Using Chandra X-ray observations and optical imaging and spectroscopy of a flux-limited sample of 5 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. In general, all the fossils we study show regular and symmetric X-ray emission, indicating an absence of recent major group mergers. 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 L_X-T_X 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 L_X-sigma and T_X-sigma 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 M_X-T_X 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 conclude that the cuspy potential raises the luminosity and temperature of the IGM in fossils. However, this works in conjunction with lower gas entropy, which may arise from less effective preheating of the gas.Comment: 12 pages, 10 figures, Accepted for publication in MNRA

We have used new deep observations of the Coma cluster from Galaxy Evolution Explorer to visually identify 13 star-forming galaxies with asymmetric morphologies in the ultraviolet (UV). Aided by wide-field optical broad-band and Hα imaging, we interpret the asymmetric features as being due to star formation within gas stripped from the galaxies by interaction with the cluster environment. The selected objects display a range of structures from broad fanshaped systems of filaments and knots ('jellyfish') to narrower and smoother tails extending up to 100 kpc in length. Some of the features have been discussed previously in the literature, while others are newly identified here. We assess the ensemble properties of the sample. The candidate stripping events are located closer to the cluster centre than other star-forming galaxies; their radial distribution is more similar to that of all cluster members, dominated by passive galaxies. The fraction of blue galaxies which are undergoing stripping falls from 40 per cent in the central 500 kpc to less than 5 per cent beyond 1 Mpc. We find that tails pointing away from (i.e. galaxies moving towards) the cluster centre are strongly favoured (11/13 cases). From the small number of 'outgoing' galaxies with stripping signatures, we conclude that the stripping events occur primarily on first passage towards the cluster centre, and are short-lived compared to the cluster crossing time. Using galaxy infall trajectories extracted from a cosmological simulation, we find that the observed fraction of blue galaxies undergoing stripping can be reproduced if the events are triggered at a threshold radius of ∼1 Mpc and detectable for ∼500 Myr. Hubble Space Telescope images are available for two galaxies from our sample and reveal compact blue knots coincident with UV and Hα emission, apparently forming stars within the stripped material. Our results confirm that stripping of gas from infalling galaxies, and associated star formation in the stripped material, is a widespread phenomenon in rich clusters. Deep UV imaging of additional clusters is a promising route to constructing a statistically powerful sample of stripping events and constraining models for the truncation of star formation in clusters.

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.

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(R 200 ) ≥ 10 13 h −1 M , and trace their properties until the present time (z = 0). We consider only the systems with X-ray luminosity L X,bol ≥ 0.25 × 10 42 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 m 12 ≥ 2 (in the R-band, within 0.5 R 200 of the centre of the group), an alternative criterion m 14 ≥ 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 earlyformed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.

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−170 kpc (∼0.1 r200) using ACIS spatially resolved spectroscopy. The unusually high LX/Lopt 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 0.1r200. 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 0.1r200 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 ≤108 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.

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 µ b (0) and with effective radius r e . Using bivariate analysis techniques, we find that log n, log r e and µ b (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. r e and the disk scale length r d are well correlated for these early type disk galaxies, but with large scatter. We examine the implications of our results to various bulge formation scenarios in disk galaxies.

We present the results of a search for galaxy clusters and groups in the ∼ 2 square degree of the COSMOS field using all available X-ray observations from the XMM-Newton and Chandra observatories. We reach an X-ray flux limit of 3 × 10 −16 ergs cm −2 s −1 in 0.5-2 keV range, and identify 247 X-ray groups with M 200c = 8 × 10 12 − 3 × 10 14 M at a redshift range of 0.08 z < 1.53, using the multiband photometric redshift and the master spectroscopic redshift catalogues of the COSMOS. The X-ray centres of groups are determined using high-resolution Chandra imaging. We investigate the relations between the offset of the brightest group galaxies (BGGs) from halo X-ray centre and group properties and compare with predictions from semi-analytic models and hydrodynamical simulations. We find that BGG offset decreases with both increasing halo mass and decreasing redshift with no strong dependence on the X-ray flux and SNR. We show that the BGG offset decreases as a function of increasing magnitude gap with no considerable redshift dependent trend. The stellar mass of BGGs in observations extends over a wider dynamic range compared to model predictions. At z < 0.5, the central dominant BGGs become more massive than those with large offsets by up to 0.3dex, in agreement with model prediction. The observed and predicted lognormal scatter in the stellar mass of both lowand large-offset BGGs at fixed halo mass is ∼ 0.3dex.

We study the history from z ∼ 2 to z ∼ 0 of the stellar mass assembly of quiescent and star-forming galaxies in a spatially resolved fashion. For this purpose we use multi-wavelength imaging data from the Hubble Space Telescope (HST) over the GOODS fields and the Sloan Digital Sky Survey (SDSS) for the local population. We present the radial stellar mass surface density profiles of galaxies with M * > 10 10 M ⊙ , corrected for mass-to-light ratio (M * /L) variations, and derive the half-mass radius (R m ), central stellar mass surface density within 1 kpc (Σ 1 ) and surface density at R m (Σ m ) for star-forming and quiescent galaxies and study their evolution with redshift. At fixed stellar mass, the half-mass sizes of quiescent galaxies increase from z ∼ 2 to z ∼ 0 by a factor of ∼ 3 − 5, whereas the half-mass sizes of star-forming galaxies increase only slightly, by a factor of ∼ 2. The central densities Σ 1 of quiescent galaxies decline slightly (by a factor of 1.7) from z ∼ 2 to z ∼ 0, while for star-forming galaxies Σ 1 increases with time, at fixed mass. We show that the central density Σ 1 has a tighter correlation with specific star-formation rate (sSFR) than Σ m and for all masses and redshifts galaxies with higher central density are more prone to be quenched. Reaching a high central density (Σ 1 10 10 M ⊙ kpc 2 ) seems to be a prerequisite for the cessation of star formation, though a causal link between high Σ 1 and quenching is difficult to prove and their correlation can have a different origin.