# Posts Tagged cluster core

## Recent Postings from cluster core

### Large scale gas sloshing out to half the virial radius in the strongest cool core REXCESS galaxy cluster, RXJ2014.8-2430

We search the cool core galaxy clusters in the REXCESS sample for evidence of large scale gas sloshing, and find clear evidence for sloshing in RXJ2014.8-2430, the strongest cool core cluster in the REXCESS cluster sample. The residuals of the surface brightness distribution from the azimuthal average for RXJ2014 show a prominent swirling excess feature extending out to an abrupt surface brightness discontinuity at 800 kpc from the cluster core (half the virial radius) to the south, which the XMM-Newton observations confirm to be cold, low entropy gas. The gas temperature is significantly higher outside this southern surface brightness discontinuity, indicating that this is a cold front 800 kpc from the cluster core. Chandra observations of the central 200 kpc show two clear younger cold fronts on opposite sides of the cluster. The scenario appears qualitatively consistent with simulations of gas sloshing due to minor mergers which raise cold, low entropy gas from the core to higher radius, resulting in a swirling distribution of opposing cold fronts at increasing radii. However the scale of the observed sloshing is much larger than that which has been simulated at present, and is similar to the large scale sloshing recently observed in the Perseus cluster and Abell 2142.

### Galaxy properties in clusters. II. Backsplash Galaxies

We explore the properties of galaxies on the outskirts of clusters and their dependence on recent dynamical history in order to understand the real impact that the cluster core has on the evolution of galaxies. We analyse the properties of more than 1000 galaxies brighter than $M_{^{0.1}r}$=-19.6 on the outskirts of 90 clusters ($1<r/r_{vir}<2$) in the redshift range $0.05<z<0.10$. Using the line of sight velocity, we selected high and low velocity subsamples. Theoretical predictions indicate that a significant fraction of the first subsample should be backsplash galaxies, that is, objects that have already orbited near the cluster centre. A significant proportion of the sample of high relative velocity HV galaxies seems to be composed of infalling objects. Our results suggest that, at fixed stellar mass, late type galaxies in the low velocity LV sample are systematically older, redder and have formed fewer stars during the last 3 Gyrs than galaxies in the HV sample. This result is consistent with models that assume that the central regions of clusters are effective in quenching the star formation by means of processes such as ram pressure stripping or strangulation. At fixed stellar mass, LV galaxies show some evidence of having higher surface brightness and smaller size than HV galaxies. These results are consistent with the scenario where galaxies that have orbited the central regions of clusters are more likely to suffer tidal effects, producing loss of mass as well as a redistribution of matter towards more compact configurations. Finally, we found a higher fraction of ET galaxies in the LV sample, supporting the idea that the central region of clusters of galaxies may contribute to the transformation of morphological types towards earlier types.

### Photometric studies of Abell 1664: The subtle effect a minor merger has on cluster galaxies

A combination of $BRI$ photometry and archival Chandra X-ray data have been used to analyse the effects a minor merger has on the galaxy population of A1664. We utilise adaptive smoothing techniques in the 2D spatial distribution of cluster galaxies to reveal substructure $\sim$ 800 kpc South of the cluster core. We identify this substructure as most likely the remnant core of a merging group which has passed pericentre and responsible for triggering a cold front in the cluster core. We define two samples to represent two different environments within A1664 in accordance with the location of the substructure. We apply a morphological analysis using CAS, M$_{20}$ and Gini to these samples to deduce if there has been any significant effect on the cluster galaxies due to this interaction. We find there are more asymmetric galaxies found in the inner sample (at the 3.7$\sigma$ level) which is likely due to galaxy-galaxy interactions as the merging group passed through core passage. No other differences were found between the inner and outer cluster in our morphological analysis, which we attribute to the limited resolution of our imagery. The colour profiles of the galaxies are found to be consistent with the morphology-density relation suggesting there is no unique environmental effect in A1664 that has enhanced galaxy transformations. This study favours the star formation of cluster galaxies being quenched well before it is able to interact with the merging group and demonstrates that a minor cluster merger has little effect on the observable parameters of cluster galaxies such as morphology and colour.

### Diffuse optical intracluster light as a measure of stellar tidal stripping: the cluster CL0024+17 at $z\sim$0.4 observed at LBT

We have evaluated the diffuse intracluster light (ICL) in the central core of the galaxy cluster CL0024+17 at $z\sim 0.4$ observed with the prime focus camera (LBC) at LBT. The measure required an accurate removal of the galaxies light within $\sim 200$ kpc from the center. The residual background intensity has then been integrated in circular apertures to derive the average ICL intensity profile. The latter shows an approximate exponential decline as expected from theoretical cold dark matter models. The radial profile of the ICL over the galaxies intensity ratio (ICL fraction) is increasing with decreasing radius but near the cluster center it starts to bend and then decreases where the overlap of the halos of the brightest cluster galaxies becomes dominant. Theoretical expectations in a simplified CDM scenario show that the ICL fraction profile can be estimated from the stripped over galaxy stellar mass ratio in the cluster. It is possible to show that the latter quantity is almost independent of the properties of the individual host galaxies but mainly depends on the average cluster properties. The predicted ICL fraction profile is thus very sensitive to the assumed CDM profile, total mass and concentration parameter of the cluster. Adopting values very similar to those derived from the most recent lensing analysis in CL0024+17 we find a good agreement with the observed ICL fraction profile. The galaxy counts in the cluster core have then been compared with that derived from composite cluster samples in larger volumes, up to the clusters virial radius. The galaxy counts in the CL0024+17 core appear flatter and the amount of bending respect to the average cluster galaxy counts imply a loss of total emissivity in broad agreement with the measured ICL fraction.

### Diffuse optical intracluster light as a measure of stellar tidal stripping: the cluster CL0024+17 at $z\sim$0.4 observed at LBT [Replacement]

We have evaluated the diffuse intracluster light (ICL) in the central core of the galaxy cluster CL0024+17 at $z\sim 0.4$ observed with the prime focus camera (LBC) at LBT. The measure required an accurate removal of the galaxies light within $\sim 200$ kpc from the center. The residual background intensity has then been integrated in circular apertures to derive the average ICL intensity profile. The latter shows an approximate exponential decline as expected from theoretical cold dark matter models. The radial profile of the ICL over the galaxies intensity ratio (ICL fraction) is increasing with decreasing radius but near the cluster center it starts to bend and then decreases where the overlap of the halos of the brightest cluster galaxies becomes dominant. Theoretical expectations in a simplified CDM scenario show that the ICL fraction profile can be estimated from the stripped over galaxy stellar mass ratio in the cluster. It is possible to show that the latter quantity is almost independent of the properties of the individual host galaxies but mainly depends on the average cluster properties. The predicted ICL fraction profile is thus very sensitive to the assumed CDM profile, total mass and concentration parameter of the cluster. Adopting values very similar to those derived from the most recent lensing analysis in CL0024+17 we find a good agreement with the observed ICL fraction profile. The galaxy counts in the cluster core have then been compared with that derived from composite cluster samples in larger volumes, up to the clusters virial radius. The galaxy counts in the CL0024+17 core appear flatter and the amount of bending respect to the average cluster galaxy counts imply a loss of total emissivity in broad agreement with the measured ICL fraction.

### Linear Structures in the Core of the Coma Cluster of Galaxies

The hot X-ray emitting plasma in galaxy clusters is predicted to have turbulent motions which can contribute around ten percent of the cluster’s central energy density. We report deep Chandra X-ray Observatory observations of the Coma cluster core, showing the presence of quasi-linear high-density arms spanning 150 kpc, consisting of low-entropy material likely stripped from merging subclusters. Two appear to be connected with a subgroup of galaxies at 650 kpc radius that is merging into the cluster, implying coherence over several hundred Myr. Such long lifetime implies that strong isotropic turbulence and conduction are suppressed in the core, despite the unrelaxed state of the cluster. Magnetic fields are presumably responsible. The structures seen in Coma present insight into the past Gyr of subcluster merger activity.

