Fossil groups in the Millennium Simulation - Evolution of the brightest galaxies
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Aims. We create a catalogue of simulated fossil groups and study their properties, in particular the merging histories of their first-ranked galaxies. We compare the simulated fossil group properties with those of both simulated non-fossil and observed fossil groups. Methods. Using simulations and a mock galaxy catalogue, we searched for massive (>5 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$) fossil groups in the Millennium Simulation Galaxy Catalogue. In addition, we attempted to identify observed fossil groups in the Sloan Digital Sky Survey Data Release 6 using identical selection criteria. Results. Our predictions on the basis of the simulation data are: (a) fossil groups comprise about 5.5% of the total population of groups/clusters with masses larger than 5 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$. This fraction is consistent with the fraction of fossil groups identified in the SDSS, after all observational biases have been taken into account; (b) about 88% of the dominant central objects in fossil groups are elliptical galaxies that have a median R -band absolute magnitude of ~$-23.5{-}5~\log~h$, which is typical of the observed fossil groups known in the literature; (c) first-ranked galaxies of systems with $ {\cal M} >$ 5 $\times$ 10$^{13}~h^{-1}~{\cal M}_\odot$, regardless of whether they are either fossil or non-fossil, are mainly formed by gas-poor mergers; (d) although fossil groups, in general, assembled most of their virial masses at higher redshifts in comparison with non-fossil groups, first-ranked galaxies in fossil groups merged later, i.e. at lower redshifts , compared with their non-fossil-group counterparts. Conclusions. We therefore expect to observe a number of luminous galaxies in the centres of fossil groups that show signs of a recent major merger.Keywords:
Virial mass
We investigate the origin and evolution of fossil groups in a concordance ΛCDM cosmological simulation. We consider haloes with masses between 1 × 1013 and 5 × 1013h−1M⊙, and study the physical mechanisms that lead to the formation of the large gap in magnitude between the brightest and the second most bright group member, which is typical for these fossil systems. Fossil groups are found to have high dark matter concentrations, which we can relate to their early formation time. The large magnitude gaps arise after the groups have built up half of their final mass, due to merging of massive group members. We show that the existence of fossil systems is primarily driven by the relatively early infall of massive satellites, and that we do not find a strong environmental dependence for these systems. In addition, we find tentative evidence for fossil group satellites falling in on orbits with typically lower angular momentum, which might lead to a more efficient merger on to the host. We find a population of groups at higher redshifts that go through a 'fossil phase': a stage where they show a large magnitude gap, which is terminated by renewed infall from their environment.
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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.
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We use SDSS-DR4 photometric and spectroscopic data out to redshift z ∼ 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of 25 fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample, and the main pitfalls are also discussed. The spatial density of FGs, estimated via the V/VMAX test, is 2.83 × 10−6 h375 Mpc−3 for LX > 0.89 × 1042 h−275 erg s−1 consistent with Vikhlinin et al., who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same data set. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color–magnitude diagram, and structural parameters such as a4 and internal color gradients. Furthermore, examination of stellar populations shows that our 25 FGs have similar ages, metallicities, and α-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift. Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the universe.
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Defined as X-ray bright galaxy groups with large differences between the luminosities of their brightest and second brightest galaxies, 'fossil groups' are believed to be some of the oldest galaxy systems in the Universe. They have therefore been the subject of much recent research.
