arXiv subjects

Simon D. M. White

Publications and source records attributed to Simon D. M. White.

At least 217 records · Page 12Linked to original sources

arXiv2005

Supernova outflows in galaxy formation

We investigate the generation of galactic outflows by supernova feedback in the context of SPH cosmological simulations. We use a modified version of the code GADGET-2 which includes chemical enrichment and energy feedback by Supernova. We find that energy feedback plays a fundamental role in the evolution of galaxies, heating up the cold material in the centre of the haloes and triggering outflows which efficiently transport gas from the centre to the outskirts of galaxies. The impact of feedback is found to depend on the virial mass of the system with smaller systems, such as dwarf galaxies, being more strongly affected. The outflows help to establish a self-regulated star formation process, and to transport a significant amount of metals into the haloes and even out of the systems. According to our results, energy feedback by supernovae could be the mechanism responsible for the chemical enrichment of the intergalactic medium.

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arXiv2005

Early structure in LCDM

We use a novel technique to simulate the growth of the most massive progenitor of a protocluster region from redshift z~80, when its mass was about 10 Msun until the present day. Our nested sequence of N-body resimulations allows us to study in detail the structure both of the dark matter object itself and of its environment. Our effective resolution is optimal at redshifts of 49, 29, 12, 5 and 0 when the dominant object has mass 1.2e5, 5e7, 2e10, 3e12 and 8e14 Msun/hrespectively, and contains ~1e6 simulation particles within its virial radius. Extended Press-Schechter theory correctly predicts both this rapid growth and the substantial overabundance of massive haloes we find at early times in regions surrounding the dominant object. Although the large-scale structure in these regions differs dramatically from a scaled version of its present-day counterpart, the internal structure of the dominant object is remarkably similar. Molecular hydrogen cooling could start as early as z~49 in this object, while cooling by atomic hydrogen becomes effective at z~39. If the first stars formed in haloes with virial temperature ~2000, their comoving abundance by z=49 would be similar to that of dwarf galaxies today, while their comoving correlation length would be ~2.5Mpc/h.

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arXiv2005

The age dependence of halo clustering

We use a very large simulation of the concordance LCDM cosmogony to study the clustering of dark matter haloes. For haloes less massive than about 1e13Msun/h, the amplitude of the two-point correlation function on large scales depends strongly on halo formation time. Haloes that assembled at high redshift are substantially more clustered than those that assembled more recently. The effect is a smooth function of halo formation time and its amplitude increases with decreasing halo mass. At 1e11 Msun/h the ``oldest'' 10% of haloes are more than 5 times more strongly correlated than the ``youngest'' 10%. This unexpected result is incompatible with the standard excursion set theory for structure growth, and it contradicts a fundamental assumption of the halo occupation distribution models often used to study galaxy clustering, namely that the galaxy content of a halo of given mass is statistically independent of its larger scale environment.

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arXiv2005

Quantitative Morphology of Galaxies from the SDSS I: Luminosity in Bulges and Disks

In the first paper of this series we use the publicly available code Gim2D to model the r- and i-band images of all galaxies in a magnitude-limited sample of roughly 1800 morphologically classified galaxies taken from the Sloan Digital Sky Survey. The model is a concentric superposition of two components, each with elliptical isophotes with constant flattening and position angle. The disk luminosity profile is assumed exponential, while the bulge is assumed to have a de Vaucouleurs or a Sersic profile. We find that the parameters returned by Gim2D depend little on the waveband or bulge profile used; their formal uncertainties are usually small. Nevertheless, for bright galaxies the measured distribution of b/a, the apparent disk flattening, deviates strongly from the expected uniform distribution, showing that the `disk' identified by the code frequently corresponds to an intrinsically 3-dimensional structure rather than to a true thin disk. We correct approximately for this systematic problem using the observed statistics of the b/a distribution and estimate, as a function of absolute magnitude, the mean fractions of galaxy light in disks and in `pure bulge' systems (those with no detectable disk). For the brightest galaxies the disk light fraction is about 10% and about 80% are `pure bulge' systems. For faint galaxies most of the light is in disks and we do not detect a `pure bulge' population. Averaging over the galaxy population as a whole, we find that 54 \pm 2% of the local cosmic luminosity density at both r and i comes from disks and 32 \pm 2% from `pure bulge' systems. The remaining 14 \pm 2% comes from bulges in galaxies with detectable disks.

