Gabriella De Lucia

The hierarchical formation of the brightest cluster galaxies

We use semi-analytic techniques to study the formation and evolution of brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of these objects and discuss the limitations of simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by active galactic nucleus (AGN) activity, the stars of BCGs are formed very early (50 per cent at z ∼ 5, 80 per cent at z ∼ 3) and in many small galaxies. The high star formation rates in these high-z progenitors are fuelled by rapid cooling, not by merger-triggered starbursts. We find that model BCGs assemble surprisingly late: half their final mass is typically locked up in a single galaxy after z ∼ 0.5. Because most of the galaxies accreted on to BCGs have little gas content and red colours, late mergers do not change the apparent age of BCGs. It is this accumulation of a large number of old stellar populations - driven mainly by the merging history of the dark matter halo itself - that yields the observed homogeneity of BCG properties. In the second part of the paper, we discuss the evolution of BCGs to high redshifts, from both observational and theoretical viewpoints. We show that our model BCGs are in qualitative agreement with high-z observations. We discuss the hierarchical link between high-z BCGs and their local counterparts. We show that high-z BCGs belong to the same population as the massive end of local BCG progenitors, although they are not in general the same galaxies. Similarly, high-z BCGs end up as massive galaxies in the local Universe, although only a fraction of them are actually BCGs of massive clusters.

From dwarf spheroidals to cD galaxies: simulating the galaxy population in a ΛCDM cosmology

We have updated and extended our semi-analytic galaxy formation modelling capabilities and applied them simultaneously to the stored halo/subhalo merger trees of the Millennium and Millennium-II simulations. These differ by a factor of 125 in mass resolution, allowing explicit testing of resolution effects on predicted galaxy properties. We have revised the treatments of the transition between the rapid infall and cooling flow regimes of gas accretion, of the sizes of bulges and of gaseous and stellar disks, of supernova feedback, of the transition between central and satellite status as galaxies fall into larger systems, and of gas and star stripping once they become satellites. Plausible values of efficiency and scaling parameters yield an excellent fit not only to the observed abundance of low-redshift galaxies over 5 orders of magnitude in stellar mass and 9 magnitudes in luminosity, but also to the observed abundance of Milky Way satellites. This suggests that reionisation effects may not be needed to solve the “missing satellite” problem except, perhaps, for the faintest objects. The same model matches the observed large-scale clustering of galaxies as a function of stellar mass and colour. The fit remains excellent down to � 30 kpc for massive galaxies. For M∗ < 6×10 10 M⊙, however, the model overpredicts clustering at scales below � 1 Mpc, suggesting that the assumed fluctuation amplitude, σ8 = 0.9, is too high. The observed difference in clustering between active and passive galaxies is matched quite well for all masses. Galaxy distributions within rich clusters agree between the simulations and match those observed, but only if galaxies without dark matter subhalos (so-called orphans) are included. Even at MS-II resolution, schemes which assign galaxies only to resolved dark matter subhalos cannot match observed clusters. Our model predicts a larger passive fraction among low-mass galaxies than is observed, as well as an overabundance of � 10 10 M⊙ galaxies beyond z � 0.6. (The abundance of � 10 11 M⊙ galaxies is matched out to z � 3.) These discrepancies appear to reflect deficiencies in the way star-formation rates are modelled.

The many manifestations of downsizing: hierarchical galaxy formation models confront observations

It has been widely claimed that several lines of observational evidence point towards a ‘downsizing’ of the process of galaxy formation over cosmic time. This behaviour is sometimes termed ‘antihierarchical’, and contrasted with the ‘bottom-up’ (small objects form first) assembly of the dark matter structures in cold dark matter (CDM) models. In this paper, we address three different kinds of observational evidence that have been described as ‘downsizing’: the stellar mass assembly (i.e. more massive galaxies assemble at higher redshift with respect to low-mass ones), star formation rate (SFR) (i.e. the decline of the specific star formation rate is faster for more massive systems) and the ages of the stellar populations in local galaxies (i.e. more massive galaxies host older stellar populations). We compare a broad compilation of available data sets with the predictions of three different semi-analytic models

The Evolution of the Star Formation Activity in Galaxies and Its Dependence on Environment

