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Register a new userThe Evolution of Galaxy Clustering in Hierarchical Models
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The main ingredients of recent semi-analytic models of galaxy formation are summarised. We present predictions for the galaxy clustering properties of a well specified LCDM model whose parameters are constrained by observed local galaxy properties. We present preliminary predictions for evolution of clustering that can be probed with deep pencil beam surveys.
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Advances in extragalactic astronomy have prompted the development of increasingly realistic models which aim to describe the formation and evolution of galaxies. We review the philosophy behind one such technique, called semi-analytic modelling, and explain the relation between this approach and direct simulations of gas dynamics. Finally, we present model predictions for the evolution of the stellar mass of galaxies in a universe in which structure formation is hierarchical.
We investigate the star formation activity in galaxy pairs in chemical hydrodynamical simulations consistent with a Lambda-CDM scenario. A statistical analysis of the effects of galaxy interactions on the star formation activity as a function of orbital parameters shows that close encounters (r < 30 kpc/h) can be effectively correlated with an enhancement of star formation activity with respect to galaxies without a close companion. Our results suggest that the stability properties of systems are also relevant in this process. We found that the passive star forming galaxies pairs tend to have deeper potential wells, older stellar populations, and less leftover gas than active star forming ones. In order to assess the effects that projection and interlopers could introduce in observational samples, we have also constructed and analysed projected simulated catalogs of galaxy pairs. In good agreement with observations, our results show a threshold (rp < 25 kpc/h) for interactions to enhance the star formation activity with respect to galaxies without a close companion. Finally, analysing the environmental effect, we detect the expected SFR-local density relation for both pairs and isolated galaxy samples, although the density dependence is stronger for galaxies in pairs suggesting a relevant role for interactions in driving this relation.
In hierarchical models, density fluctuations on different scales are correlated. This induces correlations between dark halo masses, their formation histories, and their larger-scale environments. In turn, this produces a correlation between galaxy properties and environment. This correlation is entirely statistical in nature. We show how the observed clustering of galaxies can be used to quantify the importance of this statistical correlation relative to other physical effects which may also give rise to correlations between the properties of galaxies and their surroundings. We also develop a halo model description of this environmental dependence of clustering.
The baryon fraction of galaxy clusters in numerical simulations is found to be dependant on the cluster formation method. In all cases, the gas is anti-biased compared with the dark matter. However, clusters formed hierarchically are found to be more depleted in baryons than clusters formed non-hierarchically. There is a depletion of 10 to 15% for hierarchically formed clusters while the depletion is less than 10% for those clusters formed non-hierarchically. This difference is dependent on the mass of the clusters. The mean baryon enrichment profile for the hierarchically formed clusters shows an appreciable baryon enhancement around the virial radius not seen in the clusters formed without substructure. If this phenomenon applies to real clusters, it implies that determinations of the baryon fractions in clusters of galaxies require data extending beyond the virial radius of the clusters in order to achieve an unbiased value.
We predict the structure and dynamics of disc galaxies using galaxy evolution models within a hierarchical formation scenario The halo mass aggregation histories, for a Lambda CDM model, were generated and used to calculate the virialization of dark matter (DM) haloes. A diversity of halo density profiles were obtained, the most typical one being close to the NFW profile. We modeled the formation of discs in centrifugal equilibrium within the evolving DM haloes using gas accretion rates proportional to the halo mass aggregation rates, and assuming detailed angular momentum conservation. We calculated the gravitational interactions between halo and disc, and the hydrodynamics, star formation, and evolution of the galaxy discs. We found that the slope and zero-point of the infrared Tully-Fisher relations (TFR) may be explained as a direct consequence of the cosmological initial conditions. This relation is almost independent of the assumed disc mass fraction. The rms scatter of the TFR originates mainly from the scatter in the DM halo structure and, to a minor extension, from the dispersion of the primordial spin parameter. The scatter obtained does not disagree with the observational estimates. Our models allow us to understand why the residuals of the TFR do not correlate significantly with disc size or surface brightness (SB), and why low and high SB galaxies have the same TFR. The correlations between gas fraction and SB, and between scale length and V_max agree with those observed. The discs present nearly exponential SB distributions. The shape of the rotation curves changes with the SB and is nearly flat for most cases. The rotation curve decompositions show a dominance of DM down to very small radii. The introduction of shallow cores in the DM halo attenuates this difficulty.
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