arXiv subjects

Gabriella De Lucia

Publications and source records attributed to Gabriella De Lucia.

162 records · Page 9Linked to original sources

arXiv2008

The Galaxy and its stellar halo - insights from a hybrid cosmological approach

We use a series of high-resolution N-body simulations of a `Milky-Way' halo, coupled to semi-analytic techniques, to study the formation of our own Galaxy and of its stellar halo. Our model Milky Way galaxy is a relatively young system whose physical properties are in quite good agreement with observational determinations. In our model, the stellar halo is mainly formed from a few massive satellites accreted early on during the galaxy's lifetime. The stars in the halo do not exhibit any metallicity gradient, but higher metallicity stars are more centrally concentrated than stars with lower abundances. This is due to the fact that the most massive satellites contributing to the stellar halo are also more metal rich, and dynamical friction drags them closer to the inner regions of the host halo.

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arXiv2008

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

We investigate how the [OII] 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 [OII] 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 a) that the average star-forming galaxy forms about 1 Msun/yr in all clusters; b) that the total number of galaxies scales with cluster mass and c) 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 types 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.(abr.)

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arXiv2008

The evolution of the colour-magnitude relation and of the star formation activity in galaxy clusters since z~0.8

We present recent results on the evolution of the colour-magnitude relation and of the star formation activity in galaxy clusters since z~0.8. Results are based on the ESO Distant Cluster Survey (EDisCS) - an ESO large programme aimed at the study of cluster structure and cluster galaxy evolution over a significant fraction of cosmic time - and are discussed in the framework of the current standard paradigm of structure formation.

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arXiv2008

Spectroscopy of clusters in the ESO distant cluster survey (EDisCS).II. Redshifts, velocity dispersions, and substructure for clusters in the last 15 fields

AIMS. We present spectroscopic observations of galaxies in 15 survey fields as part of the ESO Distant Cluster Survey (EDisCS). We determine the redshifts and velocity dispersions of the galaxy clusters located in these fields, and we test for possible substructure in the clusters. METHODS. We obtained multi-object mask spectroscopy using the FORS2 instrument at the VLT. We reduced the data with particular attention to the sky subtraction. We implemented the method of Kelson for performing sky subtraction prior to any rebinning/interpolation of the data. From the measured galaxy redshifts, we determine cluster velocity dispersions using the biweight estimator and test for possible substructure in the clusters using the Dressler-Shectman test. RESULTS. The method of subtracting the sky prior to any rebinning/interpolation of the data delivers photon-noise-limited results, whereas the traditional method of subtracting the sky after the data have been rebinned/interpolated results in substantially larger noise for spectra from tilted slits. Redshifts for individual galaxies are presented and redshifts and velocity dispersions are presented for 21 galaxy clusters. For the 9 clusters with at least 20 spectroscopically confirmed members, we present the statistical significance of the presence of substructure obtained from the Dressler-Shectman test, and substructure is detected in two of the clusters. CONCLUSIONS. Together with data from our previous paper, spectroscopy and spectroscopic velocity dispersions are now available for 26 EDisCS clusters with redshifts in the range 0.40-0.96 and velocity dispersions in the range 166-1080 km/s.

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arXiv2007

The Dependence of Galaxy Formation on Cosmological Parameters: Can we distinguish 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 1-year and 3-year results from the Wilkinson Microwave Anisotropy Probe 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 AGN feedback efficiencies can be played off against each other to give similar results. Models which fit observed luminosity functions predict projected 2-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 current modelling uncertainties, it is not easy to distinguish the WMAP1 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.

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arXiv2007

Realistic simulations of gravitational lensing by galaxy clusters: extracting arc parameters from mock DUNE images

We present a newly developed code that allows simulations of optical observations of galaxy fields with a variety of instruments. The code incorporates gravitational lensing effects and is targetted at simulating lensing by galaxy clusters. Our goal is to create the tools required for comparing theoretical expectations with observations to obtain a better understanding of how observational noise affects lensing applications such as mass estimates, studies on the internal properties of galaxy clusters and arc statistics. Starting from a set of input parameters, characterizing both the instruments and the observational conditions, the simulator provides a virtual observation of a patch of the sky. It includes several sources of noise such as photon-noise, sky background, seeing, and instrumental noise. Ray-tracing through simulated mass distributions accounts for gravitational lensing. Source morphologies are realistically simulated based on shapelet decompositions of galaxy images retrieved from the GOODS-ACS archive. According to their morphological class, spectral-energy-distributions are assigned to the source galaxies in order to reproduce observations of each galaxy in arbitrary photometric bands. We illustrate our techniques showing virtual observations of a galaxy-cluster core as it would be observed with the space telescope DUNE, which was recently proposed to ESA within its "Cosmic vision" programme. (Abridged)

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arXiv2007

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 supernovae 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, that allows an estimate of the amount of mass and metals that haloes can permanently deposit into the 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 intergalactic medium. We find that galaxies that reside in haloes with M_vir < 10^12 M_sun/h may deposit most of their metal mass into the intergalactic medium, while groups and clusters at z=0 have lost at most a few percent of their metals before the bulk of the halo mass was accreted.

