beneath radio bubbles inflated by the central AGN. Smooth velocity gradients found along the filaments are consistent with gas flowing along streamlines surrounding these bubbles. Evidence suggests most of the molecular clouds formed from low entropy X-ray gas that became thermally unstable and cooled when lifted by the buoyant bubbles. Uplifted gas will stall and fall back to the galaxy in a circulating flow. The distribution in morphologies from filament to disk-dominated sources therefore implies slowly evolving molecular structures driven by the episodic activity of the AGN.

astro-ph.GA↗

arXiv2019

Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster

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.

astro-ph.GA↗

arXiv2019

Discovery of a diffuse optical line emitting halo in the core of the Centaurus cluster of galaxies: Line emission outside the protection of the filaments

We present the discovery of diffuse optical line emission in the Centaurus cluster seen with the MUSE IFU. The unparalleled sensitivity of MUSE allows us to detect the faint emission from these structures which extend well beyond the bounds of the previously known filaments. Diffuse structures (emission surrounding the filaments, a northern shell and an extended Halo) are detected in many lines typical of the nebulae in cluster cores ([NII]$_{λ6548\&6583}$ ,[SII]$_{λ6716\&6731}$, [OI]$_{λ6300}$, [OIII]$_{λ4959\&5007}$ etc.) but are more than an order of magnitude fainter than the filaments, with the faint halo only detected through the brightest line in the spectrum ([NII]$_{λ6583}$). These structures are shown to be kinematically distinct from the stars in the central galaxy and have different physical and excitation states to the filaments. Possible origins are discussed for each structure in turn and we conclude that shocks and/or pressure imbalances are resulting in gas dispersed throughout the cluster core, formed from either disrupted filaments or direct cooling, which is not confined to the bright filaments.

astro-ph.CO↗

arXiv2018

An enormous molecular gas flow in the RXJ0821+0752 galaxy cluster

We present recent {\it Chandra} X-ray observations of the RXJ0821.0+0752 galaxy cluster in addition to ALMA observations of the CO(1-0) and CO(3-2) line emission tracing the molecular gas in its central galaxy. All of the CO line emission, originating from a $10^{10}\,M_{\odot}$ molecular gas reservoir, is located several kpc away from the nucleus of the central galaxy. The cold gas is concentrated into two main clumps surrounded by a diffuse envelope. They form a wide filament coincident with a plume of bright X-ray emission emanating from the cluster core. This plume encompasses a putative X-ray cavity that is only large enough to have uplifted a few percent of the molecular gas. Unlike other brightest cluster galaxies, stimulated cooling, where X-ray cavities lift low entropy cluster gas until it becomes thermally unstable, cannot have produced the observed gas reservoir. Instead, the molecular gas has likely formed as a result of sloshing motions in the intracluster medium induced by a nearby galaxy. Sloshing can emulate uplift by dislodging gas from the galactic center. This gas has the shortest cooling time, so will condense if disrupted for long enough.

astro-ph.GA↗

arXiv2018

Molecular gas filaments and star-forming knots beneath an X-ray cavity in RXC J1504-0248

We present recent ALMA observations of the CO(1-0) and CO(3-2) emission lines in the brightest cluster galaxy of RXCJ1504.1$-$0248, which is one of the most extreme cool core clusters known. The central galaxy contains $1.9\times 10^{10}~M_{\odot}$ of molecular gas. The molecular gas morphology is complex and disturbed, showing no evidence for a rotationally-supported structure in equilibrium. $80\%$ of the gas is situated within the central 5 kpc of the galactic center, while the remaining gas is located in a 20 kpc long filament. The cold gas has likely condensed out of the hot atmosphere. The filament is oriented along the edge of a putative X-ray cavity, suggesting that AGN activity has stimulated condensation. This is enegetically feasible, although the morphology is not as conclusive as systems whose molecular filaments trail directly behind buoyant radio bubbles. The velocity gradient along the filament is smooth and shallow. It is only consistent with free-fall if it lies within $20^{\circ}$ of the plane of the sky. The abundance of clusters with comparably low velocities suggests that the filament is not free-falling. Both the central and filamentary gas are coincident with bright UV emission from ongoing star formation. Star formation near the cluster core is consistent with the Kennicutt-Schmidt law. The filament exhibits increased star formation surface densities, possibly resulting from either the consumption of a finite molecular gas supply or spatial variations in the CO-to-H$_2$ conversion factor.