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arXiv2005

The ages and metallicities of galaxies in the local universe

We derive stellar metallicities, light-weighted ages and stellar masses for a magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the alpha/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively star-forming galaxies. We show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000AA-break strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption at fixed D4000 is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration galaxies follow these relations, but there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass. Despite the large scatter, the relation between stellar metallicity and stellar mass is similar to the correlation between gas-phase oxygen abundance and stellar mass for star-forming galaxies. [abriged]

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arXiv2005

Diffuse light in z~0.25 galaxy clusters: constraining tidal damage and the faint end of the Luminosity Function

The starlight coming from the intergalactic space in galaxy clusters and groups witnesses the violent tidal interactions that galaxies experience in these dense environments. Such interactions may be (at least partly) responsible for the transformation of normal star-forming galaxies into passive dwarf ellipticals (dEs). In this contribution we present the first systematic study of the IntraCluster Light (ICL) for a statistically representative sample (Zibetti et al. 2005), which comprises 683 clusters selected between z=0.2 and 0.3 from ~1500 deg^2 in the SDSS. Their ICL is studied by stacking the images in the g-, r-, and i-band after masking out all galaxies and polluting sources. In this way a very uniform background illumination is obtained, that allows us to measure surface brightnesses as faint as 31 mag/arcsec^2 and to trace the ICL out to 700 kpc from the central galaxy. We find that the local fraction of light contributed by intracluster stars rapidly decreases as a function of the clustercentric distance, from ~40% at 100 kpc to ~5% at 500 kpc. By comparing the distribution and colours of the ICL and of the clusters galaxies, we find indication that the main source of ICL are the stars stripped from galaxies that plunge deeply into the cluster potential well along radial orbits. Thus, if dEs are the remnants of these stripped progenitors we should expect similar radial orbital anisotropies and correlations between the dE luminosity function and the amount of ICL in different clusters. The diffuse emission we measure is contaminated by faint unresolved galaxies: this makes our flux estimate depend to some extent on the assumed luminosity function, but, on the other hand, allows us to constrain the number of faint galaxies. Our present results disfavour steep (alpha<-1.35) faint-end powerlaw slopes.

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arXiv2005

Simulating the joint evolution of quasars, galaxies and their large-scale distribution

The cold dark matter model has become the leading theoretical paradigm for the formation of structure in the Universe. Together with the theory of cosmic inflation, this model makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a novel framework for the quantitative physical interpretation of such surveys. This combines the largest simulation of the growth of dark matter structure ever carried out with new techniques for following the formation and evolution of the visible components. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with next generation surveys.

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arXiv2005

Semi-analytic Simulations of Galactic Winds: Volume Filling Factor, Ejection of Metals and Parameter Study

We present a semi-analytic treatment of galactic winds within high resolution, large scale cosmological N-body simulations of a LCDM Universe. The evolution of winds is investigated by following the expansion of supernova driven superbubbles around the several hundred thousand galaxies that form in an approximately spherical region of space with diameter 52 Mpc/h and mean density close to the mean density of the Universe. We focus our attention on the impact of winds on the diffuse intergalactic medium. Initial conditions for mass loss at the base of winds are taken from Shu, Mo and Mao (2003). Results are presented for the volume filling factor and the mass fraction of the IGM affected by winds and their dependence on the model parameters is carefully investigated. The mass loading efficiency of bubbles is a key factor to determine the evolution of winds and their global impact on the IGM: the higher the mass loading, the later the IGM is enriched with metals. Galaxies with 10^9 < M_* < 10^10 M_sun are responsible for most of the metals ejected into the IGM at z=3, while galaxies with M_* < 10^9 M_sun give a non negligible contribution only at higher redshifts, when larger galaxies have not yet assembled. We find a higher mean IGM metallicity than Lyalpha forest observations suggest and we argue that the discrepancy may be explained by the high temperatures of a large fraction of the metals in winds, which may not leave detectable imprints in absorption in the Lyalpha forest.

