Habib G. Khosroshahi

Scaling relations in fossil galaxy groups

Using Chandra X-ray observations and optical imaging and spectroscopy of a flux-limited sample of five fossil groups, supplemented by additional systems from the literature, we provide the first detailed study of the scaling properties of fossils compared to normal groups and clusters. Fossil groups are dominated by a single giant elliptical galaxy at the centre of an extended bright X-ray halo. In general, all the fossils we study show regular and symmetric X-ray emission, indicating an absence of recent major group mergers. We study the scaling relations involving total gravitational mass, X-ray temperature, X-ray luminosity, group velocity dispersion and the optical luminosity of the fossil groups. We confirm that, for a given optical luminosity of the group, fossils are more X-ray luminous than non-fossil groups. Fossils, however, fall comfortably on the conventional LX‐ TX relation of galaxy groups and clusters, suggesting that their X-ray luminosity and their gas temperature are both boosted, arguably, as a result of their early formation. This is supported by other scaling relations including the LX‐σ and TX‐σ relations in which fossils show higher X-ray luminosity and temperature for a given group velocity dispersion. We find that mass concentration in fossils is higher than in non-fossil groups and clusters. In addition, the MX ‐TX relation suggests that fossils are hotter, for a given total gravitational mass, both consistent with an early formation epoch for fossils. We show that the mass-to-light ratio in fossils is rather high but not exceptional, compared to galaxy groups and clusters. The entropy of the gas in low-mass fossils appears to be systematically lower than that in normal groups, which may explain why the properties of fossils are more consistent with an extension of cluster properties. We discuss possible reasons for this difference in fossil properties and conclude that the cuspy potential raises the luminosity and temperature of the intergalactic medium (IGM) in fossils. However, this works in conjunction with lower gas entropy, which may arise from less effective pre-heating of the gas.

An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482

We present the first detailed X-ray observations, using Chandra, of NGC 6482 - the nearest known fossil group. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 h -1 70 kpc (∼0.1 r 200 ) using ACIS spatially resolved spectroscopy. The unusually high L X /L opt ratio is found to result from a high central gas density. The temperature profile shows a continuous decrease outward, dropping to 0.63 of its central value at 0.1r 200 . The derived total mass profile is strongly centrally peaked, suggesting an early formation epoch. These results support a picture in which fossil groups are old, giving time for the most massive galaxies to have merged (via the effects of dynamical friction) to produce a central giant elliptical galaxy. Although the cooling time within 0.1r 200 is less than a Hubble time, no decrease in central temperature is detected. The entropy of the system lies toward the low side of the distribution seen in poor groups and drops all the way into the centre of the system, reaching very low values. No isentropic core, such as those predicted in simple pre-heating models, is present. Given the lack of any central temperature drop in the system, it seems unlikely that radiative cooling can be invoked to explain this low central entropy. The lack of any signature of central cooling is especially striking in a system that appears to be old and relaxed, and to have a central cooling time ≤10 8 yr. We find that the centrally peaked temperature profile is consistent with a steady-state cooling-flow solution with an accretion rate of 2 M ○. yr -1 , given the large P dV work arising from the cuspy mass profile. However, solutions involving distributed or non-steady heating cannot be ruled out.

Correlations among global photometric properties of disk galaxies

Using a two-dimensional galaxy image decomposition technique, we extract global bulge and disk parameters for a complete sample of early-type disk galaxies in the near-infrared K band. We find significant correlation of the bulge parameter n with the central bulge surface brightness μb(0) and with effective radius re. Using bivariate analysis techniques, we find that log n, log re, and μb(0) are distributed in a plane with small scatter. We do not find a strong correlation of n with bulge-to-disk luminosity ratio, contrary to earlier reports. For these early-type disk galaxies, re and the disk scale length rd are well correlated, but with large scatter. We examine the implications of our results for various bulge formation scenarios in disk galaxies.

LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters

We study the luminosity gap, ∆m_(12), between the first- and second-ranked galaxies in a sample of 59 massive (~10^(15)M_⊙) galaxy clusters, using data from the Hale Telescope, the Hubble Space Telescope, Chandra and Spitzer. We find that the ∆m_(12) distribution, p(∆m_(12)), is a declining function of ∆m_(12) to which we fitted a straight line: p(∆m_(12))∝−(0.13 ± 0.02)∆m_(12). The fraction of clusters with ‘large’ luminosity gaps is p(∆m_(12) ≥ 1) = 0.37 ± 0.08, which represents a 3σ excess over that obtained from MonteCarlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with ‘extreme’ luminosity gaps, ∆m_(12) ≥ 2, giving a fraction of p(∆m_(12) ≥ 2) = 0.07^(+0.05)_(−0.03). More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/discy brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/discy BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch and the recent infall history of the cluster. ‘BCG dominance’ is therefore a phase that a cluster may evolve through and is not an evolutionary ‘cul-de-sac’. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models is able to reproduce all of the observational results on ∆m_(12), underlining the inability of the current generation of models to match the empirical properties of BCGs. We identify the strength of active galactic nucleus feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies.

The mass assembly of fossil groups of galaxies in the Millennium simulation

The evolution of present-day fossil galaxy groups is studied in the Millennium simulation. Using the corresponding Millennium gas simulation and semi-analytic galaxy catalogues, we select fossil groups at redshift zero according to the conventional observational criteria, and trace the haloes corresponding to these groups backwards in time, extracting the associated dark matter, gas and galaxy properties. The space density of the fossils from this study is remarkably close to the observed estimates and various possibilities for the remaining discrepancy are discussed. The fraction of X-ray bright systems which are fossils appears to be in reasonable agreement with observations, and the simulations predict that fossil systems will be found in significant numbers (3-4 per cent of the population) even in quite rich clusters. We find that fossils assemble a higher fraction of their mass at high redshifts, compared to non-fossil groups, with the ratio of the currently assembled halo mass to final mass, at any epoch, being about 10-20 per cent higher for fossils. This supports the paradigm whereby fossils represent undisturbed, early-forming systems in which large galaxies have merged to form a single dominant elliptical.

THE HST/ACS COMA CLUSTER SURVEY. II. DATA DESCRIPTION AND SOURCE CATALOGS ∗

The Coma cluster, Abell 1656, was the target of an HST-ACS Treasury program designed for deep imaging in the F475W and F814W passbands. Although our survey was interrupted by the ACS instrument failure in early 2007, the partially completed survey still covers ~50% of the core high-density region in Coma. Observations were performed for 25 fields that extend over a wide range of cluster-centric radii (~1.75 Mpc or 1°) with a total coverage area of 274 arcmin2. The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the southwest region of the cluster. In this paper, we present reprocessed images and SEXTRACTOR source catalogs for our survey fields, including a detailed description of the methodology used for object detection and photometry, the subtraction of bright galaxies to measure faint underlying objects, and the use of simulations to assess the photometric accuracy and completeness of our catalogs. We also use simulations to perform aperture corrections for the SEXTRACTOR Kron magnitudes based only on the measured source flux and its half-light radius. We have performed photometry for ~73,000 unique objects; approximately one-half of our detections are brighter than the 10σ point-source detection limit at F814W = 25.8 mag (AB). The slight majority of objects (60%) are unresolved or only marginally resolved by ACS. We estimate that Coma members are 5%-10% of all source detections, which consist of a large population of unresolved compact sources (primarily globular clusters but also ultra-compact dwarf galaxies) and a wide variety of extended galaxies from a cD galaxy to dwarf low surface brightness galaxies. The red sequence of Coma member galaxies has a color-magnitude relation with a constant slope and dispersion over 9 mag (–21 < M F814W < –13). The initial data release for the HST-ACS Coma Treasury program was made available to the public in 2008 August. The images and catalogs described in this study relate to our second data release.

The mass assembly of galaxy groups and the evolution of the magnitude gap

We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated Millennium gas simulations, and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z= 1 with dark matter halo mass M(R200) ≥ 1013 h−1 M⊙, and trace their properties until the present time (z= 0). We consider only the systems with X-ray luminosity LX,bol≥ 0.25 × 1042 h−2 erg s−1 at redshift z= 0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90 per cent of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0 < z < 1) cease to be fossils within 4 Gyr, mostly because other massive galaxies are assembled within their cores, even though most of the mass in their haloes might have been assembled at early times. We show that compared to the conventional definition of fossil galaxy groups based on the magnitude gap Δm12≥ 2 (in the R-band, within 0.5 R200 of the centre of the group), an alternative criterion Δm14≥ 2.5 (within the same radius) finds 50 per cent more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.