astro-ph.GA↗

arXiv2018

A Universal Entropy Profile for the Hot Atmospheres of Galaxies and Clusters within $R_{2500}$

We present atmospheric gas entropy profiles for 40 early type galaxies and 110 clusters spanning several decades of halo mass, atmospheric gas mass, radio jet power, and galaxy type. We show that within $\sim 0.1R_{2500}$ the entropy profiles of low-mass systems, including ellipticals, brightest cluster galaxies, and spiral galaxies, scale approximately as $K\propto R^{2/3}$. Beyond $\sim 0.1R_{2500}$ entropy profiles are slightly shallower than the $K \propto R^{1.1}$ profile expected from gravitational collapse alone, indicating that heating by AGN feedback extends well beyond the central galaxy. We show that the $K\propto R^{2/3}$ entropy profile shape indicates that thermally unstable cooling is balanced by heating where the inner cooling and free-fall timescales approach a constant ratio. Hot atmospheres of elliptical galaxies have a higher rate of heating per gas particle compared to central cluster galaxies. This excess heating may explain why some central cluster galaxies are forming stars while most early-type galaxies have experienced no significant star formation for billions of years. We show that the entropy profiles of six lenticular and spiral galaxies follow the $R^{2/3}$ form. The continuity between central galaxies in clusters, giant ellipticals, and spirals suggests perhaps that processes heating the atmospheres of elliptical and brightest cluster galaxies are also active in spiral galaxies.

astro-ph.CO↗

arXiv2018

The imprints of AGN feedback within a supermassive black hole's sphere of influence

We present a new 300 ks Chandra observation of M87 that limits pileup to only a few per cent of photon events and maps the hot gas properties closer to the nucleus than has previously been possible. Within the supermassive black hole's gravitational sphere of influence, the hot gas is multiphase and spans temperatures from 0.2 to 1 keV. The radiative cooling time of the lowest temperature gas drops to only 0.1-0.5 Myr, which is comparable to its free fall time. Whilst the temperature structure is remarkably symmetric about the nucleus, the density gradient is steep in sectors to the N and S, with $ρ{\propto}r^{-1.5\pm0.1}$, and significantly shallower along the jet axis to the E, where $ρ{\propto}r^{-0.93\pm0.07}$. The density structure within the Bondi radius is therefore consistent with steady inflows perpendicular to the jet axis and an outflow directed E along the jet axis. By putting limits on the radial flow speed, we rule out Bondi accretion on the scale resolved at the Bondi radius. We show that deprojected spectra extracted within the Bondi radius can be equivalently fit with only a single cooling flow model, where gas cools from 1.5 keV down below 0.1 keV at a rate of 0.03 M$_{\odot}$/yr. For the alternative multi-temperature spectral fits, the emission measures for each temperature component are also consistent with a cooling flow model. The lowest temperature and most rapidly cooling gas in M87 is therefore located at the smallest radii at ~100 pc and may form a mini cooling flow. If this cooling gas has some angular momentum, it will feed into the cold gas disk around the nucleus, which has a radius of ~80 pc and therefore lies just inside the observed transition in the hot gas structure.

astro-ph.GA↗

arXiv2018

X-ray scaling relations of early-type galaxies

X-ray luminosity, temperature, gas mass, total mass, and their scaling relations are derived for 94 early-type galaxies using archival $Chandra$ X-ray Observatory observations. Consistent with earlier studies, the scaling relations, $L_X \propto T^{4.5\pm0.2}$, $M \propto T^{2.4\pm0.2}$, and $L_X \propto M^{2.8\pm0.3}$, are significantly steeper than expected from self similarity. This steepening indicates that their atmospheres are heated above the level expected from gravitational infall alone. Energetic feedback from nuclear black holes and supernova explosions are likely heating agents. The tight $L_X - T$ correlation for low-luminosities systems (i.e., below 10$^{40}$ erg/s) are at variance with hydrodynamical simulations which generally predict higher temperatures for low luminosity galaxies. We also investigate the relationship between total mass and pressure, $Y_X = M_g \times T$, finding $M \propto Y_{X}^{0.45\pm0.04}$. We explore the gas mass to total mass fraction in early-type galaxies and find a range of $0.1-1.0\%$. We find no correlation between the gas-to-total mass fraction with temperature or total mass. Higher stellar velocity dispersions and higher metallicities are found in hotter, brighter, and more massive atmospheres. X-ray core radii derived from $β$-model fitting are used to characterize the degree of core and cuspiness of hot atmospheres.

