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Register a new userFossil Groups in the Millennium Simulation: Their environment and its evolution
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Eugenia Diaz-Gimenez
Publication date
2010
fields
Physics
and research's language is
English
Authors
Eugenia Diaz-Gimenez
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Fossil systems are defined to be X-ray bright galaxy groups with a 2-magnitude difference between their two brightest galaxies within half the projected virial radius,and represent an interesting extreme of the population of galaxy agglomerations.However,the physical conditions and processes leading to their formation are still poorly constrained.We compare the outskirts of fossil systems with that of normal groups to understand whether environmental conditions play a significant role in their formation.We study galaxy groups in both,numerical simulations and observations.We use a variety of statistical tools including the spatial cross-correlation function and the local density parameter Delta_5 to probe differences in the density and structure of the environments of normal and fossil systems in the Millennium simulation.We find that the number density of galaxies surrounding fossil systems evolves from greater than that observed around normal systems at z=0.69, to lower than the normal systems by z=0.Both fossil and normal systems exhibit an increment in their otherwise radially declining local density measure (Delta_5) at distances of order 2.5r_{vir} from the system centre.We show that this increment is more noticeable for fossil systems than normal systems and demonstrate that this difference is linked to the earlier formation epoch of fossil groups.Despite the importance of the assembly time, we show that the environment is different for fossil and non-fossil systems with similar masses and formation times along their evolution.We also confirm that the physical characteristics identified in the Millennium simulation can also be detected in SDSS observations.Our results confirm the commonly held belief that fossil systems assembled earlier than normal systems but also show that the surroundings of fossil groups could be responsible for the formation of their large magnitude gap.
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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. 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, attempted to identify observed fossil groups in the Sloan Digital Sky Survey Data Release 6 using identical selection criteria. 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 x 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 $sim -23.5-5 log h$, which is typical of the observed fossil groups known in the literature; (c)first-ranked galaxies of systems with $ {cal M} >$ 5 x 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. We therefore expect to observe a number of luminous galaxies in the centres of fossil groups that show signs of a recent major merger.
The Millennium N-body simulation and the Sloan Digital Sky Survey seventh data release (SDSS DR7) galaxy and galaxy group catalogues are compared to study the properties of galaxy groups and the distribution of galaxies in groups. We construct mock galaxy group catalogues for a Millennium semi-analytical galaxy catalogue by using the same friends-of-friends method, which was used by Tago et al to analyse the SDSS data. We analyse in detail the group luminosities, group richnesses, virial radii, sizes of groups and their rms velocities for four volume-limited samples from observations and simulations. Our results show that the spatial densities of groups agree within one order of magnitude in all samples with a rather good agreement between the mock catalogues and observations. All group property distributions have similar shapes and amplitudes for richer groups. For galaxy pairs and small groups, the group properties for observations and simulations are clearly different. In addition, the spatial distribution of galaxies in small groups is different: at the outskirts of the groups the galaxy number distributions do not agree, although the agreement is relatively good in the inner regions. Differences in the distributions are mainly due to the observational limitations in the SDSS sample and to the problems in the semi-analytical methods that produce too compact and luminous groups.