The central elliptical galaxy in fossil groups and formation of brightest cluster galaxies

We study the dominant central giant elliptical galaxies in “Fossil groups” using deep optical (R-band) and near infrared (Ks-band) photometry. These galaxies are as luminous as the brightest cluster galaxies (BCGs), raising immediate inte rest in their link to the formation of BCGs and galaxy clusters. However, despite apparent similarities, the dominant fossil galaxies show non-boxy isophotes, in contrast to the most luminous BCGs. This study suggests that the structure of the brightest group galaxies produced in fossil groups are systematically different to the majority of BCGs. If the fossils do indeed fo rm from the merger of major galaxies including late-types within a group, then their di sky nature is consistent with the results of recent numerical simulations of semi-analytical models which suggest that gas rich mergers result in disky isophote ellipticals. We show that fossils form a homogeneous population in which the velocity dispersion of the fossil group is tightly correlated with the luminosity o f the dominant elliptical galaxy. This supports the scenario in which the giant elliptical galaxie s in fossils can grow to the size and luminosity of BCGs in a group environment. However, the boxy structure of luminous BCGs indicate that they are either not formed as fossils, or have u ndergone later gas-free mergers within the cluster environment.

A fossil galaxy cluster: an X-ray and optical study of RX J1416.4+2315

We present a detailed X-ray and optical study of a distant fossil system RX J1416.4+2315 (z � 0.137), combining Chandra and XMM-Newton observations, optical photometry and spectroscopy. X-ray emitting hot gas imaged by both the Chandraand XMM-Newtonshows a globally relaxed spatial distribution, supporting the ide a that fossil groups are old galaxy systems with no recent mergers. However, the diffuse X-ray emission shows signs of asymmetries in the core of the system. With a mean gas temperature of � 4.0 keV and total gravitational mass of 3.1×10 14 M⊙, within the virial radius, this is better described as a foss il galaxy cluster rather than a fossil group. The temperature profile shows no sign of a significant cooler core despite a cooling time dropping to 5 Gyr within the resolved core. We find a mass concentration parameter c200 � 11 which is relatively high for a cluster of this mass, indicati ve of an early formation epoch. Using the spectroscopically identified cluster members we p resent the galaxy luminosity function for this fossil system. We measure the velocity dispersion of the galaxies to be � 700 km s −1 based on 18 confirmed members. The dynamical mass is nearly twice the total gravitational mass derived from the X-ray analysis. The measured R-band mass-to-light ratio, within the virial radius, is � 440 M⊙/L⊙ which is not unusual for clusters of galaxies. The central giant elliptical galaxy has discy isophotes and spe ctral features typical of elliptical galaxies.

Scaling relations in early-type galaxies belonging to groups

We present a photometric analysis of a large sample of early-type galaxies in 16 nearby groups, imaged with the Wide-Field Camera on the Isaac Newton Telescope. Using a two-dimensional surface brightness decomposition routine, we fit Sersic (r1/n) and exponential models to their bulge and disc components, respectively. Dividing the galaxies into three subsamples according to the X-ray luminosities of their parent groups, we compare their photometric properties. Galaxies in X-ray luminous groups tend to be larger and more luminous than those in groups with undetected or low X-ray luminosities, but no significant differences in n are seen. Both normal and dwarf elliptical galaxies in the central regions of groups are found to have cuspier profiles than their counterparts in group outskirts. Structural differences between dwarf and normal elliptical galaxies are apparent in terms of an offset between their ‘photometric planes’ in the space of n, re and μ0. Dwarf ellipticals are found to populate a surface, with remarkably low scatter, in this space with significant curvature, somewhat similar to the surfaces of constant entropy proposed by Marquez et al. Normal ellipticals are offset from this distribution in a direction of higher specific entropy. This may indicate that the two populations are distinguished by the action of galaxy merging on larger galaxies.