arXiv subjects

William Forman

Publications and source records attributed to William Forman.

70 records · Page 4Linked to original sources

arXiv2012

The Universal Gas Mass Fraction in Clusters of Galaxies

We obtained a deep 150ksec Chandra observation of the optically selected cluster of galaxies, RCS 2318+0034, to investigate the gas mass fraction in this system. Combining our deep {\it Chandra} observation with an archival 50ksec observation, we derive gas mass fractions of fgas=0.06 +- 0.02 and 0.10 +- 0.02 within r2500 and r500, respectively. The gas mass fraction in RCS 2318+0034 within r500 is typical of X-ray selected clusters. Further study shows that the large scale properties of RCS 2318+0034, including the relations between gas mass, X-ray luminosity and gas temperature are also consistent with the observed correlations of X-ray selected clusters. However, the gas mass fraction within r2500 is less than most X-ray selected clusters, as previously reported. The deep Chandra image of RCS 2318+0034 shows that this system is presently undergoing a major merger which may have an impact on the inferred gas mass fraction within r2500.

astro-ph.CO↗

arXiv2010

AGN Driven Weather and Multiphase Gas in the Core of the NGC 5044 Galaxy Group

A deep Chandra observation of the X-ray bright group, NGC 5044, shows that the central region of this group has been strongly perturbed by repeated AGN outbursts. These recent AGN outbursts have produced many small X-ray cavities, cool filaments and cold fronts. We find a correlation between the coolest X-ray emitting gas and the morphology of the Ha filaments. The Ha filaments are oriented in the direction of the X-ray cavities, suggesting that the warm gas responsible for the Halpha emission originated near the center of NGC 5044 and was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed spectroscopic analysis shows that the central region of NGC 5044 contains spatially varying amounts of multiphase gas. The regions with the most inhomogeneous gas temperature distribution tend to correlate with the extended 235 MHz and 610 MHz radio emission detected by the GMRT. This may result from gas entrainment within the radio emitting plasma or mixing of different temperature gas in the regions surrounding the radio emitting plasma by AGN induced turbulence. Accounting for the effects of multiphase gas, we find that the abundance of heavy elements is fairly uniform within the central 100 kpc, with abundances of 60-80% solar for all elements except oxygen, which has a significantly sub-solar abundance. In the absence of continued AGN outbursts, the gas in the center of NGC 5044 should attain a more homogeneous distribution of gas temperature through the dissipation of turbulent kinetic energy and heat conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044 indicates that the time between recent AGN outbursts has been less than approximately 10e8 yr.

astro-ph.CO↗

arXiv2009

Groups of Galaxies at Intermediate Redshift

Galaxy groups are key tracers of galaxy evolution, cluster evolution, and structure formation, yet they are difficult to study at even moderate redshift. We have undertaken a project to observe a flux-limited sample of intermediate-redshift (0.1 < z < 0.5) group candidates identified by the XBootes Chandra survey. When complete, this project will nearly triple the current number of groups with measured temperatures in this redshift range. Here we present deep Suzaku/XIS and Chandra/ACIS follow-up observations of the first 10 targets in this project; all are confirmed sources of diffuse, thermal emission with derived temperatures and luminosities indicative of rich groups/poor clusters. By exploiting the multi-wavelength coverage of the XBootes/NOAO Deep Wide Field Survey (NDWFS) field, we aim to (1) constrain non-gravitational effects that alter the energetics of the intragroup medium, and (2) understand the physical connection between the X-ray and optical properties of groups. We discuss the properties of the current group sample in the context of observed cluster scaling relations and group and cluster evolution and outline the future plans for this project.

astro-ph.CO↗

arXiv2009

Radio Mode Outbursts in Giant Elliptical Galaxies

Outbursts from active galactic nuclei (AGN) affect the hot atmospheres of isolated giant elliptical galaxies (gE's), as well as those in groups and clusters of galaxies. Chandra observations of a sample of nearby gE's show that the average power of AGN outbursts is sufficient to stop their hot atmospheres from cooling and forming stars, consistent with radio mode feedback models. The outbursts are intermittent, with duty cycles that increases with size.