astro-ph.GA↗

arXiv2017

Mystery solved: discovery of extended radio emission in the merging galaxy cluster Abell 2146

Abell 2146 ($z=0.232$) is a massive galaxy cluster currently undergoing a spectacular merger in the plane of the sky with a bullet-like morphology. It was the first system in which both the bow and upstream shock fronts were detected at X-ray wavelengths (Mach$\sim2$), yet deep GMRT 325 MHz observations failed to detect extended radio emission associated with the cluster as is typically seen in such systems. We present new, multi-configuration $1-2$ GHz Karl G. Jansky Very Large Array (VLA) observations of Abell 2146 totalling 16 hours of observations. These data reveal for the first time the presence of an extended ($\approx850$ kpc), faint radio structure associated with Abell 2146. The structure appears to harbour multiple components, one associated with the upstream shock which we classify as a radio relic and one associated with the subcluster core which is consisted as being a radio halo bounded by the bow shock. The newly detected structures have some of the lowest radio powers detected thus far in any cluster ($P_{\rm 1.4 GHz, halo}=2.4\pm0.2\times10^{23}$ W Hz$^{-1}$ and $P_{\rm 1.4 GHz, relic}=2.2\pm0.2\times10^{23}$ W Hz$^{-1}$). The flux measurement of the halo, as well as its morphology, also suggest that the halo was recently created ($\approx0.3$ Gyr after core passage), consistent with the dynamical state of the cluster. These observations demonstrate the capacity of the upgraded VLA to detect extremely faint and extended radio structures. Based on these observations, we predict that many more radio relics and radio halos in merging clusters should be detected by future radio facilities such as the Square Kilometre Array (SKA).

astro-ph.GA↗

arXiv2017

Hydrostatic Chandra X-ray analysis of SPT-selected galaxy clusters - I. Evolution of profiles and core properties

We analyse Chandra X-ray Observatory observations of a set of galaxy clusters selected by the South Pole Telescope using a new publicly-available forward-modelling projection code, MBProj2, assuming hydrostatic equilibrium. By fitting a powerlaw plus constant entropy model we find no evidence for a central entropy floor in the lowest-entropy systems. A model of the underlying central entropy distribution shows a narrow peak close to zero entropy which accounts for 60 per cent of the systems, and a second broader peak around 130 keV cm^2. We look for evolution over the 0.28 to 1.2 redshift range of the sample in density, pressure, entropy and cooling time at 0.015 R_500 and at 10 kpc radius. By modelling the evolution of the central quantities with a simple model, we find no evidence for a non-zero slope with redshift. In addition, a non-parametric sliding median shows no significant change. The fraction of cool-core clusters with central cooling times below 2 Gyr is consistent above and below z=0.6 (~30-40 per cent). Both by comparing the median thermodynamic profiles, centrally biased towards cool cores, in two redshift bins, and by modelling the evolution of the unbiased average profile as a function of redshift, we find no significant evolution beyond self-similar scaling in any of our examined quantities. Our average modelled radial density, entropy and cooling-time profiles appear as powerlaws with breaks around 0.2 R_500. The dispersion in these quantities rises inwards of this radius to around 0.4 dex, although some of this scatter can be fit by a bimodal model.