### UB CCD photometry of the old, metal rich, open clusters NGC 6791, NGC 6819 and NGC 7142

We report on a UV-oriented imaging survey in the fields of the old, metal-rich open clusters, NGC 6791, NGC 6819 and NGC 7142. These three clusters represent both very near and ideal stellar aggregates to match the distinctive properties of the evolved stellar populations, as in elliptical galaxies and bulges of spirals. The CMD of the three clusters is analyzed in detail, with special emphasis to the hot stellar component. We report, in this regard, one new extreme horizontal-branch star candidate in NGC 6791. For NGC 6819 and 7142, the stellar luminosity function points to a looser radial distribution of faint lower Main Sequence stars, either as a consequence of cluster dynamical interaction with the Galaxy or as an effect of an increasing fraction of binary stars toward the cluster core, as actually observed in NGC 6791 too.

### Suzaku observations of the type 2 QSO in the central galaxy of the Phoenix cluster

We report the \Suzaku/XIS and HXD and \Chandra/ACIS-I results on the X-ray spectra of the Phoenix cluster at the redshift $z=0.596$. The spectrum of the intracluster medium (ICM) is well-reproduced with the emissions from a low temperature ($\sim3.0$\,keV and $\sim0.76$\,solar) and a high temperature ($\sim11$\,keV and $\sim0.33$\,solar) plasmas; the former is localized at the cluster core, while the latter distributes over the cluster. In addition to these ICM emissions, a strongly absorbed power-law component is found, which is due to an active galactic nucleus (AGN) in the cluster center. The absorption column density and unobscured luminosity of the AGN are $\sim3.2\times10^{23}$\,cm$^{-2}$ and $\sim4.7\times10^{45}$\,ergs\,s$^{-1}$ (2-10\,keV), respectively. Furthermore, a neutral iron (\ion{Fe}{1}) K-shell line is discovered for the first time with the equivalent width (EW) of $\sim150$\,eV at the rest frame. The column density and the EW of the \ion{Fe}{1} line are exceptionally large for such a high luminosity AGN, and hence the AGN is classified as a type 2 quasi-stellar object (QSO). We speculate that the significant fraction of the ICM cooled gas would be consumed to maintain the torus and to activate the type 2 QSO. The Phoenix cluster has a massive starburst in the central galaxy, indicating suppression in the cooling flow is less effective. This may be because the onset of the latest AGN feedback has occurred recently and it has not yet been effective. Alternatively, the AGN feedback is predominantly in radiative-mode not in kinetic-mode and the torus may work as a shield to reduce its effect.

### Understanding the Unusual X-Ray Emission Properties of the Massive, Close Binary WR 20a: A High Energy Window into the Stellar Winds Initiation Region

The problem of explaining the X-ray emission properties of the massive, close binary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR 20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solar masses orbiting with a period of only 3.7 days. Although Chandra observations were taken during the secondary optical eclipse, the X-ray light curve shows no signs of a flux decrement. In fact, WR 20a appears slightly more X-ray luminous and softer during the optical eclipse, opposite to what has been observed in other binary systems. To aid in our interpretation of the data, we compare with the results of hydrodynamical simulations using the adaptive mesh refinement code Mezcal that includes radiative cooling and a radiative acceleration force term. It is shown that the X-ray emission can be successfully explained in models where the wind-wind collision interface in this system occurs while the outflowing material is still being accelerated. Consequently, WR 20a serves as a critical test-case for how radiatively-driven stellar winds initiate and interact. Our models not only procure a robust description of current Chandra data, which cover the orbital phases between 0.3 to 0.6, but provide detailed predictions over the entire orbit.

### Understanding the Unusual X-Ray Emission Properties of the Massive, Close Binary WR 20a: A High Energy Window into the Stellar Wind Initiation Region [Replacement]

The problem of explaining the X-ray emission properties of the massive, close binary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR 20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solar masses orbiting with a period of only 3.7 days. Although Chandra observations were taken during the secondary optical eclipse, the X-ray light curve shows no signs of a flux decrement. In fact, WR 20a appears slightly more X-ray luminous and softer during the optical eclipse, opposite to what has been observed in other binary systems. To aid in our interpretation of the data, we compare with the results of hydrodynamical simulations using the adaptive mesh refinement code Mezcal that includes radiative cooling and a radiative acceleration force term. It is shown that the X-ray emission can be successfully explained in models where the wind-wind collision interface in this system occurs while the outflowing material is still being accelerated. Consequently, WR 20a serves as a critical test-case for how radiatively-driven stellar winds initiate and interact. Our models not only procure a robust description of current Chandra data, which cover the orbital phases between 0.3 to 0.6, but provide detailed predictions over the entire orbit.

### A Tidal Flare Candidate in Abell 1795

As part of our ongoing archival X-ray survey of galaxy clusters for tidal flares, we present evidence of an X-ray transient source within 1 arcmin of the core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity (> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal disruption event according to theoretical predictions and to existing X-ray observations, implying a massive >~10^5 M_sun black hole at the centre of that galaxy. The large number of X-ray source counts (~700) and long temporal baseline (~12 years with Chandra and XMM-Newton) make this one of the best-sampled examples of any tidal flare candidate to date. The transient may be the same EUV source originally found contaminating the diffuse ICM observations of Bowyer et al. (1999), which would make it the only tidal flare candidate with reported EUV observations and implies an early source luminosity 1-2 orders of magnitude greater. If the host galaxy is a cluster member then it must be a dwarf galaxy, an order of magnitude less massive than the quiescent galaxy Henize 2-10 which hosts a massive black hole that is difficult to reconcile with its low mass. The unusual faintness of the host galaxy may be explained by tidal stripping in the cluster core.

### Chandra View of the Ultra-Steep Spectrum Radio Source in Abell 2443: Merger Shock-Induced Compression of Fossil Radio Plasma?

We present a new Chandra X-ray observation of the intracluster medium in the galaxy cluster Abell 2443, hosting an ultra-steep spectrum radio source. The data reveal that the intracluster medium is highly disturbed. The thermal gas in the core is elongated along a northwest to southeast axis and there is a cool tail to the north. We also detect two X-ray surface brightness edges near the cluster core. The edges appear to be consistent with an inner cold front to the northeast of the core and an outer shock front to the southeast of the core. The southeastern edge is coincident with the location of the radio relic as expected for shock (re)acceleration or adiabatic compression of fossil relativistic electrons.

### Searching for the missing iron in the core of the Centaurus cluster

We re-analyse a combined 198 ks Chandra observation of NGC4696, the brightest galaxy of the Centaurus cluster. We extract temperature and metallicity profiles from the data, and we confirm the presence of a sharp drop in iron abundance, from ~1.8 Zsolar to ~0.4 Zsolar, within the central 5 kpc of the cluster. We estimate that this abundance drop corresponds to a total "missing" iron mass of 1.4e06 Msolar. We propose that part of this missing iron is locked up in cool (~19 K) far-IR emitting dust, as found by Spitzer and Herschel observations. This can occur if the iron injected by stellar mass loss in the central region is in grains, which remain in that form as the injected dusty cold gas mixes and joins the cold dusty filamentary nebula observed within the same region. The bubbling feedback process observed in the cluster core then drags filaments outward and dumps them at 10-20 kpc radius, where the metallicity is high.

### Searching for the missing iron in the core of the Centaurus cluster [Replacement]

We re-analyse a combined 198 ks Chandra observation of NGC4696, the brightest galaxy of the Centaurus cluster. We extract temperature and metallicity profiles from the data, and we confirm the presence of a sharp drop in iron abundance, from ~1.8 Zsolar to ~0.4 Zsolar, within the central 5 kpc of the cluster. We estimate that this abundance drop corresponds to a total "missing" iron mass of 1.4e06 Msolar. We propose that part of this missing iron is locked up in cool (~19 K) far-IR emitting dust, as found by Spitzer and Herschel observations. This can occur if the iron injected by stellar mass loss in the central region is in grains, which remain in that form as the injected dusty cold gas mixes and joins the cold dusty filamentary nebula observed within the same region. The bubbling feedback process observed in the cluster core then drags filaments outward and dumps them at 10-20 kpc radius, where the metallicity is high.