astro-ph.GA↗

arXiv2009

Chandra Observations of the X-ray Point Source Population in NGC 4636

We present the X-ray point source population in the nearby Virgo elliptical galaxy NGC 4636 from 3 Chandra/ACIS observations, totaling 193 ks, taken over 3 years. Using wavelet decomposition, we detect 318 point sources. Here, we use a subset of the 277 sources with >10 net cts (>1.2E37 erg/s, 0.5-2 keV, >1.5 arcmin of center). Between 1.5-6 arcmin from the center, 25% are likely background sources (AGN) and the rest are LMXBs within the galaxy, while at >6 arcmin, background sources dominate. We present a color-color diagram based on ratios of X-ray flux, which yields a tighter source distribution, and shows a large population of sources which are likely LMXBs and black hole candidates. We explore short and long-term variability (over time-scales of 1 day to 3 yr): 54 sources (24%) in the common ACIS fields show significant variability between observations. Of these, 37 are detected with >10 net counts only once and thus may be "transient". About 10% of sources in each observation show significant short-term variability. The cumulative luminosity function for the point sources can be represented as a power-law of slope 1.14. Between 1.5-6 arcmin of the center, 48 of the 129 X-ray point sources (37%) with >10 net counts are matched with optical globular cluster (GC) candidates (matches could be as high as 50% since many of the optical sources are AGN). 70% of matched sources are associated with the redder GC candidates, those that are thought to have near-solar metal abundance. The fraction of GC candidates matched with point sources decreases with decreasing GC luminosity. The X-ray luminosities of the matched point sources are not correlated with the luminosity or color of the host GC candidates. Finally, we estimate an upper limit of 4.5E37 erg/s to the current X-ray luminosity of the historical supernova SN1939A.

astro-ph↗

arXiv2006

The Hot Gas Content of Low-Luminosity Early-Type Galaxies and the Implications Regarding Supernova Heating and AGN Feedback

We have analyzed Chandra observations of 18 low-luminosity early-type galaxies with L_B < 3 x 10e10 Lo. Thermal emission from hot gas with temperatures between 0.2 and 0.8 keV comprises 5-70% of the total 0.5-2.0 keV emission from these galaxies. We find that the total X-ray luminosity from LMXBs (resolved plus the power-law component of the unresolved emission) scales roughly linearly with the K-band luminosity of the galaxies with a normalization comparable to that found in more luminous early-type galaxies. All of the galaxies in our sample are gas poor with gas masses much less than that expected from the accumulation of stellar mass loss over the life time of the galaxies. The average ratio of gas mass to stellar mass in our sample is M_{gas}/M_*=0.001, compared to more luminous early-type galaxies which typically have M_{gas}/M_*=0.01. The time required to accumulate the observed gas mass from stellar mass loss in these galaxies is typically 3 x 10e8 yr. Since the cooling time of the gas is longer than the replenishment time, the gas cannot be condensing out of the hot phase and forming stars, implying that the gas is most likely being expelled from these galaxies in a wind (abridged).

astro-ph↗

arXiv2006

Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56

The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004,2006a).