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(Abridged) Fossil systems are galaxy associations dominated by a relatively isolated, bright elliptical galaxy, surrounded by a group of smaller galaxies lacking L* objects. We analyzed the near-infrared photometric and structural properties of a sample of 20 BGGs present in FGs in order to better understand their formation mechanisms. Their surface-brightness distribution was fitted to a Sersic profile using the GASP2D algorithm. Then, the standard scaling relations were derived for the first time for these galaxies and compared with those of normal ellipticals and brightest cluster galaxies in non-fossil systems. The BGGs presented in this study represent a subset of the most massive galaxies in the Universe. We found that their ellipticity profiles are continuously increasing with the galactocentric radius. Our fossil BCGs follow closely the fundamental plane described by normal ellipticals. However, they depart from both the log \sigma_0 vs. log L_{K_{s}} and log r_{\rm e} vs. log L_{K_{s}} relations described by intermediate mass ellipticals. This occurs in the sense that our BGGs have larger effective radii and smaller velocity dispersions than those predicted by these relations. We also found that more elliptical galaxies systematically deviate from the previous relations while more rounder object do not. No similar correlation was found with the Sersic index. The derived scaling relations can be interpreted in terms of the formation scenario of the BGGs. Because our BGGs follow the fundamental plane tilt but they have larger effective radii than expected for intermediate mass ellipticals, we suggest that they only went through dissipational mergers in a early stage of their evolution and then assembled the bulk of their mass through subsequent dry mergers, contrary to previous claims that BGGs in FGs were formed mainly by the merging of gas-rich galaxies.
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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.
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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) ≥ 1013h−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 × 1042h−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.
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We report the discovery of seven new fossil systems in the 400d cluster survey. Our search targets nearby, z ⩽ 0.2, and X-ray bright, LX ⩾ 1043 erg s−1, clusters of galaxies. Where available, we measure the optical luminosities from Sloan Digital Sky Survey images, thereby obtaining uniform sets of both X-ray and optical data. Our selection criteria identify 12 fossil systems, out of which five are known from previous studies. While in general agreement with earlier results, our larger sample size allows us to put tighter constraints on the number density of fossil clusters. It has been previously reported that fossil groups are more X-ray bright than other X-ray groups of galaxies for the same optical luminosity. We find, however, that the X-ray brightness of massive fossil systems is consistent with that of the general population of galaxy clusters and follows the same LX–Lopt scaling relation.
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A search for fossil groups in the Sloan Digital Sky Survey was performed using virtual observatory tools. A cross-match of the positions of all SDSS luminous red galaxies (with r < 19 and measured spectroscopic redshifts) with sources in the ROSAT All-Sky Survey catalog resulted in a list of elliptical galaxies with extended X-ray emission (with a galaxy/ROSAT-source distance of less than 0.5' in all cases). A search for neighbors of the selected elliptical galaxies within a radius of 0.5 h Mpc was conducted, taking into account the r-band magnitudes and spectroscopic or photometric redshifts of all objects within this area, leading to a sample of 34 candidate fossil groups. Considering this sample, the estimated space density of fossil systems is n = (1.0 ± 0.6) × 10-6 h Mpc-3.
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We study the properties of the fossil cluster RX J1416.4+2315 through g' and i'-band imaging and spectroscopy of 25 member galaxies. The system is at a mean redshift of 0.137 and has a velocity dispersion of 584 km s^-1. Superimposed onto one quadrant of the cluster field there is a group of five galaxies at a mean redshift of 0.131, which, if included as part of the cluster, increases the velocity dispersion to 846 km/s. The central object of RX J1416.4+2315 is a normal elliptical galaxy, with no cD envelope. The luminosity function of the system, estimated by the number counts, and statistical background correction, in the range -22.6< M_g'< -16.6, is well fitted by a Schechter function with M_g'^* = -21.2 +/- 0.8 and alpha = -1.2 +/- 0.2 (H_0 = 70 km s^-1 Mpc^-1, Omega_M=0.3, Omega_Lambda=0.7). The luminosity function obtained from the spectroscopically confirmed members in both g' and i' bands agrees with the photometric results. The mass of the system, M 0.9 \times 10^14 h^-1_70 M_sun, its M/L of 445 h_70 M_sun/L_B_sun and L_X of 11 10^43 h^-2_70 ergs s^-1 (bolometric) suggest that this system is the second example of known fossil cluster, after RX J1552.2+2013, confirmed in the literature.
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