arXiv subjects

Guinevere Kauffmann

Publications and source records attributed to Guinevere Kauffmann.

At least 181 records · Page 10Linked to original sources

arXiv2002

The Dependence of Star Formation History and Internal Structure on Stellar Mass for 10^5 Low-Redshift Galaxies

We study the relations between stellar mass, star formation history, size and internal structure for a complete sample of 122,808 galaxies drawn from the Sloan Digital Sky Survey. We show that low-redshift galaxies divide into two distinct families at a stellar mass of 3 \times 10^10 M_sol. Lower mass galaxies have young stellar populations, low surface mass densities and the low concentrations typical of disks. A significant fraction of the lowest mass galaxies in our sample have experienced recent starbursts. At given stellar mass, the sizes of low mass galaxies are log- normally distributed with dispersion sigma(ln R_50) \sim 0.5, in excellent agreement with the idea that they form with little angular momentum loss through cooling and condensation in a gravitationally dominant dark matter halo. Their median stellar surface mass density scales with stellar mass as mu* propto M_*^0.54, suggesting that the stellar mass of a disk galaxy is proprtional to the three halves power of its halo mass. This suggests that the efficiency of the conversion of baryons into stars in low mass galaxies increases in propor- tion to halo mass, perhaps as a result of supernova feedback processes. At stellar masses above 3 \times 10^10 M_sol, there is a rapidly increasing frac- tion of galaxies with old stellar populations, high surface mass densities and high concentrations typical of bulges. In this regime, the size distribution is log-normal, but its dispersion decreases rapidly with increasing stellar mass and the median mass surface density is approximately constant. This suggests that the star formation efficiency decreases in the highest mass halos, and that little star formation occurs in massive galaxies once they have assembled.

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arXiv2002

Multiple black holes in galactic bulges

We study the number and interaction rates of supermassive black holes in galactic bulges as predicted by hierarchical models of galaxy formation in which the spheroidal components of galaxies are formed by mergers. In bright ellipticals, the number of events that can eject a central supermassive binary black hole is large. Central binaries must therefore merge in less than a Hubble time -- otherwise there will be too much scatter in the M-sigma relation and too many off-center galactic nuclei. We propose that binary black holes are able to merge during the major gas accretion events that trigger QSO activity in galaxies. If this is the case, less than 10 percent of faint ellipticals and 40 percent of bright ellipticals are predicted to harbour binary black holes with near equal masses at their centres. This binary may be ejected away from the centre of the galaxy or even into intergalactic space in up to 20 percent of the most luminous ellipticals. The number of low mass black holes that can interact with the central object is predicted to be a strong function of galaxy luminosity. In most faint ellipticals, no black holes fall into the centre of the galaxy after the last major gas accretion event, but in the most luminous ellipticals, an average of 10 low mass black holes interact with the central supermassive object after this time. It is expected that stars will be ejected from galaxy cores as these low mass-ratio binaries harden. Multiple black holes in galactic bulges thus provide a natural explanation for the strong systematic trends in the observed central density profiles of ellipticals as a function of luminosity.

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arXiv2002

Understanding the Results of Galaxy-Galaxy Lensing using Galaxy-Mass Correlation in Numerical Simulations

McKay et al. (2002) have recently used measurements of weak galaxy-galaxy lensing in the Sloan Digital Sky Survey to estimate the cross correlation between galaxies and the projected dark matter density field. They derive a relation between aperture mass within a radius of $260\kpch$, $M_{260}$, and lens galaxy luminosity, that does not depend on galaxy luminosity, type or environment. In this paper, we study the cross-correlation between galaxies and dark matter using galaxy catalogs constructed from a high-resolution N-body simulation of a $Λ$CDM Universe. We show that our simulations reproduce the McKay et al. results reasonably well. In the simulation, $M_{260}$ is approximately equal to the halo virial mass for $L_*$ galaxies. $M_{260}$ overestimates the virial mass for fainter galaxies and underestimates it for brighter galaxies. We use the simulations to show that under certain circumstances the halo virial mass may be recovered by fitting an NFW profile to the projected galaxy-mass correlation function. If we apply our method to the observations, we find that $L_*$ galaxies typically reside in halos of $\sim 2\times 10^{12} h^{-1} \msun$, consistent with halo masses estimated from the observed Tully-Fisher relation. In the simulations, the halo virial mass scales with galaxy luminosity as $L^{1.5}$ for central galaxies in halos and for galaxies in low-density regions. For satellite galaxies, and for galaxies in high-density regions, there is no simple relation between galaxy luminosity and halo mass and care must be exercised when interpreting the lensing results.