astro-ph↗

arXiv2005

The Diffuse Emission and a Variable ULX in the Elliptical Galaxy NGC 3379

A Chandra observation of the intermediate luminosity (M_B=-20) elliptical galaxy NGC 3379 resolves 75% of the X-ray emission within the central 5kpc into point sources. Spectral analysis of the remaining unresolved emission within the central 770pc indicates that 90% of the emission probably arises from undetected point sources, while 10% arises from thermal emission from kT=0.6keV gas. Assuming a uniform density distribution in the central region of the galaxy gives a gas mass of 5.0e5 Mo.Such a small amount of gas can be supplied by stellar mass loss in only 1.0e7 years. Thus, the gas must be accreting into the central supermassive black hole at a very low radiative efficiency as in the ADAF or RIAF models, or is being expelled in a galactic wind driven by the same AGN feedback mechanism as that observed in cluster cooling flows. If the gas is being expelled in an AGN driven wind, then the ratio of mechanical to radio power of the AGN must be 1.0e4, which is comparable to that measured in cluster cooling flows which have recently been perturbed by radio outbursts. The brightest point source in NGC 3379 is located 360pc from the central AGN with a peak luminosity of 3.5e39 ergs/s, which places it in the class of ultra-luminous X-ray point sources (ULX). Analysis of an archival ROSAT HRI observation of NGC 3379 shows that this source was at a comparable luminosity 5 years prior to the Chandra observation. During the Chandra observation, the source intensity smoothly varies by a factor of two with the suggestion of an 8-10 hour period.

astro-ph↗

arXiv2005

The merging cluster Abell 85 caught between meals by XMM-Newton

Our XMM-Newton observations of Abell 85 confirm the extended 4 Mpc filament first detected with ROSAT, which has an X-ray temperature of about 2 keV and is probably made of groups falling on to the cluster. A comparison of the temperature map with numerical simulations show that Abell 85 had intense merging activity in the past and is not fully relaxed, even in the central region. Finally, a deprojected temperature profile has been calculated and used, together with the suface brightness, to estimate the entropy and pressure profiles. Abell 85 only presents a mild flattening of the entropy profile in the center, showing no evidence of an ``entropy floor''.

astro-ph↗

arXiv2004

A Deep Chandra Observation of the Distant Galaxy Cluster MS1137.5+6625

We present results from a deep Chandra observation of MS1137.5+66, a distant (z=0.783) and massive cluster of galaxies. Only a few similarly massive clusters are currently known at such high redshifts; accordingly, this observation provides much-needed information on the dynamical state of these rare systems. The cluster appears both regular and symmetric in the X-ray image. However, our analysis of the spectral and spatial X-ray data in conjunction with interferometric Sunyaev-Zel'dovich effect data and published deep optical imaging suggests the cluster has a fairly complex structure. The angular diameter distance we calculate from the Chandra and Sunyaev-Zel'dovich effect data assuming an isothermal, spherically symmetric cluster implies a low value for the Hubble constant for which we explore possible explanations.

astro-ph↗

arXiv2003

The 160 Square Degree ROSAT Survey - Revised Catalog & Cluster Evolution

We have constructed a large, statistically complete sample of galaxy clusters serendipitously detected as extended X-ray sources in 647 ROSAT PSPC pointed observations. The survey covers 158 square degrees with a median sample flux limit of 1.2 x 10^-13 erg cm^-2 s^-1 (0.5-2.0 keV). Our sample consists of 201 clusters of galaxies characterized by a median redshift of z=0.25 and a maximum of z=1.26. With 22 clusters at z > 0.5, the 160 Square Degree ROSAT Survey is the largest high-redshift sample of X-ray-selected clusters published to date. Here we describe the revised sample which features spectroscopic redshifts for 99.5% of the clusters and discuss the implications for evolution in the cluster abundance.

astro-ph↗

arXiv1999

ROSAT PSPC Observations of the Richest ($R \geq 2$) ACO Clusters

We have compiled an X-ray catalog of optically selected rich clusters of galaxies observed by the PSPC during the pointed GO phase of the ROSAT mission. This paper contains a systematic X-ray analysis of 150 clusters with an optical richness classification of $R \geq 2$ from the ACO catalog (Abell, Corwin, and Olowin 1989). All clusters were observed within 45' of the optical axis of the telescope during pointed PSPC observations. For each cluster, we calculate: the net 0.5-2.0 keV PSPC count rate (or $4 σ$ upper limit) in a 1 Mpc radius aperture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray centroid. The cluster sample is then used to examine correlations between the X-ray and optical properties of clusters, derive the X-ray luminosity function of clusters with different optical classifications, and obtain a quantitative estimate of contamination (i.e, the fraction of clusters with an optical richness significantly overestimated due to interloping galaxies) in the ACO catalog.