We detected 10 compact galaxy groups (CGs) at $z=0$ in the semi-analytic galaxy catalog of Guo et al. (2011) for the milli-Millennium Cosmological Simulation (sCGs in mGuo2010a). We aimed to identify potential canonical pathways for compact group evolution and thus illuminate the history of observed nearby compact groups. By constructing merger trees for $z=0$ sCG galaxies, we studied the cosmological evolution of key properties, and compared them with $z=0$ Hickson CGs (HCGs). We found that, once sCG galaxies come within 1 (0.5) Mpc of their most massive galaxy, they remain within that distance until $z=0$, suggesting sCG birth redshifts. At $z=0$ stellar masses of sCG most-massive galaxies are within $10^{10} lesssim M_{ast}/M_{odot} lesssim 10^{11}$. In several cases, especially in the two 4- and 5-member systems, the amount of cold gas mass anti-correlates with stellar mass, which in turn correlates with hot gas mass. We define the angular difference between group members 3D velocity vectors, $Deltatheta_{rm vel}$, and note that many of the groups are long-lived because their small values of $Deltatheta_{rm vel}$ indicate a significant parallel component. For triplets in particular, $Deltatheta_{rm vel}$ values range between $20^{circ}$ and $40^{circ}$ so that galaxies are coming together along roughly parallel paths, and pairwise separations do not show large pronounced changes after close encounters. The best agreement between sCG and HCG physical properties is for $M_{ast}$ galaxy values, but HCG values are higher overall, including for SFRs. Unlike HCGs, due to a tail at low SFR and $M_{ast}$, and a lack of $M_{ast}gtrsim 10^{11}M_{odot}$ galaxies, only a few sCG galaxies are on the star-forming main sequence.
(Abridged) Fossil systems are group- or cluster-sized objects whose luminosity is dominated by a very massive central galaxy. In the current cold dark matter scenario, these objects formed hierarchically at an early epoch of the Universe and then slowly evolved until present day. That is the reason why they are called {it fossils}. We started an extensive observational program to characterize a sample of 34 fossil group candidates spanning a broad range of physical properties. Deep $r-$band images were taken for each candidate and optical spectroscopic observations were obtained for $sim$ 1200 galaxies. This new dataset was completed with SDSS DR7 archival data to obtain robust cluster membership and global properties of each fossil group candidate. For each system, we recomputed the magnitude gaps between the two brightest galaxies ($Delta m_{12}$) and the first and fourth ranked galaxies ($Delta m_{14}$) within 0.5 $R_{{rm 200}}$. We consider fossil systems those with $Delta m_{12} ge 2$ mag or $Delta m_{14} ge 2.5$ mag within the errors. We find that 15 candidates turned out to be fossil systems. Their observational properties agree with those of non-fossil systems. Both follow the same correlations, but fossils are always extreme cases. In particular, they host the brightest central galaxies and the fraction of total galaxy light enclosed in the central galaxy is larger in fossil than in non-fossil systems. Finally, we confirm the existence of genuine fossil clusters. Combining our results with others in the literature, we favor the merging scenario in which fossil systems formed due to mergers of $L^ast$ galaxies. The large magnitude gap is a consequence of the extreme merger ratio within fossil systems and therefore it is an evolutionary effect. Moreover, we suggest that at least one candidate in our sample could represent a transitional fossil stage.
Future galaxy surveys require realistic mock catalogues to understand and quantify systematics in order to make precise cosmological measurements. We present a halo lightcone catalogue and halo occupation distribution (HOD) galaxy catalogue built using the Millennium-XXL (MXXL) simulation. The halo catalogue covers the full sky, extending to z = 2 with a mass resolution of ~1e11 Msun/h . We use this to build a galaxy catalogue, which has an r-band magnitude limit of r < 20.0, with a median redshift of z~0.2. A Monte Carlo HOD method is used to assign galaxies to the halo lightcone catalogue, and we evolve the HODs to reproduce a target luminosity function; by construction, the luminosity function of galaxies in the mock is in agreement with the Sloan Digital Sky Survey (SDSS) at low redshifts and the Galaxy and Mass Assembly (GAMA) survey at high redshifts. A Monte Carlo method is used to assign a 0.1(g-r) colour to each galaxy, and the colour distribution of galaxies at different redshifts agrees with measurements from GAMA. The clustering of galaxies in the mock for galaxies in different magnitude and redshift bins is in good agreement with measurements from SDSS and GAMA, and the colour-dependent clustering is in reasonable agreement. We show that the baryon acoustic oscillation (BAO) can be measured in the mock catalogue, and the redshift space distortions (RSDs) are in agreement with measurements from SDSS, illustrating that this catalogue will be useful for upcoming surveys.
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