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arXiv2001

The Clustering of Galaxies around Quasars

We study the cross-correlation between quasars and galaxies by embedding models for the formation and evolution of the two populations in cosmological N-body simulations. We adopt the quasar evolution model of Kauffmann & Haehnelt (2000), in which supermassive black holes are formed and fuelled during major mergers. We define the `bias' parameter b_QG as the ratio of the cross-correlation function xi_QG to the galaxy auto-correlation function xi_GG. On scales larger than 1 h^{-1} Mpc, the values of b_QG predicted by our models at low redshift depend very little on galaxy selection. They measure the characteristic mass of the dark matter halos that host quasars and can be used to estimate the typical quasar lifetime. In current redshift surveys, such measurements will constrain the lifetimes of low z quasars more accurately than measurements of the quasar auto-correlation function, because galaxies have much higher space densities than quasars. On scales smaller than 1 h^{-1} Mpc, the main contribution to xi_QG comes from quasar/galaxy pairs in the same dark matter halo. The amplitude of xi_QG depends both on the location of the host galaxy and on the density profile of other galaxies within the halo. As a result, measurements on these scales yield information about the processes responsible for fuelling super- massive black holes. At high redshifts, our models predict that quasars of fixed luminosity are located in less massive halos than at low redshift. They are therefore less biased relative to galaxies of given luminosity or stellar mass. We have used the simulations to calculate the evolution of the quasar auto- correlation function. We find that models with quasar lifetimes in the range 10^6-10^7 years provide a good match to the results of the 2dF QSO survey.

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arXiv2001

The Star Formation Histories of Local Galaxies: Continuous or Intermittent?

We explore a model in which efficient star formation in galaxies is triggered by merging satellites. We show that the merger/interaction rates predicted by hierarchical galaxy formation models depend strongly on galaxy mass. If a merger of a satellite larger than 1% the mass of the primary triggers efficient star formation, low-mass dwarf galaxies experience strong bursts separated by quiescent periods lasting several Gyr. Massive galaxies, such as our own Milky Way, are perturbed by a 1% satellite every few hundred million years, and thus have fluctuating, but relatively continuous star formation histories. We study the spectral signatures of a population of galaxies undergoing intermittent star formation by combining the models with the latest version of the Bruzual & Charlot spectral synthesis code. We concentrate on spectral indicators that are sensitive primarily to stellar age, rather than to metallicity and extinction. We show that if a population undergoes intermittent rather than continuous star formation, the signatures of the bursts should be evident from the observed dispersion in the star formation rates, H-delta equivalent widths and gas mass fractions of galaxies with 4000 Angstrom break strengths indicative of recent or ongoing star formation.

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arXiv2000

Populating a cluster of galaxies - I. Results at z=0

We simulate the assembly of a massive rich cluster and the formation of its constituent galaxies in a flat, low-density universe. Our most accurate model follows the collapse, the star-formation history and the orbital motion of all galaxies more luminous than the Fornax dwarf spheroidal, while dark halo structure is tracked consistently throughout the cluster for all galaxies more luminous than the SMC. Within its virial radius this model contains about 2.0e7 dark matter particles and almost 5000 distinct dynamically resolved galaxies. Simulations of this same cluster at a variety of resolutions allow us to check explicitly for numerical convergence both of the dark matter structures produced by our new parallel N-body and substructure identification codes, and of the galaxy populations produced by the phenomenological models we use to follow cooling, star formation, feedback and stellar aging. This baryonic modelling is tuned so that our simulations reproduce the observed properties of isolated spirals outside clusters. Without further parameter adjustment our simulations then produce a luminosity function, a mass-to-light ratio, luminosity, number and velocity dispersion profiles, and a morphology-radius relation which are similar to those observed in real clusters. In particular, since our simulations follow galaxy merging explicitly, we can demonstrate that it accounts quantitatively for the observed cluster population of bulges and elliptical galaxies.

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arXiv2000

The correlation between black hole mass and bulge velocity dispersion in hierarchical galaxy formation models

Recent work has demonstrated that there is a tight correlation between the mass of a black hole and the velocity dispersion of the bulge of its host galaxy. We show that the model of Kauffmann & Haehnelt, in which bulges and supermassive black holes both form during major mergers, produces a correlation between M_bh and sigma with slope and scatter comparable to the observed relation. In the model, the M_bh - sigma relation is significantly tighter than the correlation between black hole mass and bulge luminosity or the correlation between bulge luminosity and velocity dispersion. There are two reasons for this: i) the gas masses of bulge progenitors depend on the velocity dispersion but not on the formation epoch of the bulge, whereas the stellar masses of the progenitors depend on both; ii) mergers between galaxies move black holes along the observed M_bh - sigma relation, even at late times when the galaxies are gas-poor and black holes grow mainly by merging of pre-existing black holes. We conclude that the small scatter in the observed M_bh - sigma relation is consistent with a picture in which bulges and black holes form over a wide range in redshift.