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arXiv2007

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 M_{infall}, 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 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.

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arXiv2007

Modelling and interpreting the dependence of clustering on the spectral energy distributions of galaxies

We extend our previous physically-based halo occupation distribution models to include the dependence of clustering on the spectral energy distributions of galaxies. The high resolution Millennium Simulation is used to specify the positions and the velocities of the model galaxies. The stellar mass of a galaxy is assumed to depend only on M_{infall}, the halo mass when the galaxy was last the central dominant object of its halo. Star formation histories are parametrized using two additional quantities that are measured from the simulation for each galaxy: its formation time (t_{form}), and the time when it first becomes a satellite (t_{infall}). Central galaxies begin forming stars at time t_{form} with an exponential time scale tau_c. If the galaxy becomes a satellite, its star formation declines thereafter with a new time scale tau_s. We compute 4000 Åbreak strengths for our model galaxies using stellar population synthesis models. By fitting these models to the observed abundances and projected correlations of galaxies as a function of break strength in the Sloan Digital Sky Survey, we constrain tau_c and tau_s as functions of galaxy stellar mass. We find that central galaxies with large stellar masses have ceased forming stars. At low stellar masses, central galaxies display a wide range of different star formation histories, with a significant fraction experiencing recent starbursts. Satellite galaxies of all masses have declining star formation rates, with similar e-folding times, tau_s ~ 2.5Gyr. One consequence of this long e-folding time is that the colour-density relation is predicted to flatten at redshifts > 1.5, because star formation in the majority of satellites has not yet declined by a significant factor. This is consistent with recent observational results from the DEEP and VVDS surveys.

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arXiv2006

Substructure in lensing clusters and simulations

We present high-resolution mass reconstructions for five massive cluster-lenses spanning a redshift range from $z = 0.18$--0.57 utilising archival {\it Hubble Space Telescope} ({\it HST}) data and applying galaxy-galaxy lensing techniques. These detailed mass models were obtained from the observations by combining constraints from the strong and weak lensing regimes. We ascribe local weak distortions in the shear maps to perturbations induced by the presence of galaxy haloes around individual bright early-type cluster member galaxies. This technique constrains the mass enclosed within an aperture for these subhaloes. We are sensitive to a specific mass range for these subhaloes, $10^{11}$ -- $10^{12.5} \msun$, which we associate with galaxy-scale subhaloes. Adopting a parametric model for the subhaloes, we also derive their velocity dispersion function and the aperture radius function. The mass spectrum of substructure in the inner regions of the observed clusters is directly compared with that in simulated clusters extracted from the {\it Millennium Simulation}. The massfunction, aperture radii and velocity dispersion function are compared in detail. Overall, we find good agreement between the distribution of substructure properties retrieved using the lensing analysis and those obtained from the simulation (truncated abstract).

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arXiv2006

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 limits of simple ways to capture their evolution. In a model where cooling flows are suppressed at late times by 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 onto 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 counter-parts. 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.

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arXiv2006

How `heredity' and `environment' shape galaxy properties

In this review, I give a brief summary of galaxy evolution processes in hierarchical cosmologies and of their relative importance at different masses, times, and environments. I remind the reader of the processes that are commonly included in modern semi-analytic models of galaxy formation, and I comment on recent results and open issues.

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arXiv2006

The build-up of the colour-magnitude relation in galaxy clusters since z~0.8

Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z~0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high redshift EDisCS clusters exhibit a significant "deficit" of faint red galaxies. Combining clusters in three different redshift bins, and defining as `faint' all galaxies in the range 0.4 > L/L* > 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z~0.8 to z~0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved onto the red-sequence below z~0.8.

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arXiv2005

The formation history of elliptical galaxies

We take advantage of the largest high-resolution simulation of cosmic structure growth ever carried out -- the Millennium Simulation of the concordance LambdaCDM cosmogony -- to study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass. We concentrate on a galaxy formation model which is tuned to fit the joint luminosity/colour/morphology distribution of low redshift galaxies. Massive ellipticals in this model have higher metal abundances, older luminosity-weighted ages, shorter star formation timescales, but lower assembly redshifts than less massive systems. Within clusters the typical masses, ages and metal abundances of ellipticals are predicted to decrease, on average, with increasing distance from the cluster centre. We also quantify the effective number of progenitors of ellipticals as a function of present stellar mass, finding typical numbers below 2 for M* < 10^{11} Msun, rising to about 5 for the most massive systems. These findings are consistent with recent observational results that suggest ``down-sizing'' or ``anti-hierarchical'' behaviour for the star formation history of the elliptical galaxy population, despite the fact that our model includes all the standard elements of hierarchical galaxy formation and is implemented on the standard, LambdaCDM cosmogony.