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arXiv2005

Intergalactic stars in z~0.25 galaxy clusters: systematic properties from stacking of Sloan Digital Sky Survey imaging data

We analyse the spatial distribution and colour of the intracluster light (ICL) in 683 clusters of galaxies between z=0.2 and 0.3, selected from approx 1500 deg^2 of the SDSS-DR1. Surface photometry in the g, r and i bands is conducted on stacked images of the clusters, after rescaling them to the same metric size and masking out resolved sources. We are able to trace the average surface brightness profile of the ICL out to 700 kpc, where it is less than 1/10,000 of the mean surface brightness of the dark night sky. The ICL appears as a clear surface brightness excess with respect to an inner R^1/4 profile which characterises the mean profile of the BCG. The surface brightness (SB) of the ICL ranges from 27.5 mag/arcsec^2 at 100 kpc to roughly 32 at 700 kpc in the observed r-band (26.5 to 31 in the rest-frame g-band). We find that, on average, the ICL contributes only a small fraction of the total optical emission in a cluster (10.9+-5.0% within 500 kpc). The radial distribution of the ICL is more centrally concentrated than that of the cluster galaxies, but the colours of the two components are identical within the statistical uncertainties. In the mean the ICL is aligned with and more flattened than the BCG itself. This alignment is substantially stronger than that of the cluster light as a whole. The SB of the ICL correlates both with BCG luminosity and with cluster richness, while the fraction of the total light in the ICL is almost independent of these quantities. These results support the idea that the ICL is produced by stripping and disruption of galaxies as they pass through the central regions of clusters. Our measurements of the diffuse light also constrain the faint-end slope of the cluster LF. Slopes alpha<-1.35 would imply more light from undetected galaxies than is observed in the diffuse component.

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arXiv2004

The subhalo populations of LCDM dark haloes

We use a large cosmological simulation and a variety of high resolution resimulations of individual cluster and galaxy haloes to study the systematics of subhalo populations over ranges of 1000 in halo mass and 1000 in the ratio of subhalo to parent halo mass. The subhalo populations of different haloes are not scaled copies of each other, but vary systematically with halo properties. On average, the amount of substructure increases with halo mass. At fixed mass, it decreases with halo concentration and with halo formation redshift. These trends are comparable in size to the scatter in subhalo abundance between similar haloes. Averaged over all haloes of given mass, the abundance of low mass subhaloes per unit parent halo mass is independendent of parent mass.It is very similar to the abundance per unit mass of low mass haloes in the universe as a whole, once differing boundary definitions for subhaloes and haloes are accounted for. The radial distribution of subhaloes within their parent haloes is substantially less centrally concentrated than that of the dark matter. It varies at most weakly with the mass(or concentration) of the parent halo and not at all with subhalo mass. It does depend on the criteria used to define the subhalo population considered. About 90% of present-day subhaloes were accreted after z=1 and about 70% after z=0.5. Only about 8% the total mass of all haloes accreted at z=1 survives as bound subhaloes at z=0. For haloes accreted at z=2, the survival mass fraction is just 2%. Subhaloes seen near the centre of their parent typically were accreted earlier and retain less of their original mass than those seen near the edge. These strong systematics mean that comparison with galaxies in real clusters is only possible if the formation of the luminous component is modelled appropriately.