astro-ph.CO↗

arXiv2017

The Origin of Molecular Clouds In Central Galaxies

We present an analysis of 55 central galaxies in clusters and groups with molecular gas masses and star formation rates lying between $10^{8}-10^{11}\ M_{\odot}$ and $0.5-270$ $M_{\odot}\ yr^{-1}$, respectively. We have used Chandra observations to derive profiles of total mass and various thermodynamic variables. Molecular gas is detected only when the central cooling time or entropy index of the hot atmosphere falls below $\sim$1 Gyr or $\sim$35 keV cm$^2$, respectively, at a (resolved) radius of 10 kpc. This indicates that the molecular gas condensed from hot atmospheres surrounding the central galaxies. The depletion timescale of molecular gas due to star formation approaches 1 Gyr in most systems. Yet ALMA images of roughly a half dozen systems drawn from this sample suggest the molecular gas formed recently. We explore the origins of thermally unstable cooling by evaluating whether molecular gas becomes prevalent when the minimum of the cooling to free-fall time ratio ($t_{\rm cool}/t_{\rm ff}$) falls below $\sim10$. We find: 1) molecular gas-rich systems instead lie between $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$, where $t_{\rm cool}/t_{\rm ff}=25$ corresponds approximately to cooling time and entropy thresholds $t_{\rm cool} \lesssim 1$ Gyr and 35 keV~cm$^2$, respectively, 2) $min(t_{\rm cool}/t_{\rm ff}$) is uncorrelated with molecular gas mass and jet power, and 3) the narrow range $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$ can be explained by an observational selection effect. These results and the absence of isentropic cores in cluster atmospheres are in tension with "precipitation" models, particularly those that assume thermal instability ensues from linear density perturbations in hot atmospheres. Some and possibly all of the molecular gas may instead have condensed from atmospheric gas lifted outward either by buoyantly-rising X-ray bubbles or merger-induced gas motions.

astro-ph.GA↗

arXiv2017

A $^{13}$CO Detection in a Brightest Cluster Galaxy

We present ALMA Cycle 4 observations of CO(1-0), CO(3-2), and $^{13}$CO(3-2) line emission in the brightest cluster galaxy of RXJ0821+0752. This is one of the first detections of $^{13}$CO line emission in a galaxy cluster. Half of the CO(3-2) line emission originates from two clumps of molecular gas that are spatially offset from the galactic center. These clumps are surrounded by diffuse emission that extends $8~{\rm kpc}$ in length. The detected $^{13}$CO emission is confined entirely to the two bright clumps, with any emission outside of this region lying below our detection threshold. Two distinct velocity components with similar integrated fluxes are detected in the $^{12}$CO spectra. The narrower component ($60~{\rm km}~{\rm s}^{-1}$ FWHM) is consistent in both velocity centroid and linewidth with $^{13}$CO(3-2) emission, while the broader ($130-160~{\rm km}~{\rm s}^{-1}$), slightly blueshifted wing has no associated $^{13}$CO(3-2) emission. A simple local thermodynamic model indicates that the $^{13}$CO emission traces $2.1\times 10^{9}~{\rm M}_\odot$ of molecular gas. Isolating the $^{12}$CO velocity component that accompanies the $^{13}$CO emission yields a CO-to-H$_2$ conversion factor of $α_{\rm CO}=2.3~{\rm M}_{\odot}~({\rm K~km~s^{-1}})^{-1}$, which is a factor of two lower than the Galactic value. Adopting the Galactic CO-to-H$_2$ conversion factor in brightest cluster galaxies may therefore overestimate their molecular gas masses by a factor of two. This is within the object-to-object scatter from extragalactic sources, so calibrations in a larger sample of clusters are necessary in order to confirm a sub-Galactic conversion factor.

astro-ph.GA↗

arXiv2017

Close entrainment of massive molecular gas flows by radio bubbles in the central galaxy of Abell 1795

We present new ALMA observations tracing the morphology and velocity structure of the molecular gas in the central galaxy of the cluster Abell 1795. The molecular gas lies in two filaments that extend 5 - 7 kpc to the N and S from the nucleus and project exclusively around the outer edges of two inner radio bubbles. Radio jets launched by the central AGN have inflated bubbles filled with relativistic plasma into the hot atmosphere surrounding the central galaxy. The N filament has a smoothly increasing velocity gradient along its length from the central galaxy's systemic velocity at the nucleus to -370 km/s, the average velocity of the surrounding galaxies, at the furthest extent. The S filament has a similarly smooth but shallower velocity gradient and appears to have partially collapsed in a burst of star formation. The close spatial association with the radio lobes, together with the ordered velocity gradients and narrow velocity dispersions, show that the molecular filaments are gas flows entrained by the expanding radio bubbles. Assuming a Galactic $X_{\mathrm{CO}}$ factor, the total molecular gas mass is $3.2\pm0.2\times10^{9}$M$_{\odot}$. More than half lies above the N radio bubble. Lifting the molecular clouds appears to require an infeasibly efficient coupling between the molecular gas and the radio bubble. The energy required also exceeds the mechanical power of the N radio bubble by a factor of two. Stimulated feedback, where the radio bubbles lift low entropy X-ray gas that becomes thermally unstable and rapidly cools in situ, provides a plausible model. Multiple generations of radio bubbles are required to lift this substantial gas mass. The close morphological association then indicates that the cold gas either moulds the newly expanding bubbles or is itself pushed aside and shaped as they inflate.