### Searching for the missing iron in the core of the Centaurus cluster [Replacement]

We re-analyse a combined 198 ks Chandra observation of NGC4696, the brightest galaxy of the Centaurus cluster. We extract temperature and metallicity profiles from the data, and we confirm the presence of a sharp drop in iron abundance, from ~1.8 Zsolar to ~0.4 Zsolar, within the central 5 kpc of the cluster. We estimate that this abundance drop corresponds to a total "missing" iron mass of 1.4e06 Msolar. We propose that part of this missing iron is locked up in cool (~19 K) far-IR emitting dust, as found by Spitzer and Herschel observations. This can occur if the iron injected by stellar mass loss in the central region is in grains, which remain in that form as the injected dusty cold gas mixes and joins the cold dusty filamentary nebula observed within the same region. The bubbling feedback process observed in the cluster core then drags filaments outward and dumps them at 10-20 kpc radius, where the metallicity is high.

### The galaxy population of the complex cluster system Abell 3921 [Replacement]

We present a spectrophotometric analysis of the galaxy pop. in the area of the merging cluster Abell 3921 at redshift 0.093. We investigate the impact of the complex cluster environment on galaxy properties such as morphology or star formation rate. We combine multi-object spectroscopy from the 2dF spectrograph with optical imaging taken with the ESO WFI. We carry out a redshift analysis and determine cluster velocity dispersions using biweight statistics. Applying a Dressler-Shectman (DS-)test we seek evidence for cluster substructure. Cluster and field galaxies are investigated with respect to [OII] and H{\alpha} equivalent width, SFR and morphological descriptors such as concentration index and Gini coefficient. We study these cluster galaxy properties as a function of clustercentric distance and investigate the spatial distribution of various galaxy types. Applying the DS-test we find a 3rd component (A3921-C) in addition to the two main subclusters (A3921-A and A3921-B) already known. The re-determined mass ratio between the main components A and B is approx. 2:1. Similar to prev. studies of galaxy clusters, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. These are likely systems that are quenched due to ram pressure stripping. We also find quenched spirals at rather large distances of 3 to 4 Mpc from the cluster core. A3921-C might be a group of galaxies falling onto the main cluster components. We speculate that the unexpected population of quenched spirals at large clustercentric radii in A3921-A and A3921-B might be an effect of the ongoing cluster merger: shocks in the ICM might give raise to enhanced ram pressure stripping and at least in part be the cause for the quenching of star formation. These quenched spirals might be an interm. stage in the morphological transformation of field spirals into cluster S0s.

### The galaxy population of the complex cluster system Abell 3921

We present a spectrophotometric analysis of the galaxy pop. in the area of the merging cluster Abell 3921 at redshift 0.093. We investigate the impact of the complex cluster environment on galaxy properties such as morphology or star formation rate. We combine multi-object spectroscopy from the 2dF spectrograph with optical imaging taken with the ESO WFI. We carry out a redshift analysis and determine cluster velocity dispersions using biweight statistics. Applying a Dressler-Shectman (DS-)test we seek evidence for cluster substructure. Cluster and field galaxies are investigated with respect to [OII] and H{\alpha} equivalent width, SFR and morphological descriptors such as concentration index and Gini coefficient. We study these cluster galaxy properties as a function of clustercentric distance and investigate the spatial distribution of various galaxy types. Applying the DS-test we find a 3rd component (A3921-C) in addition to the two main subclusters (A3921-A and A3921-B) already known. The re-determined mass ratio between the main components A and B is approx. 2:1. Similar to prev. studies of galaxy clusters, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. These are likely systems that are quenched due to ram pressure stripping. We also find quenched spirals at rather large distances of 3 to 4 Mpc from the cluster core. A3921-C might be a group of galaxies falling onto the main cluster components. We speculate that the unexpected population of quenched spirals at large clustercentric radii in A3921-A and A3921-B might be an effect of the ongoing cluster merger: shocks in the ICM might give raise to enhanced ram pressure stripping and at least in part be the cause for the quenching of star formation. These quenched spirals might be an interm. stage in the morphological transformation of field spirals into cluster S0s.

### The galaxy population of the complex cluster system Abell 3921 [Replacement]

We present a spectrophotometric analysis of the galaxy pop. in the area of the merging cluster Abell 3921 at redshift 0.093. We investigate the impact of the complex cluster environment on galaxy properties such as morphology or star formation rate. We combine multi-object spectroscopy from the 2dF spectrograph with optical imaging taken with the ESO WFI. We carry out a redshift analysis and determine cluster velocity dispersions using biweight statistics. Applying a Dressler-Shectman (DS-)test we seek evidence for cluster substructure. Cluster and field galaxies are investigated with respect to [OII] and H{\alpha} equivalent width, SFR and morphological descriptors such as concentration index and Gini coefficient. We study these cluster galaxy properties as a function of clustercentric distance and investigate the spatial distribution of various galaxy types. Applying the DS-test we find a 3rd component (A3921-C) in addition to the two main subclusters (A3921-A and A3921-B) already known. The re-determined mass ratio between the main components A and B is approx. 2:1. Similar to prev. studies of galaxy clusters, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. These are likely systems that are quenched due to ram pressure stripping. We also find quenched spirals at rather large distances of 3 to 4 Mpc from the cluster core. A3921-C might be a group of galaxies falling onto the main cluster components. We speculate that the unexpected population of quenched spirals at large clustercentric radii in A3921-A and A3921-B might be an effect of the ongoing cluster merger: shocks in the ICM might give raise to enhanced ram pressure stripping and at least in part be the cause for the quenching of star formation. These quenched spirals might be an interm. stage in the morphological transformation of field spirals into cluster S0s.

### The distance to the young open cluster Westerlund 2

A new X-ray, {\it UBVR}$I_c$, and {\it JHK$s$} study of the young cluster Westerlund 2 was undertaken to resolve discrepancies tied to the cluster’s distance. Existing spectroscopic observations for bright cluster members and new multi-band photometry imply a reddening relation towards Westerlund~2 described by $E_{U-B}/E_{B-V}=0.63 + 0.02\;E_{B-V}$. Variable-extinction analyses for Westerlund~2 and nearby IC 2581 based upon spectroscopic distance moduli and ZAMS fitting yield values of $R_V=A_V/E_{B-V}=3.88\pm0.18$ and $3.77\pm0.19$, respectively, and confirm prior assertions that anomalous interstellar extinction is widespread throughout Carina (e.g., Turner 2012). The results were confirmed by applying the color difference method to {\it UBVR$I_c$JH$K_s$} data for 19 spectroscopically-observed cluster members, yielding $R_V=3.85\pm0.07$. The derived distance to Westerlund~2 of $d=2.85\pm0.43$ kpc places the cluster on the far side of the Carina spiral arm. The cluster’s age is no more than $\tau \sim2\times10^6$ yr as inferred from the cluster’s brightest stars and an X-ray (Chandra) cleaned analysis of its pre-main-sequence demographic. Four Wolf-Rayet stars in the cluster core and surrounding corona (WR20a, WR20b, WR20c, and WR20aa) are likely cluster members, and their inferred luminosities are consistent with those of other late-WN stars in open clusters. The color-magnitude diagram for Westerlund~2 also displays a gap at spectral type B0.5 V with associated color spread at higher and lower absolute magnitudes that might be linked to close binary mergers. Such features, in conjunction with the evidence for mass loss from the WR stars, may help to explain the high flux of $\gamma$ rays, cosmic rays, and X-rays from the direction towards Westerlund~2.