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arXiv2004

Early Formation and Late Merging of the Giant Galaxies

The most luminous galaxies in the present Universe are found at the centers of the most massive dark matter haloes, rich galaxy clusters. In the LCDM cosmology, such massive halo cores are present at redshift z=6 with a comoving number density (as a function of mass interior to ~10 kpc) that is comparable to today's value. The identity of the matter in these central regions is, however, predicted to change as major mergers bring together stars and dark matter from initially well separated sub-units. We use N-body simulations to investigate how these mergers push pre-existing matter outwards in the dominant galaxy while preserving the inner density profile of collisionless matter. It appears that the central regions of large galaxies end up dominated by stars formed in a number of dense cores, well before the last major mergers. The density profile of collisionless matter (stars and dark matter combined) in these central regions appears to be stable and to have attractor-like behavior under merging. This suggests that the baryon loading associated with dissipative contraction and star formation may be erased as subsequent mergers drive the mass distribution back to a universal profile. Such suppression of the effects of baryon loading, along with the early assembly of mass concentrations, may help resolve some apparent challenges to the CDM model for structure formation.

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arXiv2004

Stellar masses, star formation rates, metallicities and AGN properties for 200,000 galaxies in the SDSS Data Release Two (DR2)

By providing homogeneous photometric and spectroscopic data of high quality for very large and objectively selected samples of galaxies, the Sloan Digital Sky Survey allows statistical studies of the physical properties of galaxies and AGN to be carried out at an unprecedented level of precision and detail. Here we publicly release catalogues of derived physical properties for 211,894 galaxies, including 33,589 narrow-line AGN. These are complete samples with well understood selection criteria drawn from the normal galaxy spectroscopic sample in the second SDSS public data release (DR2). We list properties obtained from the SDSS spectroscopy and photometry using modelling techniques presented in papers already published by our group. These properties include: stellar masses; stellar mass-to-light ratios; attenuation of the starlight by dust; indicators of recent major starbursts; current total and specific star-formation rates, both for the regions with spectroscopy and for the galaxies as a whole; gas-phase metallicities; AGN classifications based on the standard emission line ratio diagnostic diagrams and AGN [OIII] emission line luminosities. We also list our own measurements of absorption line indices and emission line fluxes from which these quantities were derived, together with a few quantities from the standard SDSS pipelines which play an important role in our work. Many other observed properties of these galaxies can be obtained from the SDSS DR2 catalogues themselves. We will add further physical properties to this release site as the relevant papers are accepted for publication. Catalogies containing these parameters may be accessed at http://www.mpa-garching.mpg.de/SDSS/.

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arXiv2004

Present-Day Growth of Black Holes and Bulges: the SDSS Perspective

We investigate the accretion-driven growth of supermassive black holes in the low-redshift Universe using 23,000 ``Type 2'' AGN and the complete sample of 123,000 galaxies in the SDSS from which they were drawn. We use the stellar velocity dispersions of the early type galaxies and AGN hosts to estimate their black hole masses and we use the AGN [OIII]5007 emission line luminosities to estimate black hole accretion rates. We find that most present-day accretion occurs onto black holes with masses < 10E8 Msun that reside in moderately massive galaxies (M ~ 10E10 to 10E11.5 Msun) with high stellar densities (~10E8.5 to 10E9.5 Msun/kpc^2) and young stellar populations. The volume-averaged accretion rates of low mass black holes (<10E7.5 Msun) imply that this population is growing on a timescale that is comparable to the age of the Universe. Around half this growth takes place in AGN that are radiating within a factor of five of the Eddington luminosity. Such systems are rare, making up only 0.2% of the low mass black hole population at the present day. The rest of the growth occurs in lower luminosity AGN. The growth timescale is more than two orders of magnitude longer for the population of the most massive black holes in our sample. The volume averaged ratio of star formation to black hole accretion in bulge-dominated galaxies is ~1000, in remarkable agreement with the observed ratio of stellar mass to black hole mass in nearby galaxy bulges. We conclude: a) that bulge formation and black hole formation are tightly coupled, even in present-day galaxies; and b) that the evolution of the AGN luminosity function documented in recent optical and x-ray surveys is driven by a decrease in the characteristic mass scale of accreting black holes.