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arXiv2000

The Spatial and Kinematic Distributions of Cluster Galaxies in a LCDM Universe -- Comparison with Observations

We combine dissipationless N-body simulations and semi-analytic models of galaxy formation to study the spatial and kinematic distributions of cluster galaxies in a LCDM cosmology. We investigate how the star formation rates, colours and morphologies of galaxies vary as a function of distance from the cluster centre and compare our results with the CNOC1 survey of galaxies from 15 X-ray luminous clusters in the redshift range 0.18 to 0.55. In our model, gas no longer cools onto galaxies after they fall into the cluster and their star formation rates decline on timescales of 1-2 Gyr. Galaxies in cluster cores have lower star formation rates and redder colours than galaxies in the outer regions because they were accreted earlier. Our colour and star formation gradients agree with those those derived from the data. The difference in velocity dispersions between red and blue galaxies observed in the CNOC1 clusters is also well reproduced by the model. We assume that the morphologies of cluster galaxies are determined solely by their merging histories. Morphology gradients in clusters arise naturally, with the fraction of bulge- dominated galaxies highest in cluster cores. We compare these gradients with the CNOC1 data and find excellent agreement for bulge-dominated galaxies. The simulated clusters contain too few galaxies of intermediate bulge-to-disk ratio, suggesting that additional processes may influence the morphological evolution of disk-dominated galaxies in clusters. Although the properties of the cluster galaxies in our model agree extremely well with the data, the same is not true of field galaxies. Both the star formation rates and the colours of bright field galaxies appear to evolve much more strongly from redshift 0.2 to 0.4 in the CNOC1 field sample than in our simulations.

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arXiv1999

The formation and evolution of supermassive black holes and their host galaxies

We discuss constraints on the assembly history of supermassive black holes from the observed remnant black holes in nearby galaxies and from the emission caused by accretion onto these black holes. We also summarize the results of a specific model for the evolution of galaxies and their central black holes which traces their hierachical build-up in CDM-like cosmogonies. The model assumes (i) that black holes, ellipticals and starburts form during major mergers of galaxies (ii) that the gas fraction in galaxies decreases with decreasing redshift (iii) that the optical bright phase of a QSO lasts for about 10^7 years. The model succesfully reproduces the evolution of cold gas as traced by damped damped Lyman alpha systems, the evolution of optically bright QSOs, the remnant black hole mass distribution and the host galaxy luminosities of QSOs.

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arXiv1999

A Unified Model for the Evolution of Galaxies and Quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few percent of the gas in the merger remnant is accreted by the new black hole over a timescale of a few times 10^7 years. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z=2 to z=0 is due to the combination of three effects: i) a decrease in the merging rate, ii) a decrease in the amount of cold gas available to fuel black holes, and iii) an increase in the timescale for gas accretion. In a LCDM cosmology the predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Lyman-alpha systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starbursts is inextricably linked to the hierarchical build-up of galaxies.

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arXiv1999

Probing Star Formation Timescales in Elliptical Galaxies

In models of galaxy formation in a hierarchical Universe, elliptical galaxies form through the merging of smaller disk systems. These models yield a number of testable predictions if reliable techniques for determining the relative ages and compositions of the stellar populations of different galaxies can be found: 1) ellipticals in low-density environments form later than ellipticals in clusters, 2) more massive ellipticals form later, 3) more massive ellipticals form in dissipationless mergers from disk galaxies with low gas content. While colours and the Balmer line strengths of galaxies can be used to infer the average ages of the stellar populations of ellipticals, alpha/Fe element ratios carry information about the timescale over which star formation took place. Here we present preliminary results from semi-analytic models for the distribution of Mg/Fe ratios in galaxies as a function of morphological type, luminosity and environment.