### Cold fronts and metal anisotropies in the X-ray cool core of the galaxy cluster Zw1742+3306

(Context) In recent years, our understanding of the cool cores of galaxy clusters from a relatively simple place where gas is cooling and flowing toward the center has changed to a very dynamic place, where heating from the central AGN and cooling, as inferred from active star formation, molecular gas and Halpha nebulosity, find an uneasy energetic balance. (Aims) We want to characterize the X-ray properties of the nearby cool-core cluster Zw1742+3306, selected for being bright at X-ray (with a flux greater than 1e-11 erg/s/cm2 in the 0.1-2.4 keV band) and Halpha wavelength (Halpha lumininosity > 1e40 erg/s). (Methods) We use Chandra data to analyze the spatial and spectral properties of the cool core of Zw1742+3306, a galaxy cluster at z=0.0757 which emits in Halpha and presents the Brightest Central Galaxy located in a diffuse X-ray emission with multiple peaks in surface brightness. (Results) We show that the X-ray cool core of the galaxy cluster Zw1742+3306 is thermodynamically very active with evidence of cold fronts and a weak shock in the surface brightness map and of an apparently coherent, elongated structure with metallicity larger by about 50 per cent than the value measured in the surrounding ambient gas. This anisotropic structure has size 280 x 90 kpc2 and is aligned with the cold fronts and with the X-ray emission on larger scales. We suggest that all these peculiarities in the X-ray emission of Zw1742+3306 are either a very fine-tuned output of a sloshing gas in the cluster core or the product of a metal-rich outflow from the central AGN.

### Cold fronts and metal anisotropies in the X-ray cool core of the galaxy cluster Zw1742+3306 [Replacement]

(Context) In recent years, our understanding of the cool cores of galaxy clusters has changed. Once thought to be relatively simple places where gas cools and flows toward the centre, now they are believed to be very dynamic places where heating from the central Active Galactic Nucleus (AGN) and cooling, as inferred from active star formation, molecular gas, and Halpha nebulosity, find an uneasy energetic balance. (Aims) We want to characterize the X-ray properties of the nearby cool-core cluster Zw1742+3306, selected because it is bright at X-ray (with a flux greater than 1e-11 erg/s/cm2 in the 0.1-2.4 keV band) and Halpha wavelengths (Halpha luminosity > 1e40 erg/s). (Methods) We used Chandra data to analyze the spatial and spectral properties of the cool core of Zw1742+3306, a galaxy cluster at z=0.0757 that emits in Halpha and presents the brightest central galaxy located in a diffuse X-ray emission with multiple peaks in surface brightness. (Results) We show that the X-ray cool core of the galaxy cluster Zw1742+3306 is thermodynamically very active with evidence of cold fronts and a weak shock in the surface brightness map and of an apparently coherent, elongated structure with metallicity greater than the value measured in the surrounding ambient gas by about 50 per cent. This anisotropic structure is 280 x 90 kpc2 and is aligned with the cold fronts and with the X-ray emission on larger scales. We suggest that all these peculiarities in the X-ray emission of Zw1742+3306 are either a very fine-tuned output of a sloshing gas in the cluster core or the product of a metal-rich outflow from the central AGN.

### Galaxy evolution in overdense environments at high redshift: passive early-type galaxies in a cluster at redshift 2

We present a study of galaxy populations in the central region of the IRAC-selected, X-ray detected galaxy cluster Cl J1449+0856 at z=2. Based on a sample of spectroscopic and photometric cluster members, we investigate stellar populations and morphological structure of cluster galaxies over an area of ~0.7Mpc^2 around the cluster core. The cluster stands out as a clear overdensity both in redshift space, and in the spatial distribution of galaxies close to the center of the extended X-ray emission. The cluster core region (r<200 kpc) shows a clearly enhanced passive fraction with respect to field levels. However, together with a population of massive passive galaxies mostly with early-type morphologies, it also hosts massive actively star-forming, often highly dust-reddened sources. Close to the cluster center, a multi-component system of passive and star-forming galaxies could be the future BCG still assembling. We observe a clear correlation between passive stellar populations and an early-type morphology, in agreement with field studies at similar redshift. Passive early-type galaxies in this clusters are typically a factor 2-3 smaller than similarly massive early-types at z~0, but also on average larger by a factor ~2 than their field analogs at z~2, lending support to recent claims of an accelerated structural evolution in high-redshift dense environments. These results point towards the early formation of a population of massive galaxies, already evolved both in their structure and stellar populations, coexisting with still-actively forming massive galaxies in the central regions of young clusters 10 billion years ago.

### WFC3 grism confirmation of the distant cluster Cl J1449+0856 at z=2.00: Quiescent and star-forming galaxy populations

We present deep HST/WFC3 slitless spectroscopic observations of the distant cluster Cl J1449+0856. These cover a single pointing with 18 orbits of G141 spectroscopy and F140W imaging, allowing us to derive secure redshifts down to m_140~25.5 AB and 3sigma line fluxes of 5*10^(-18) erg/s/cm^2. In particular, we were able to spectroscopically confirm ten early-type galaxies in the field up to z~3, five of which in the cluster core, which represents the first direct spectroscopic confirmation of passive galaxies in a z=2 cluster environment. With 140 redshifts in a ~6 arcmin^2 field, we can trace the spatial and redshift galaxy distribution in the cluster core and background field. We find two strong peaks at z=2.00 and z=2.07, where only one was seen in our previously published ground-based data. Thanks to the spectroscopic confirmation of the cluster ETGs, we can now re-evaluate the redshift of Cl J1449+0856 at z=2.00, rather than z=2.07, with the background overdensity being revealed to be sparse and "sheet"-like. This presents an interesting case of chance alignment of two close yet unrelated structures, each one preferentially selected by different observing strategies. With 7 passive or early-type spectroscopic members and 20 star-forming ones, Cl J1449+0856 is now reliably confirmed to be at z=2.00. The identified members can now allow for a detailed study of galaxy properties in the densest environment at z=2.

### WFC3 grism confirmation of the distant cluster Cl J1449+0856 at z=2.00: Quiescent and star-forming galaxy populations [Replacement]

We present deep Hubble Space Telescope Wide Field Camera 3 slitless spectroscopic observations of the distant cluster Cl J1449+0856. These cover a single pointing with 18 orbits of G141 spectroscopy and F140W imaging, allowing us to derive secure redshifts down to m_140~25.5 AB and 3sigma line fluxes of 5*10^(-18) erg/s/cm^2. In particular, we were able to spectroscopically confirm 12 early-type galaxies in the field up to z~3, 6 of which in the cluster core, which represents the first direct spectroscopic confirmation of passive galaxies in a z=2 cluster environment. With 140 redshifts in a ~6 arcmin^2 field, we can trace the spatial and redshift galaxy distribution in the cluster core and background field. We find two strong peaks at z=2.00 and z=2.07, where only one was seen in our previously published ground-based data. Thanks to the spectroscopic confirmation of the cluster ETGs, we can now re-evaluate the redshift of Cl J1449+0856 at z=2.00, rather than z=2.07, with the background overdensity being revealed to be sparse and "sheet"-like. This presents an interesting case of chance alignment of two close yet unrelated structures, each one preferentially selected by different observing strategies. With 6 quiescent or early-type spectroscopic members and 20 star-forming ones, Cl J1449+0856 is now reliably confirmed to be at z=2.00. The identified members can now allow for a detailed study of galaxy properties in the densest environment at z=2.

### Ultra-Compact Dwarfs around NGC 3258 in Antlia

We present the first compact stellar systems with luminosities in the range of ultra-compact dwarfs (UCDs), discovered in the Antlia galaxy cluster (-10.5 < M_V < -11.6). The magnitude limit between UCDs and globular clusters (CGs) is discussed. By means of imaging from VLT (FORS1), CTIO (MOSAIC), and the HST (ACS) archive, eleven UCDs/bright GCs are selected on the basis of photometry and confirmed as Antlia members through radial velocities measured on new GEMINI (GMOS-S) spectra. In addition, nine UCD candidates are identified taking into account properties derived from their surface brightness profiles. All of them, members and candidates, are located in the proximity of NGC\,3258, one of the two brightest elliptical galaxies in the cluster core. Antlia UCDs in this sample present absolute magnitudes fainter than M_V ~ -11.6 mag and most of them have colours within the blue GC range, falling only two within the red GC range. Effective radii measured for the ones lying on the ACS field are in the range R_eff = 3 – 11 pc and are similar to equivalent objects in other clusters, obtained from the literature. The UCD sample shares the same behaviour on the size-luminosity plane: a linear relation between R_eff and M_V is present for UCDs brighter than M_V ~ -10.5 – -11 mag while no trend is detected for fainter ones, that have an approximately constant R_eff. The projected spatial distribution of UCDs, GCs and X-ray emission points to an ongoing merger between two Antlia groups, dominated by NGC 3258 and NGC 3268. Nuclei of dwarf elliptical galaxies and blue UCDs share the same locus on the colour-magnitude diagram, supporting the hypothesis that some blue UCDs may be remnants of stripped nucleated dwarfs.