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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

Intracluster light at z~0.25 from SDSS imaging data

We investigate the broadband optical emission of diffuse intergalactic stars in galaxy clusters at z~0.25 by means of an image stacking technique. The images of 654 clusters, selected with the max-BCG algorithm from a subsample of the SDSS-Data Release 1 (DR1), have been stacked after masking all the sources detected down to very low S/N. The resulting images in the g, r and i bands provide reliable photometric data at the level of >=30 mag arcsec^-2 (r band), out to >~ 600 kpc from the Brightest Cluster Galaxy (BCG). Our analysis shows that: i) the IntraCluster Light (ICL) is much more concentrated than the galaxy light, contributing ~30% of the total cluster optical emission at R=100 kpc, but less than 10% at R>500 kpc; ii) the ICL contributes between 15 and 20% of the total cluster optical luminosity between the optical radius (mu_r=25 mag arcsec^-2) of the BCG and 500 kpc; iii) the colours of the ICL are consistent with the global colours of the cluster galaxies, with little evidence for redder g-r.

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arXiv2004

The Environmental Dependence of the Relations between Stellar Mass, Structure, Star Formation and Nuclear Activity in Galaxies

We use a complete sample of galaxies drawn from the SDSS to study how structure, star formation and nuclear activity depend on local density and on stellar mass. Local density is estimated by counting galaxies above a fixed absolute magnitude limit within cylinders 2 Mpc in projected radius and +-500 km/s in depth. The stellar mass distribution of galaxies shifts by nearly a factor of two towards higher masses between low and high density regions. At fixed stellar mass, both star formation and nuclear activity depend strongly on local density, while structural parameters such as size and concentration are almost independent of it. The galaxy property most sensitive to environment is specific star formation rate. For galaxies with stellar masses less than 3 x 10^10 M_sun, the median SFR/M* decreases by more than a factor of 10 from low to high densities. This decrease is less marked for massive galaxies. At fixed stellar mass, twice as many galaxies host AGN with strong [OIII] emission in low density regions as in high. Massive galaxies in low-density environments also contain more dust. We have analyzed correlations between spectroscopic indicators that probe SFH on different timescales (D4000, Hdelta_A and SFR/M*). The correlations do not depend on environment, suggesting that the decrease in star formation has ocurred over long (>1 Gyr) timescales. Since structure does not depend on environment for more massive galaxies, trends in recent SFH, dust and AGN for these systems cannot be driven by processes that alter structure, e.g. mergers. The SFH-density correlation is strongest for small scale (< 1 Mpc) estimates of local density. Finally, we highlight a striking similarity between changes in the galaxy population as a function of density and as a function of redshift and we interpret this using N-body simulations.

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arXiv2004

Dark matter annihilation in the halo of the Milky Way

If the dark matter in the Universe is made of weakly self-interacting particles, they may self-annihilate and emit gamma-rays. We use high resolution numerical simulations to estimate directly the annihilation flux from the central regions of the Milky Way and from dark matter substructures in its halo. Although such estimates remain uncertain because of their strong dependence on the structure of the densest regions, our numerical experiments suggest that less direct calculations have overestimated the emission both from the centre and from halo substructure. We estimate a maximal enhancement of at most a factor of a few with respect to a smooth spherical halo of standard Navarro-Frenk-White (NFW) structure. We discuss detection strategies for the next generation of gamma-ray detectors and find that the annihilation flux may be detectable, regardless of uncertainties about the densest regions, for the annihilation cross-sections predicted by currently popular elementary particle models for the dark matter.

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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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