We study how the proportion of star-forming galaxies evolves between z ¼ 0:8 and 0 as a function of galaxy environment,usingtheOiilineinemissionasasignatureofongoingstarformation.Our high-zdatasetcomprises16 clusters, 10 groups, and another 250 galaxies in poorer groups and the field at z ¼ 0:4 0:8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use galaxy systems selected from the Sloan Digital Sky Survey (SDSS) at 0:04 < z < 0:08. At high z most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z ¼ 0:4 0:8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the O ii equivalent width) in star-forming galaxies is also found to vary systematically with environment. SDSS clusters have much lower fractions of starforming galaxies than clusters at z ¼ 0:4 0:8 and, in contrast with the distant clusters, show a plateau for velocity dispersions � 550kms � 1 ,where thefraction ofgalaxieswithOiiemission doesnotvarysystematicallywithvelocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find that it is strongest in intermediate-mass systems (� � 500 600 km s � 1 at z ¼ 0). To understandtheoriginoftheobservedtrends,weusethePress-Schechter formalismandtheMillenniumSimulationandshow thatgalaxystarformationhistoriesmaybecloselyrelatedtothegrowthhistoryofclustersandgroups.Ifthescenariowe propose is roughly correct, the link between galaxy properties and environment is extremely simple to predict purely from a knowledge of the growth of dark matter structures. Subject headings: cosmology: observations — galaxies: clusters: general — galaxies: evolution — galaxies: fundamental parameters — galaxies: stellar content

Modelling galaxy clustering in a high-resolution simulation of structure formation

We use the Millennium Simulation, a 10 billion particle simulation of the growth of cosmic structure, to construct a new model of galaxy clustering. We adopt a methodology that falls midway between the traditional semi-analytic approach and the halo occupation distribution (HOD) approach. In our model, we adopt the positions and velocities of the galaxies that are predicted by following the orbits and merging histories of the substructures in the simulation. Rather than using star formation and feedback ‘recipes’ to specify the physical properties of the galaxies, we adopt parametrized functions to relate these properties to the quantity Minfall, defined as the mass of the halo at the epoch when the galaxy was last the central dominant object in its own halo. We test whether these parametrized relations allow us to recover the basic statistical properties of galaxies in the semi-analytic catalogues, including the luminosity function, the stellar mass function and the shape and amplitude of the two-point correlation function evaluated in different stellar mass and luminosity ranges. We then use our model to interpret recent measurements of these quantities from Sloan Digital Sky Survey (SDSS) data. We derive relations between the luminosities and the stellar masses of galaxies in the local Universe and their host halo masses. Our results are in excellent agreement with recent determinations of these relations by Mandelbaum et al. using galaxy‐galaxy weak lensing measurements from the SDSS.

The relation between star formation, morphology, and local density in high-redshift clusters and groups

We investigate how the [O II] properties and the morphologies of galaxies in clusters and groups at z = 0.4–0.8 depend on projected local galaxy density, and compare with the field at similar redshifts and clusters at low z. In both nearby and distant clusters, higher density regions contain proportionally fewer star-forming galaxies, and the average [O II] equivalent width of star-forming galaxies is independent of local density. However, in distant clusters the average current star formation rate (SFR) in star-forming galaxies seems to peak at densities ~15-40 galaxies Mpc^−2. At odds with low-z results, at high z the relation between star-forming fraction and local density varies from high- to low-mass clusters. Overall, our results suggest that at high z the current star formation (SF) activity in star-forming galaxies does not depend strongly on global or local environment, though the possible SFR peak seems at odds with this conclusion. We find that the cluster SFR normalized by cluster mass anticorrelates with mass and correlates with the star-forming fraction. These trends can be understood given (1) that the average star-forming galaxy forms about 1⊙M yr^−1 (uncorrected for dust) in all clusters; (2) that the total number of galaxies scales with cluster mass; and (3) the dependence of star-forming fraction on cluster mass. We present the morphology-density (MD) relation for our z = 0.4 − 0.8 clusters, and uncover that the decline of the spiral fraction with density is entirely driven by galaxies of type Sc or later. For galaxies of a given Hubble type, we see no evidence that SF properties depend on local environment. In contrast with recent findings at low z, in our distant clusters the SF-density relation and the MD relation are equivalent, suggesting that neither of the two is more fundamental than the other.