### R144 revealed as a double-lined spectroscopic binary

R144 is a WN6h star in the 30 Doradus region. It is suspected to be a binary because of its high luminosity and its strong X-ray flux, but no periodicity could be established so far. Here, we present new Xshooter multi-epoch spectroscopy of R144 obtained at the ESO Very Large Telescope (VLT). We detect variability in position and/or shape of all the spectral lines. We measure radial velocity variations with an amplitude larger than 250 km/s in NIV and NV lines. Furthermore, the NIII and NV line Doppler shifts are anti-correlated and the NIV lines show a double-peaked profile on six of our seven epochs. We thus conclude that R144 is a double-lined spectroscopic binary. Possible orbital periods range from 2 to 6 months, although a period up to one year is allowed if the orbit is highly eccentric. We estimate the spectral types of the components to be WN5-6h and WN6-7h, respectively. The high luminosity of the system (log Lbol/Lsun ~ 6.8) suggests a present-day total mass content in the range of about 200 to 300 Msun, depending on the evolutionary stage of the components. This makes R144 the most massive binary identified so far, with a total mass content at birth possibly as large as 400 Msun. We briefly discuss the presence of such a massive object 60 pc away from the R136 cluster core in the context of star formation and stellar dynamics.

### The moment of core collapse in star clusters with a mass function

Star clusters with multi-mass components dynamically evolve faster than those modelled with equal-mass components. Using a series of direct $N$-body simulations, we investigate the dynamical evolution of star clusters with mass functions, especially their core collapse time. Multi-mass clusters tend to behave like systems with a smaller number of particles, which we call the effective number of particles ($N_{\rm eff}$) and for which $N_{\rm eff} = M/m_{\rm max}$ (here $M$ and $m_{\rm max}$ are the total cluster mass and the mass of the most massive star in the cluster, respectively). We find that the time of core collapse is inversely proportional to the mass of the most massive star in the cluster and analytically confirm that this is because the core collapse of clusters with a mass function proceeds on the dynamical friction timescale of the most massive stars. As the mass of the most massive star increases, however, the core-collapse time, which is observed as a core bounce of the cluster core from the evolution of the core density or core radius, becomes ambiguous. We find that in that case the total binding energy of the hard binaries gives a good diagnosis for determining the moment at which the cluster core collapses. Based on the results of our simulations, we argue that the core bounce becomes ambiguous when the mass of the most massive star exceeds 0.1% of the total mass of the cluster.

### Mock Observations of Blue Stragglers in Globular Cluster Models [Replacement]

We created artificial color-magnitude diagrams of Monte Carlo dynamical models of globular clusters, and then used observational methods to determine the number of blue stragglers in those clusters. We compared these blue stragglers to various cluster properties, mimicking work that has been done for blue stragglers in Milky Way globular clusters to determine the dominant formation mechanism(s) of this unusual stellar population. We find that a mass-based prescription for selecting blue stragglers will choose approximately twice as many blue stragglers than a selection criterion that was developed for observations of real clusters. However, the two numbers of blue stragglers are well-correlated, so either selection criterion can be used to characterize the blue straggler population of a cluster. We confirm previous results that the simplified prescription for the evolution of a collision or merger product in the BSE code overestimates their lifetimes. We show that our model blue stragglers follow similar trends with cluster properties (core mass, binary fraction, total mass, collision rate) as the true Milky Way blue stragglers, as long as we restrict ourselves to model clusters with an initial binary fraction higher than 5%. We also show that, in contrast to earlier work, the number of blue stragglers in the cluster core does have a weak dependence on the collisional parameter Gamma in both our models and in Milky Way globular clusters.

### Reverse dynamical evolution of Eta Chamaeleontis

In the scope of the star formation process, it is unclear how the environment shapes the initial mass function (IMF). While observations of open clusters propose a universal picture for the IMF from the substellar domain up to a few solar masses, the young association eta Chamaeleontis presents an apparent lack of low mass objects (m<0.1 Msun). Another unusual feature of this cluster is the absence of wide binaries with a separation > 50 AU. We aim to test whether dynamical evolution alone can reproduce the peculiar properties of the association assuming a universal IMF. We use a pure N-body code to simulate the dynamical evolution of the cluster for 10 Myr, and compare the results with observations. A wide range of values for the initial parameters are tested in order to identify the initial state that would most likely lead to observations. In this context we also investigate the influence of the initial binary population on the dynamics and the possibility of having a discontinuous single IMF near the transition to the brown dwarf regime. We consider as an extreme case an IMF with no low mass systems (m<0.1 Msun). The initial configurations cover a wide range of initial density, from 10^2 to 10^8 stars/pc^3, in virialized, hot and cold dynamical state. We do not find any initial state that would evolve from a universal single IMF to fit the observations. Only when starting with a truncated IMF without any very low mass systems and no wide binaries, can we reproduce the cluster core properties with a success rate of 10% at best. Pure dynamical evolution alone cannot explain the observed properties of eta Cha from universal initial conditions. The lack of brown dwarfs and very low mass stars, and the peculiar binary properties (low binary fraction and lack of wide binaries), are probably the result of the star formation process in this association. (abridged)

### A Captured Runaway Black Hole in NGC 1277?

Recent results indicate that the compact lenticular galaxy NGC 1277 in the Perseus Cluster contains a black hole of approximately 10 billion solar masses. This far exceeds the expected mass of the central black hole in a galaxy of the modest dimensions of NGC 1277. We suggest that this giant black hole was ejected from the nearby giant galaxy NGC 1275 and subsequently captured by NGC 1277. The ejection was the result of gravitational radiation recoil when two large black holes merged following the merger of two giant ellipticals that helped to form NGC 1275. The black hole wandered in the cluster core until it was captured in a close encounter with NGC 1277. The migration of black holes in clusters may be a common occurrence.

### A Captured Runaway Black Hole in NGC 1277? [Replacement]

Recent results indicate that the compact lenticular galaxy NGC 1277 in the Perseus Cluster contains a black hole of approximately 10 billion solar masses. This far exceeds the expected mass of the central black hole in a galaxy of the modest dimensions of NGC 1277. We suggest that this giant black hole was ejected from the nearby giant galaxy NGC 1275 and subsequently captured by NGC 1277. The ejection was the result of gravitational radiation recoil when two large black holes merged following the merger of two giant ellipticals that helped to form NGC 1275. The black hole wandered in the cluster core until it was captured in a close encounter with NGC 1277. The migration of black holes in clusters may be a common occurrence.

### Thermodynamics of the Coma Cluster Outskirts

We present results from a large mosaic of Suzaku observations of the Coma Cluster, focusing on the thermodynamic properties of the ICM on large scales. The measured temperature and X-ray brightness profiles are similar along four relatively undisturbed azimuths probed, with the temperature decreasing from ~8.5 keV at the cluster center to ~2 keV at 2 Mpc. The SW merger boosts the surface brightness, allowing us to detect X-ray emission out to 2.5 Mpc along this fifth direction. The X-ray image also reveals two arc-shaped regions with excess surface brightness towards the east and west of the main cluster core. These regions appear over-pressured and most likely originate from merger induced large scale supersonic gas motions. The azimuthally averaged temperature profile, as well as the deprojected density and pressure profiles towards the E and NW, all show a sharp drop consistent with an outward propagating shock front located at the outermost edge of the giant radio halo observed at 352 MHz with the WSRT and which may be powering this radio emission. The shape of the entropy profiles along the relatively relaxed E and NW directions at large radii is consistent with the average profiles of evolved, well formed cool core clusters, suggesting similar accretion histories. Our data indicate a flat metal abundance profile at about 0.3 of the Solar value out to almost the virial radius of Coma, which would favor galactic winds over ram-pressure stripping as the dominant enrichment mechanism in the ICM.