astro-ph↗

arXiv1996

An Einstein X-ray Survey of Optically-Selected Galaxies: I. Data

We present the results of a complete Einstein Imaging Proportional Counter (IPC) X-ray survey of optically-selected galaxies from Shapley-Ames Catalog, Uppsala General Catalog and the European Southern Observatory Catalog. X-ray fluxes for 1018 galaxies have been surveyed, 21% of which are detected. 827 galaxies have either independent distance estimates or radial velocities. Associated optical, redshift and distance data has been assembled. The five large data tables in the paper are in easily-accessible ascii formats with an ADC-style README file, for easy electronic data handling. The accuracy of the X-ray fluxes has been checked in three different ways; all are consistent with the derived X-ray fluxes being of $\le$0.1 dex accuracy. In particular, there is agreement with previous published X-ray fluxes for galaxies in common with Roberts et al. (1991) and Fabbiano et al. (1992). The data presented here will be used in further studies to characterize the X-ray output of galaxies of various morphological types and thus to enable the determination of the major sources contributing to the X-ray emission from galaxies.

astro-ph↗

arXiv1996

Are Luminous cD Haloes Formed by the Disruption of Dwarf Galaxies?

From a total sample of 45 Abell clusters observed by the Einstein X-ray observatory, we present the results on the galaxy luminosity function (LF) for a group of seven clusters that were identified by the morphology of their LFs. The LFs were derived using photometric data to a completeness limit ~5.5 magnitudes below M*. We found that a single Schechter function with an average $α\approx -1.0$ gives a good fit to these individual LFs within the magnitude range. These seven clusters have common properties, which indicate they form a homogeneous class of dynamically evolved clusters that can be characterized by the presence of a dominant cD galaxy, high richness, symmetrical single-peaked X-ray emission, and high gas mass. On the other hand, steep faint-end slopes (-2.0 < α< -1.4) are usually detected in poorer clusters. Our result gives a direct indication that the faint-end slope of the galaxy LF is subject to environmental effects. We propose that the flatness of the faint-end slope in these clusters results from the disruption of a large fraction of dwarf galaxies during the early stages of cluster evolution. The stars and gas from the disrupted galaxies are redistributed throughout the cluster potential during violent relaxation. This heuristic scenario can explain the origin of the luminous haloes of cD galaxies and a large fraction of the gas content in the intracluster medium as a by-product. The correlation between the cluster gas mass determined from the modeling of the X-ray emission and the cD halo optical luminosity is presented to support the proposed model.

astro-ph↗

arXiv1995

Intracluster Globular Clusters

Globular cluster populations of supergiant elliptical galaxies are known to vary widely, from extremely populous systems like that of UGC 9799, the centrally dominant galaxy in Abell 2052, to globular-cluster-poor galaxies such as NGC 5629 in Abell 2666. Here we propose that these variations point strongly to the existence of a population of globular clusters that are not bound to individual galaxies, but rather move freely throughout the cores of clusters of galaxies. Such intracluster globular clusters may have originated as tidally stripped debris from galaxy interactions and mergers, or alternatively they may have formed in situ in some scenarios of globular cluster formation.

astro-ph↗

arXiv1995

Substructure: Clues to the Formation of Clusters of Galaxies

We have examined the spatial distribution of substructure in clusters of galaxies using Einstein X-ray observations. Subclusters are found to have a markedly anisotropic distribution that reflects the surrounding matter distribution on supercluster scales. Our results suggest a picture in which cluster formation proceeds by mergers of subclusters along large-scale filaments. The implications of such an anisotropic formation process for the shapes, orientations and kinematics of clusters are discussed briefly.

astro-ph↗