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arXiv1999

Clustering of Galaxies in a Hierarchical Universe: III. Mock Redshift Surveys

This is the third paper in a series which combines N-body simulations and semi-analytic modelling to provide a fully spatially resolved simulation of the galaxy formation and clustering processes. Here we extract mock redshift surveys from our simulations: a Cold Dark Matter model with either Omega_0=1 (tauCDM) or Omega_0=0.3 and Lambda=0.7 (LambdaCDM). We compare the mock catalogues with the northern region (CfA2N) of the Center for Astrophysics (CfA) Redshift Surveys. We study the properties of galaxy groups and clusters identified using standard observational techniques and we study the relation of these groups to real virialised systems. Most features of CfA2N groups are reproduced quite well by both models with no obvious dependence on Omega_0. Redshift space correlations and pairwise velocities are also similar in the two cosmologies. The luminosity functions predicted by our galaxy formation models depend sensitively on the treatment of star formation and feedback. For the particular choices of Paper I they agree poorly with the CfA survey. To isolate the effect of this discrepancy on our mock redshift surveys, we modify galaxy luminosities in our simulations to reproduce the CfA luminosity function exactly. This adjustment improves agreement with the observed abundance of groups, which depends primarily on the galaxy luminosity density, but other statistics, connected more closely with the underlying mass distribution, remain unaffected. Regardless of the luminosity function adopted, modest differences with observation remain. These can be attributed to the presence of the ``Great Wall'' in the CfA2N. It is unclear whether the greater coherence of the real structure is a result of cosmic variance, given the relatively small region studied, or reflects a physical deficiency of the models.

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arXiv1998

Evolution of Elliptical Galaxies in a Hierarchical Universe

In hierarchical models of galaxy formation, ellipticals form from the merging of disk galaxies drawn together by gravity as their surrounding dark halos coalesce. Using semi-analytic techniques, we are able to follow the merging, star formation and chemical enrichment histories of galaxies for a given choice of cosmological parameters. We calculate the ages and metallicities of elliptical galaxies in a standard CDM cosmology and show that we can reproduce the observed correlation between magnitude and colour, and the scatter about this relation. We also study the evolution of ellipticals to high redshift. In rich clusters, the models predict a substantial population of "evolved" ellipticals at z>1. However, the average number density of bright ellipticals must decrease at high redshift. The rate of decrease is sensitive to cosmological parameters. In a high-density model, the abundance of bright ellipticals declines by factor of 3 by z=1. In a low-density model (Omega=0.3, Lambda=0.7), the abundance drops by 30% at z=1, but is down by a factor 3 at z=2

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arXiv1998

Clustering of Galaxies in a Hierarchical Universe: II. Evolution to High Redshift

In hierarchical cosmologies the evolution of galaxy clustering depends both on cosmological quantities such as Omega and Lambda, which determine how dark matter halos form and evolve, and on the physical processes - cooling, star formation and feedback - which drive the formation of galaxies within these merging halos. In this paper, we combine dissipationless cosmological N-body simulations and semi-analytic models of galaxy formation in order to study how these two aspects interact. We focus on the differences in clustering predicted for galaxies of differing luminosity, colour, morphology and star formation rate and on what these differences can teach us about the galaxy formation process. We show that a "dip" in the amplitude of galaxy correlations between z=0 and z=1 can be an important diagnostic. Such a dip occurs in low-density CDM models because structure forms early and dark matter halos of 10**12 solar masses, containing galaxies with luminosities around L*, are unbiased tracers of the dark matter over this redshift range; their clustering amplitude then evolves similarly to that of the dark matter. At higher redshifts bright galaxies become strongly biased and the clustering amplitude increases again. In high density models, structure forms late and bias evolves more rapidly. As a result, the clustering amplitude of L* galaxies remains constant from z=0 to 1. The strength of these effects is sensitive to sample selection. The dip is weaker for galaxies with lower star formation rates, redder colours, higher luminosities and earlier morphological types. We explain why this is the case and how it is related to the variation with redshift of the abundance and and environment of the observed galaxies. Studies of clustering evolution as a function of galaxy properties should place strong constraints on models.

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arXiv1998

Clustering of Galaxies in a Hierarchical Universe: I. Methods and Results at z=0

We introduce a new technique for following the formation and evolution of galaxies in cosmological N-body simulations. Dissipationless simulations are used to track the formation and merging of dark matter halos as a function of redshift. Simple prescriptions, taken directly from semi-analytic models of galaxy formation, are adopted for cooling, star formation, supernova feedback and the merging of galaxies within the halos. This scheme enables us to study the clustering properties of galaxies and to investigate how selection by type, colour or luminosity influences the results. In this paper, we study properties of the galaxy distribution at z=0. These include luminosity functions, colours, correlation functions, pairwise peculiar velocities, cluster M/L ratios and star formation rates. We focus on two variants of a CDM cosmology: a high- density model with Gamma=0.21 (TCDM) and a low-density model with Omega=0.3 and Lambda=0.7 (LCDM). Both are normalized to reproduce the I-band Tully-Fisher relation near a circular velocity of 220 km/s. Our results depend strongly both on this normalization and on the prescriptions for star formation and feedback. Very different assumptions are required to obtain an acceptable model in the two cases. For TCDM, efficient feedback is required to suppress the growth of galaxies low-mass field halos. Without it, there are too many galaxies and the correlation function turns over below 1 Mpc. For LCDM, feedback must be weak, otherwise too few L* galaxies are produced and the correlation function is too steep. Given the uncertainties in modelling some of the key physical processes, we conclude that it is not yet possible to draw conclusions about the values of cosmological parameters from studies of this kind. Further work on global star formation and feedback effects is required to narrow the range of possibilities