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arXiv2005

Gas Infall and Stochastic Star Formation in Galaxies in the Local Universe

We study the recent star formation histories of local galaxies by analyzing the scatter in their colours and spectral properties. We present evidence that the distribution of star formation histories changes qualitatively above a characteristic stellar surface mass density of 3x10^8 M_sol kpc^-2, corresponding to the transition between disk-dominated and bulge-dominated galaxies. When we average over subpopulations of galaxies with densities below this value, we find that subpopulations of all masses form their stars at the same average rate per unit stellar mass. However, the scatter in galaxy colours and emission line strengths is larger for more compact galaxies of a given mass. This suggests that star formation occurs in shorter, higher amplitude events in galaxies with smaller sizes. Above the characteristic density, galaxy growth through star formation shuts down. We propose that star formation events are triggered when cold gas is accreted onto a galaxy. We have used the new high resolution Millennium Simulation of structure formation in a concordance LCDM Universe to quantify the incidence of these accretion events and we show that the observational data are well fit by a model in which the consumption time of accreted gas decreases with the surface density of the galaxy as t_cons \propto mu*^-1. The dark matter halos hosting massive galaxies with high densities are also expected to grow through accretion, but the observations indicate that star formation in bulge- dominated galaxies is no longer coupled to the hierarchical build-up of their surrounding dark matter halos.

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arXiv2004

Galaxies and subhaloes in LCDM galaxy clusters

We combine 10 high resolution resimulations of cluster--sized dark haloes with semi--analytic galaxy formation modelling in order to compare the number density and velocity dispersion profiles of cluster galaxies with those of dark matter substructures (subhaloes). While the radial distribution of galaxies follows closely that of the dark matter, the distribution of dark matter subhaloes is much less centrally concentrated. The velocity dispersion profiles of galaxies are also very similar to those of the dark matter, while those for subhaloes are biased high, particularly in the inner regions of clusters. We explain how these differences, already clearly visible in earlier work, are a consequence of the formation of galaxies at the centres of dark matter haloes. Galaxies and subhaloes represent different populations and are not directly comparable. Evolution produces a complex and strongly position--dependent relation between galaxies and the subhaloes in which they reside. This relation can be properly modelled only by appropriate physical representation of the galaxy formation process.

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arXiv2004

Chemical enrichment of the intra-cluster and intergalactic medium in a hierarchical galaxy formation model

(Abridged) We use a combination of high resolution N-body simulations and semi-analytic techniques to follow the formation, the evolution and the chemical enrichment of cluster galaxies in a Lambda-CDM Universe. We model the transport of metals between the stars, the cold gas in galaxies, the hot gas in dark matter haloes, and the intergalactic gas outside virialized haloes. We have compared three different feedback schemes. The `retention' model assumes that material reheated by supernova explosions is able to leave the galaxy, but not the dark matter halo. The `ejection' model assumes that this material leaves the halo and is then re-incorporated when structure collapses on larger scales. The `wind' model uses prescriptions that are motivated by observations of local starburst galaxies. We require that our models reproduce the cluster LF from the 2dF survey, the relations between stellar mass, gas mass and metallicity inferred from new SDSS data, and the observed amount of metals in the ICM. With suitable adjustment of the free parameters in the model, a reasonable fit to the observational results at redshift zero can be obtained for all three feedback schemes. All three predict that the chemical enrichment of the ICM occurs at high redshift with 60-80 per cent of the metals currently in the ICM ejected at redshifts larger than 1. Massive galaxies are important contributors to the chemical pollution. The observed decline in baryon fraction from rich clusters to galaxy groups is reproduced only in an `extreme' ejection scheme, where material ejected from dark matter haloes is re-incorporated on a timescale comparable to the age of the Universe. We explore how the metal abundance in the intergalactic medium as a function of redshift can constraint how and when galaxies ejected their metals.

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arXiv2003

Evolution of galaxies in clusters: semi-analytic models and observations

We present two different projects. The first one is related to the development of a semi-analytic model to follow the formation, the evolution and chemical enrichment of cluster galaxies in a hierarchical dark matter model. The second project concerns the comparison between high resolution N-body simulations of clusters of galaxies with the EDisCS observational dataset.

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