### Luminosity profiles and sizes of massive star clusters in NGC 7252

We present Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) images of the merger remnant NGC 7252. In particular, we focus on the surface brightness profiles and effective radii, Reff, of 36 young massive clusters (YMCs) within the galaxy. All the clusters have masses exceeding 10^5 Msun and are, despite the 64 Mpc distance to the galaxy, (partly) resolved on the HST images. Effective radii can be measured down to ~2.5 pc, and the largest clusters have Reff approaching 20 pc. The median Reff of our sample clusters is ~6-7 pc, which is larger than typical radii of YMCs (~2.5 pc). This could be due to our sample selection (only selecting resolved sources) or to an intrinsic mass–radius relation within the cluster population. We find at least three clusters that have power-law profiles of the Elson, Fall, & Freeman (1987, "EFF") type extending out to >150 pc. Among them are the two most massive clusters, W3 and W30, which have profiles that extend to at least 500 and 250 pc, respectively. Despite their extended profiles, the effective radii of the three clusters are 17.2, 12.6 and 9.1 pc for W3, W26 and W30, respectively. We compare these extended profiles with those of YMCs in the LMC (R136 in 30 Dor), the Antennae galaxies (Knot S) and in the nearby spiral galaxy NGC 6946. Extended profiles seem to be a somewhat common feature, even though many nearby YMCs show distinct truncations. A continuous distribution between these two extremes, i.e. truncated or extremely extended, is the most likely interpretation. We suggest that the presence or absence of an extended envelope in {\em very young clusters} may be due to the gas distribution of the proto-cluster giant molecular cloud, in particular if the proto-cluster core becomes distinct from the surrounding gas before star formation begins.

### Discovery of X-ray emission from young suns in the Small Magellanic Cloud

We report the discovery of extended X-ray emission within the young star cluster NGC 602 in the Wing of the Small Magellanic Cloud (SMC) based on observations obtained with the Chandra X-ray Observatory. X-ray emission is detected from the cluster core area with the highest stellar density and from a dusty ridge surrounding the HII region. We use a census of massive stars in the cluster to demonstrate that a cluster wind or wind-blown bubble is unlikely to provide a significant contribution to the X-ray emission detected from the central area of the cluster. We therefore suggest that X-ray emission at the cluster core originates from an ensemble of low- and solar-mass pre-main-sequence (PMS) stars, each of which would be too weak in X-rays to be detected individually. We attribute the X-ray emission from the dusty ridge to the embedded tight cluster of the new-born stars known in this area from infrared studies. Assuming that the levels of X-ray activity in young stars in the low-metallicity environment of NGC 602a are comparable to their Galactic counterparts, then the detected spatial distribution, spectral properties, and level of X-ray emission are largely consistent with those expected from low- and solar-mass PMS stars and young stellar objects (YSOs). This is the first discovery of X-ray emission attributable to PMS stars and YSOs in the SMC, which suggests that the accretion and dynamo processes in young, low-mass objects in the SMC resemble those in the Galaxy.

### X-ray emission from the Ultramassive Black Hole candidate NGC1277: implications and speculation on its origin

We study the X-ray emission from NGC1277, a galaxy in the core of the Perseus cluster, for which van den Bosch et al. have recently claimed the presence of an UltraMassive Black Hole (UMBH) of mass 1.7 times 10^10 Msun, unless the IMF of the stars in the stellar bulge is extremely bottom heavy. The X-rays originate in a power-law component of luminosity 1.3 times 10^40 erg/s embedded in a 1 keV thermal minicorona which has a half-light radius of about 360 pc, typical of many early-type galaxies in rich clusters of galaxies. If Bondi accretion operated onto the UMBH from the minicorona with a radiative efficiency of 10 per cent, then the object would appear as a quasar with luminosity 10^46 erg/s, a factor of almost 10^6 times higher than observed. The accretion flow must be highly radiatively inefficient, similar to past results on M87 and NGC3115. The UMBH in NGC1277 is definitely not undergoing any significant growth at the present epoch. We note that there are 3 UMBH candidates in the Perseus cluster and that the inferred present mean mass density in UMBH could be 10^5 Msun/Mpc^3, which is 20 to 30 per cent of the estimated mean mass density of all black holes. We speculate on the implied growth of UMBH and their hosts, and discuss the possibiity that extreme AGN feedback could make all UMBH host galaxies have low stellar masses at redshifts around 3. Only those which end up at the centres of groups and clusters later accrete large stellar envelopes and become Brightest Cluster Galaxies. NGC1277 and the other Perseus core UMBH, NGC1270, have not however been able to gather more stars or gas owing to their rapid orbital motion in the cluster core.

### Kinks and Dents in Protoplanetary Disks: Rapid Infrared Variability as Evidence for Large Structural Perturbations

We report on synoptic observations at 3.6 and 4.5micron of young stellar objects in IC 348 with 38 epochs covering 40 days. We find that among the detected cluster members, 338 at [3.6] and 269 at both [3.6] and [4.5], many are variable on daily to weekly timescales with typical fluctuations of ~0.1 mag. The fraction of variables ranges from 20% for the diskless pre-main sequence stars to 60% for the stars still surrounded by infalling envelopes. We also find that stars in the exposed cluster core are less variable than the stars in the dense, slightly younger, south-western ridge. This trend persists even after accounting for the underlying correlation with infrared SED type, suggesting that the change in variable fraction is not simply a reflection of the change in relative fraction of class I vs. class II sources across the cloud, but instead reflects a change in variability with age. We also see a strong correlation between infrared variability and X-ray luminosity among the class II sources. The observed variability most likely reflects large changes in the structure of the inner wall located at the dust sublimation radius. We explore the possibility that these structural perturbations could be caused by a hot spot on the star heating dust above the sublimation temperature, causing it to evaporate rapidly, and increasing the inner radius for a portion of the disk. Under a number of simplifying assumptions we show that this model can reproduce the size and timescale of the 3.6 and 4.5micron fluctuations. Regardless of its source, the infrared variability indicates that the inner disk is not a slowly evolving entity, but instead is a bubbling, warped, dented mass of gas and dust whose global size and shape fluctuate in a matter of days.

### The drivers of AGN activity in galaxy clusters: AGN fraction as a function of mass and environment

[Abridged] We present an analysis of optical spectroscopically-identified AGN to M*+1 in a sample of 6 self-similar SDSS galaxy clusters at z=0.07. These clusters are specifically selected to lack significant substructure at bright limits in their central regions so that we are largely able to eliminate the local action of merging clusters on the frequency of AGN. We demonstrate that the AGN fraction increases significantly from the cluster centre to 1.5Rvirial, but tails off at larger radii. If only comparing the cluster core region to regions at ~2Rvirial, no significant variation would be found. We compute the AGN fraction by mass and show that massive galaxies (log(stellar mass)>10.7) are host to a systematically higher fraction of AGN than lower mass galaxies at all radii from the cluster centre. We attribute this deficit of AGN in the cluster centre to the changing mix of galaxy types with radius. We use the WHAN diagnostic to separate weak AGN from retired’ galaxies in which the main ionization mechanism comes from old stellar populations. These retired AGN are found at all radii, while the mass effect is much more pronounced: we find that massive galaxies are more likely to be in the retired class. Further, we show that our AGN have no special position inside galaxy clusters – they are neither preferentially located in the infall regions, nor situated at local maxima of galaxy density. However, we find that the most powerful AGN (with [OIII] equivalent widths <-10Ang) reside at significant velocity offsets in the cluster, and this brings our analysis into agreement with previous work on X-ray selected AGN. Our results suggest that if interactions with other galaxies are responsible for triggering AGN activity, the time-lag between trigger and AGN enhancement must be sufficiently long to obfuscate the encounter site and wipe out the local galaxy density signal.