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arXiv1998

The K-band luminosity function at z=1: a powerful constraint on galaxy formation theory

There are two major approaches to modelling galaxy evolution. The traditional view is that the most massive galaxies were assembled early and have evolved with steeply declining star formation rates since a redshift of 2 or higher. According to hierarchical theories, massive galaxies were assembled much more recently from mergers of smaller subunits. Here we present a simple observational test designed to differentiate between the two. The observed K-band flux from a galaxy is a good measure of its stellar mass even at high redshift. It is probably only weakly affected by dust extinction. We compute the evolution of the observed K-band luminosity function for traditional, pure luminosity evolution (PLE) models and for hierarchical models. At z=0, both models can fit the observed local K-band luminosity function. By redshift 1, they differ greatly in the predicted abundance of bright galaxies. We calculate the redshift distributions of K-band selected galaxies and compare these with available data. We show that the number of K<19 galaxies with redshifts greater than 1 is well below the numbers predicted by the PLE models. In the Songaila et al (1994) redshift sample of 118 galaxies with 16 1. Only 2 are observed. In the Cowie et al. (1996) redshift sample of 52 galaxies with 18 1. Only 5 are observed. Both these samples are more than 90% complete. We conclude that there is already strong evidence that the abundance of massive galaxies at z=1 is well below the local value. This is inconsistent with the traditional model, but similar to the expectations of hierarchical models.

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arXiv1997

Environmental Influences on Dark Matter Halos and Consequences for the Galaxies Within Them

We use large N-body simulations of dissipationless gravitational clustering in cold dark matter (CDM) cosmologies to study whether the properties of dark matter halos are affected by their environment. We look for correlations between the masses, formation redshifts, concentrations, shapes and spins of halos and the overdensity of their local environment. We also look for correlations of these quantities with the local tidal field. Our conclusion is extremely simple. Only the mass distribution varies as a function of environment. This variation is well described by a simple analytic formula based on the conditional Press-Schechter theory. We find no significant dependence of any other halo property on environment. Our results do not depend on our choice of cosmology. According to current hierarchical models, the structure and evolutionary history of a galaxy is fully determined by the structure and evolutionary history of the dark halo in which it is embedded. If these models are correct, clustering variations between galaxies of differing morphological types, luminosities, colours and surface brightnesses, must all arise because the halo mass function is skewed towards high mass objects in overdense regions of the Universe and towards low mass objects in underdense regions.

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arXiv1997

Chemical Enrichment and the Origin of the Colour-Magnitude Relation of Elliptical Galaxies in a Hierarchical Merger Model

In this paper, we present a model of the formation and chemical enrichment of elliptical galaxies that differs from the conventional picture in two ways: 1)Ellipticals do not form in a single monolithic collapse and burst of star formation at high redshift. Instead, most of their stars form at modest rates in disk galaxies, which then merge to form the ellipticals. 2)Galaxies do not undergo closed-box chemical evolution. Instead, metals can be transferred between the stars, cold gas and the hot gas halos of the galaxies. It is assumed that metals are ejected out of disk galaxies during supernova explosions and these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass-metallicity relation for the disk systems. Big ellipticals are more metal rich because they are formed from the mergers of bigger disks. We use semi-analytic techniques to follow the formation, evolution, and chemical enrichment of cluster ellipticals in a merging hierarchy of dark matter halos. The inclusion of the new metallicity-dependent spectral synthesis models of Bruzual & Charlot allow us to compute the colours, line indices and mass-to- light ratios of these galaxies. If feedback is assumed to be efficient, we are able to reproduce the slope and scatter of the colour-magnitude and the Mg2-sigma relations. We also study the evolution of these relations to high redshift. We show that the luminosity-metallicity relation remains fixed, but the mean stellar ages of the galaxies scale with the age of the Universe. Finally, we study the enrichment history of the intracluster gas. We find that 80% of the metals were ejected by low-mass galaxies at redshifts greater than 1. The metallicity of the ICM thus evolves very little out to z > 1.

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