astro-ph.GA↗

arXiv2017

The Onset of Thermally Unstable Cooling from the Hot Atmospheres of Giant Galaxies in Clusters - Constraints on Feedback Models

We present accurate mass and thermodynamic profiles for a sample of 56 galaxy clusters observed with the Chandra X-ray Observatory. We investigate the effects of local gravitational acceleration in central cluster galaxies, and we explore the role of the local free-fall time (t$_{\rm ff}$) in thermally unstable cooling. We find that the local cooling time (t$_{\rm cool}$) is as effective an indicator of cold gas, traced through its nebular emission, as the ratio of t$_{\rm cool}$/t$_{\rm ff}$. Therefore, t$_{\rm cool}$ alone apparently governs the onset of thermally unstable cooling in hot atmospheres. The location of the minimum t$_{\rm cool}$/t$_{\rm ff}$, a thermodynamic parameter that simulations suggest may be key in driving thermal instability, is unresolved in most systems. As a consequence, selection effects bias the value and reduce the observed range in measured t$_{\rm cool}$/t$_{\rm ff}$ minima. The entropy profiles of cool-core clusters are characterized by broken power-laws down to our resolution limit, with no indication of isentropic cores. We show, for the first time, that mass isothermality and the $K \propto r^{2/3}$ entropy profile slope imply a floor in t$_{\rm cool}$/t$_{\rm ff}$ profiles within central galaxies. No significant departures of t$_{\rm cool}$/t$_{\rm ff}$ below 10 are found, which is inconsistent with many recent feedback models. The inner densities and cooling times of cluster atmospheres are resilient to change in response to powerful AGN activity, suggesting that the energy coupling between AGN heating and atmospheric gas is gentler than most models predict.

astro-ph.GA↗

arXiv2017

Star formation in a galactic outflow

Recent observations have revealed massive galactic molecular outflows that may have physical conditions (high gas densities) required to form stars. Indeed, several recent models predict that such massive galactic 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 in-situ chemical enrichment of the circumgalactic and intergalactic medium (through supernova explosions of young stars on large orbits), and some models also predict that it may contribute substantially to the global 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 new 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 Msun/yr. 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.

astro-ph.GA↗

arXiv2016

ALMA observations of massive molecular gas filaments encasing radio bubbles in the Phoenix cluster

We report new ALMA observations of the CO(3-2) line emission from the $2.1\pm0.3\times10^{10}\rm\thinspace M_{\odot}$ molecular gas reservoir in the central galaxy of the Phoenix cluster. The cold molecular gas is fuelling a vigorous starburst at a rate of $500-800\rm\thinspace M_{\odot}\rm\; yr^{-1}$ and powerful black hole activity in the form of both intense quasar radiation and radio jets. The radio jets have inflated huge bubbles filled with relativistic plasma into the hot, X-ray atmospheres surrounding the host galaxy. The ALMA observations show that extended filaments of molecular gas, each $10-20\rm\; kpc$ long with a mass of several billion solar masses, are located along the peripheries of the radio bubbles. The smooth velocity gradients and narrow line widths along each filament reveal massive, ordered molecular gas flows around each bubble, which are inconsistent with gravitational free-fall. The molecular clouds have been lifted directly by the radio bubbles, or formed via thermal instabilities induced in low entropy gas lifted in the updraft of the bubbles. These new data provide compelling evidence for close coupling between the radio bubbles and the cold gas, which is essential to explain the self-regulation of feedback. The very feedback mechanism that heats hot atmospheres and suppresses star formation may also paradoxically stimulate production of the cold gas required to sustain feedback in massive galaxies.

astro-ph.GA↗