### An HST/WFC3-UVIS View of the Starburst in the Cool Core of the Phoenix Cluster [Replacement]

We present Hubble Space Telescope Wide Field Camera 3 observations of the core of the Phoenix Cluster SPT-CLJ2344-4243 in five broadband filters spanning rest-frame 1000–5500A. These observations reveal complex, filamentary blue emission, extending for >40kpc from the brightest cluster galaxy. We observe an underlying, diffuse population of old stars, following an r^1/4 distribution, confirming that this system is somewhat relaxed. The spectral energy distribution in the inner part of the galaxy, as well as along the extended filaments, is a smooth continuum and is consistent with that of a star-forming galaxy, suggesting that the extended, filamentary emission is not due to the central AGN, either from a large-scale ionized outflow or scattered polarized UV emission, but rather a massive population of young stars. We estimate an extinction-corrected star formation rate of 798 +/- 42 Msun/yr, consistent with our earlier work based on low spatial resolution ultraviolet, optical, and infrared imaging. The lack of tidal features and multiple bulges, combine with the need for an exceptionally massive (>10^11 Msun) cold gas reservoir, suggest that this star formation is not the result of a merger of gas-rich galaxies. Instead, we propose that the high X-ray cooling rate of ~2700 Msun/yr is the origin of the cold gas reservoir. The combination of such a high cooling rate and the relatively weak radio source in the cluster core suggests that feedback has been unable to halt cooling in this system, leading to this tremendous burst of star formation.

### An HST/WFC3-UVIS View of the Starburst in the Cool Core of the Phoenix Cluster

We present the results of Hubble Space Telescope Wide Field Camera 3 observations of the core of the Phoenix Cluster (SPT-CLJ2344-4243) in five broadband filters spanning rest-frame 1000-5500A. These observations reveal complex, filamentary blue emission, extending for >40 kpc from the brightest cluster galaxy. We observe an underlying, diffuse population of old stars, following an r^1/4 distribution, confirming that this system is somewhat relaxed. The spectral energy distribution in the inner part of the galaxy, as well as along the extended filaments, is a smooth continuum and is consistent with that of a star-forming galaxy, suggesting that the extended, filamentary emission is not due to a large-scale highly-ionized outflow from the central AGN, but rather a massive population of young stars. We estimate an extinction-corrected star formation rate of 798 +/- 42 Msun/yr, consistent with our earlier work based on low spatial resolution ultraviolet, optical, and infrared imaging. We argue that such a high star formation rate is not the result of a merger, as it would require >10 mergers with gas-rich galaxies and there is no evidence for such multiple merger events. Instead, we propose that the high X-ray cooling rate of ~2850 Msun/yr is the origin of the cold gas reservoir. The combination of such a high cooling rate and the relatively weak radio source in the cluster core suggests that feedback has been unable to halt runaway cooling in this system, leading to this tremendous burst of star formation.

### Variable Stars in the Open Cluster NGC 7044

We present results of a search for variable stars in the intermediate-age open cluster NGC 7044. We found 23 variable stars in the observed field. One star turned out to be of the delta Sct type with two pulsational modes excited. From the position in the color-magnitude diagram we conclude that this star is a member of the cluster. Moreover, we found 13 eclipsing systems, of which five are W UMa stars, one is a beta Lyr variable, six are beta Per binaries showing detached configuration, and the last one is another probable beta Per system. Using the period-luminosity-color relation for W UMa stars we established the membership of the contact binaries, finding four of them to be very probable cluster members. We estimated from these four stars an apparent distance modulus (m-M)_V of NGC 7044 to be 14.2 +/- 0.4 mag, which is smaller than previous determinations of this parameter. We were able to derive orbital period for only four beta Per systems. For the remaining ones we observed only two or three eclipses. Finally, nine stars we found to show irregular light changes. Most of them are red stars not belonging to the cluster. For the cluster core we determined a reddening map, which allowed us to construct a dereddened color-magnitude diagram of NGC 7044 with a narrow main-sequence. By fitting a theoretical isochrone to this diagram we derived E(V-I_C) = 0.92 mag, (m-M)_V = 14.45 mag and log(age/yr) = 9.2.

### Retention of Stellar-Mass Black Holes in Globular Clusters

Globular clusters should be born with significant numbers of stellar-mass black holes (BHs). It has been thought for two decades that very few of these BHs could be retained through the cluster lifetime. With masses ~10 MSun, BHs are ~20 times more massive than an average cluster star. They segregate into the cluster core, where they may eventually decouple from the remainder of the cluster. The small-N core then evaporates on a short timescale. This is the so-called Spitzer instability. Here we present the results of a full dynamical simulation of a globular cluster containing many stellar-mass BHs with a realistic mass spectrum. Our Monte Carlo simulation code includes detailed treatments of all relevant stellar evolution and dynamical processes. Our main finding is that old globular clusters could still contain many BHs at present. In our simulation, we find no evidence for the Spitzer instability. Instead, most of the BHs remain well-mixed with the rest of the cluster, with only the innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution, fewer than half of the BHs are dynamically ejected through strong binary interactions in the cluster core. The presence of BHs leads to long-term heating of the cluster, ultimately producing a core radius on the high end of the distribution for Milky Way globular clusters (and those of other galaxies). A crude extrapolation from our model suggests that the BH–BH merger rate from globular clusters could be comparable to the rate in the field.

### Retention of Stellar-Mass Black Holes in Globular Clusters [Replacement]

Globular clusters should be born with significant numbers of stellar-mass black holes (BHs). It has been thought for two decades that very few of these BHs could be retained through the cluster lifetime. With masses ~10 MSun, BHs are ~20 times more massive than an average cluster star. They segregate into the cluster core, where they may eventually decouple from the remainder of the cluster. The small-N core then evaporates on a short timescale. This is the so-called Spitzer instability. Here we present the results of a full dynamical simulation of a globular cluster containing many stellar-mass BHs with a realistic mass spectrum. Our Monte Carlo simulation code includes detailed treatments of all relevant stellar evolution and dynamical processes. Our main finding is that old globular clusters could still contain many BHs at present. In our simulation, we find no evidence for the Spitzer instability. Instead, most of the BHs remain well-mixed with the rest of the cluster, with only the innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution, fewer than half of the BHs are dynamically ejected through strong binary interactions in the cluster core. The presence of BHs leads to long-term heating of the cluster, ultimately producing a core radius on the high end of the distribution for Milky Way globular clusters (and those of other galaxies). A crude extrapolation from our model suggests that the BH–BH merger rate from globular clusters could be comparable to the rate in the field.

### The growth of massive stars via stellar collisions in ensemble star clusters

Recent simulations and observations suggest that star clusters form via the assembling of smaller sub-clusters. Because of their short relaxation time, sub-clusters experience core collapse much earlier than virialized solo-clusters, which have similar properties of the merger remnant of the assembling clusters. As a consequence it seems that the assembling clusters result in efficient multiple collisions of stars in the cluster core. We performed a series of $N$-body simulations of ensemble and solitary clusters including stellar collisions and found that the efficiency of multiple collisions between stars are suppressed if sub-clusters assemble after they experience core collapse individually. In this case, sub-clusters form their own multiple collision stars which experienced a few collisions, but they fail to collide with each other after their host sub-clusters assemble. The multiple collision stars scatter each other and escape, and furthermore the central density of the remnant clusters had already been depleted for the stars to experience more collisions. On the other hand, if sub-clusters assemble before they experience core collapse, the multiple collisions of stars proceed efficiently in the remnant cluster, and the collision products are more massive than virialized solo-clusters and comparable in mass to cold solo-clusters.

### Isotropic Heating of Galaxy Cluster Cores via Rapidly Reorienting AGN Jets [Replacement]

AGN jets carry more than sufficient energy to stave off catastrophic cooling of the intracluster medium (ICM) in the cores of cool-core clusters. However, in order to prevent catastrophic cooling, the ICM must be heated in a near-isotropic fashion and narrow bipolar jets with $P_{\rm jet}=10^{44-45}$ ergs/s, typical of radio AGNs at cluster centres, are inefficient at heating the gas in the transverse direction to the jets. We argue that due to existent conditions in cluster cores, the SMBHs will, in addition to accreting gas via radiatively inefficient flows, experience short stochastic episodes of enhanced accretion via thin discs. In general, the orientation of these accretion discs will be misaligned with the spin axis of the black holes and the ensuing torques will cause the black hole’s spin axis (and therefore, the jet axis) to slew and rapidly change direction. This model not only explains recent observations showing successive generations of jet-lobes-bubbles in individual cool-core clusters that are offset from each other in the angular direction with respect to the cluster center, but also shows that AGN jets {\it can} heat the cluster core nearly isotropically on the gas cooling timescale. Our model {\it does} require that the SMBHs at the centers of cool-core clusters be spinning relatively slowly. Torques from individual misaligned discs are ineffective at tilting rapidly spinning black holes by more than a few degrees. Additionally, since SMBHs that host thin accretion discs will manifest as quasars, we predict that roughly 1–2 rich clusters within $z<0.5$ should have quasars at their centers.