The recycling of gas and metals in galaxy formation: predictions of a dynamical feedback model

We present results of a new feedback scheme implemented in the Munich galaxy formation model. The new scheme includes a dynamical treatment of galactic winds powered by supernova explosions and stellar winds in a cosmological context. We find that such a scheme is a good alternative to empirically motivated recipes for feedback in galaxy formation. Model results are in good agreement with the observed luminosity functions and stellar mass function for galaxies in the local universe. In particular, the new scheme predicts a number density of dwarfs that is lower than in previous models. This is a consequence of a new feature of the model, which allows an estimate of the amount of mass and metals that haloes can permanently deposit into the intergalactic medium (IGM). This loss of material leads to the suppression of star formation in small haloes and therefore to the decrease in the number density of dwarf galaxies. The model is able to reproduce the observed mass‐stellar metallicity and luminosity‐gas metallicity relationships. This demonstrates that our scheme provides a significant improvement in the treatment of the feedback in dwarf galaxies. Despite these successes, our model does not reproduce the observed bimodality in galaxy colours and predicts a larger number of bright galaxies than observed. Finally, we investigate the efficiency of metal injection in winds and in the IGM. We find that galaxies that reside in haloes with Mvir < 1

The dependence of galaxy formation on cosmological parameters: can we distinguish between the WMAP1 and WMAP3 parameter sets?

We combine N-body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological parameters between the first- and third-year results from the Wilkinson Microwave Anisotropy Probe (WMAP) affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering are barely altered. In both cosmologies, several combinations of physical parameters can reproduce observed, low-redshift galaxy properties; the star formation, supernova feedback and active galactic nucleus feedback efficiencies can be played off against each other to give similar results. Models which fit observed luminosity functions predict projected two-point correlation functions which scatter by about 10-20 per cent on large scale and by larger factors on small scale, depending both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given present modelling uncertainties, it is not easy to distinguish between the WMAP] and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift. Better observational data at z > 2 will better constrain galaxy formation and perhaps also cosmological parameters.

Cluster galaxies die hard

We investigate how the specific star formation rates of galaxies of different masses depend on cluster-centric radius and on the central/satellite dichotomy in both field and cluster environments. Recent data from a variety of sources, including the cluster catalogue of von der Linden et al., are compared to the semi-analytic models of De Lucia & Blaizot. We find that these models predict too many passive satellite galaxies in clusters, too few passive central galaxies with low stellar masses and too many passive central galaxies with high masses. We then outline a series of modifications to the model necessary to solve these problems: (a) instead of instantaneous stripping of the external gas reservoir after a galaxy becomes a satellite, the gas supply is assumed to decrease at the same rate that the surrounding halo loses mass due to tidal stripping and (b) the active galactic nuclei (AGN) feedback efficiency is lowered to bring the fraction of massive passive centrals in better agreement with the data. We also allow for radio mode AGN feedback in satellite galaxies. (c) We assume that satellite galaxies residing

The population of Milky Way satellites in the LambdaCDM cosmology

We present a model for the satellites of the Milky Way in which galaxy formation is followed using semi-analytic techniques applied to the six high-resolution N-body simulations of galactic halos of the Aquarius project. The model, calculated using the Galform code, incorporates improved treatments of the relevant physics in the LambdaCDM cosmogony, particularly a self-consistent calculation of reionization by UV photons emitted by the forming galaxy population, including the progenitors of the central galaxy. Along the merger tree of each halo, the model calculates gas cooling (by Compton scattering off cosmic microwave background photons, molecular hydrogen and atomic processes), gas heating (from hydrogen photoionization and supernova energy), star formation and evolution. The evolution of the intergalactic medium is followed simultaneously with that of the galaxies. Star formation in the more massive progenitor subhalos is suppressed primarily by supernova feedback, while for smaller subhalos it is suppressed primarily by photoionization due to external and internal sources. The model is constrained to match a wide range of properties of the present day galaxy population as a whole, but at high redshift it requires an escape fraction of UV photons near unity in order completely to reionize the universe by redshift z ~ 8. In the most successful model the local sources photoionize the pre-galactic region completely by z ~ 10. In addition to the luminosity function of Milky Way satellites, the model matches their observed luminosity-metallicity relation, their radial distribution and the inferred values of the mass within 300 pc, which in the models increase slowly but significantly with luminosity. There is a large variation in satellite properties from halo to halo, with the luminosity function, for example, varying by a factor of ~ 2 among the six simulations.