### Isotropic Heating of Galaxy Cluster Cores via Rapidly Reorienting AGN Jets

AGN jets carry more than sufficient energy to stave off catastrophic cooling of the intracluster medium (ICM) in the cores of cool-core clusters. However, in order to prevent catastrophic cooling, the ICM must be heated in a near-isotropic fashion and narrow bipolar jets are inefficient at heating the gas in the transverse direction to the jets. We argue that due to existent conditions in cluster cores, the SMBHs will, in addition to accreting gas via radiatively inefficient flows, experience short stochastic episodes of enhanced accretion via thin discs. In general, the orientation of these accretion discs will be misaligned with the spin axis of the black holes and the ensuing torques will cause the black hole’s spin axis (and therefore, the jet axis) to slew and rapidly change direction. This model not only explains recent observations showing successive generations of jet-lobes-bubbles in individual cool-core clusters that are offset from each other in the angular direction with respect to the cluster center, but also shows that AGN jets {\it can} heat the cluster core nearly isotropically on the gas cooling timescale. One implication of our proposed model is that since SMBHs that host thin accretion discs will manifest as quasars, we predicts that roughly 1–2 rich clusters within $z<0.5$ should have quasars at their centers. Also, recurrent accretion via misaligned accretion discs implies that as a population, the SMBHs at the centers of cool-core clusters should be spinning slowly. Our model, in fact, requires SMBHs to be spinning slowly. Torques from misaligned discs are ineffective at tilting rapidly spinning black holes by more a few degrees whereas slowly spinning SMBHs can, under optimal conditions, slew by as much as $\sim 30^\circ$ during any one accretion event.

### The central density of R136 in 30 Doradus [Replacement]

The central density rho_0 of a stellar cluster is an important physical parameter for determining its evolutionary and dynamical state. How much mass segregation there is, or whether the cluster has undergone core collapse both depends on rho_0. We reanalyze the results of a previous paper that gives the mass density profile of R136 and combine them with both a conservative upper limit for the core parameter and a more uncertain recent measurement. We thus place a lower limit on rho_0 under reasonable and defensible assumptions about the IMF, finding rho_0 >~ 1.5×10^4 Msun/pc^3 for the conservative assumption a < 0.4 pc for the cluster core parameter. If we use the lower, but more uncertain value a = 0.025 pc, the central density estimate becomes greater than 10^7 Msun/pc^3. A mechanism based on the destruction of a large number of circumstellar disks is posited to explain the hitherto unexplained increase in reddening presented in that same work.

### The central density of R136 in 30 Doradus

The central density rho_0 of a stellar cluster is an important physical parameter to determine its evolutionary and dynamical state. The degree of mass segregation, or whether the cluster has undergone core collapse both depends on rho_0. We reanalyze the results of a previous paper that gives the mass density profile of R136 and combine them with both a conservative upper limit for the core parameter and a more uncertain recent measurement. We thus place a lower limit on rho_0 under reasonable and defensible assumptions about the IMF and its extrapolation to lower masses finding rho_0 >~ 1.5×10^4 Msun/pc^3 for the conservative assumption a < 0.4 pc for the cluster core parameter. If we use the smaller, but more uncertain value a = 0.025 pc, the central density estimate becomes larger than 10^7 Msun/pc^3. A mechanism based on the destruction of a large fraction of circumstellar disks is posited to explain the hitherto unexplained increase in reddening presented in that same work.

### Plunging fireworks: Why do infalling galaxies light up on the outskirts of clusters?

(abridged)Integrated star formation rate (SFR) and specific star formation rate (sSFR), derived from the spectroscopic data obtained by SDSS DR4 are used to show that the star formation activity in galaxies (M_r<=-20.5) found on the outskirts (1-2r_{200}) of some nearby clusters (0.02<=z<=0.15) is enhanced. By comparing the mean SFR of galaxies in a sample of clusters with at least one starburst galaxy (log sSFR>=-10/yr & SFR>10 M_sun/yr) to a sample of clusters without such galaxies (comparison’ clusters), we find that despite the expected decline in the mean SFR of galaxies toward the cluster core, the SFR profile of the two samples is different, such that the galaxies in the comparison’ clusters show a lower mean SFR at all radius (<=3r_{200}) from the cluster centre. Such an increase in the SFR of galaxies is more likely to be seen in dynamically unrelaxed (sigma_v>~500 km/s) clusters. It is also evident that these unrelaxed clusters are currently being assembled via galaxies falling in through straight filaments, resulting in high velocity dispersions. On the other hand, comparison’ clusters are more likely to be fed by relatively low density filaments. We find that the starburst galaxies on the periphery of clusters are in an environment of higher local density than other cluster galaxies at similar radial distances from the cluster centre. We conclude that a relatively high galaxy density in the infalling regions of clusters promotes interactions amongst galaxies, leading to momentary bursts of star formation. Such interactions play a crucial role in exhausting the fuel for star formation in a galaxy, before it is expelled due to the environmental processes that are operational in the dense interiors of the cluster.

### Identifying Contaminated K-band Globular Cluster RR Lyrae Photometry

Acquiring near-infrared K-band (2.2 um) photometry for RR Lyrae variables in globular clusters and nearby galaxies is advantageous since the resulting distances are less impacted by reddening and metallicity. However, K-band photometry for RR Lyrae variables in M5, Reticulum, M92, omega Cen, and M15 display clustercentric trends. HST ACS data imply that multiple stars in close proximity to RR Lyrae variables located near the cluster core, where the stellar density increases markedly, are generally unresolved in ground-based images. RR Lyrae variables near the cluster cores appear to suffer from photometric contamination, thereby yielding underestimated cluster distances and biased ages. The impact is particularly pernicious since the contamination propagates a systematic uncertainty into the distance scale, and hinders the quest for precision cosmology. The clustercentric trends are probably unassociated with variations in chemical composition since an empirical K-band period-magnitude relation inferred from Araucaria/VLT data for RR Lyrae variables in the Sculptor dSph exhibits a negligible metallicity dependence: (0.059+-0.095)[Fe/H], a finding that supports prior observational results. A future multi-epoch high-resolution near-infrared survey, analogous to the optical HST ACS Galactic Globular Cluster Survey, may be employed to establish K-band photometry for the contaminating stars discussed here.

### A Weak-Lensing Mass Reconstruction of the Large-Scale Filament Feeding the Massive Galaxy Cluster MACSJ0717.5+3745

We report the first weak-lensing detection of a large-scale filament funneling matter onto the core of the massive galaxy cluster MACSJ0717.5+3745. Our analysis is based on a mosaic of 18 multi-passband images obtained with ACS aboard the HST, covering an area of \sim 10×20 arcmin^2. We use a weak-lensing pipeline developed for the COSMOS survey, modified for the analysis of galaxy clusters, to produce a weak-lensing catalogue. A mass map is then computed by applying a weak-gravitational-lensing multi-scale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong-lensing constraints and find excellent agreement. The filament is detected within the 3 sigma detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the Cosmic Web. We measure the filament’s projected length as \sim 4.5 h_{74}^{-1} Mpc, and its mean density as (2.92 \pm 0.66)10^8 h_{74} M_{\odot} kpc^{-2}. Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structure’s geometry based on its galaxy velocity field, we construct a 3D model of the large-scale filament. Within this framework, we derive the three-dimensional length of the filament to be 18 h_{74}^{-1} Mpc, and a deprojected density in terms of the critical density of the Universe of (206 \pm 46) \rho_{crit}, a value that lies at the very high end of the range predicted by numerical simulations.