arXiv subjects

Guinevere Kauffmann

Publications and source records attributed to Guinevere Kauffmann.

187 records · Page 11Linked to original sources

arXiv1996

Detection of Strong Evolution in the Population of Early-Type Galaxies

The standard picture holds that giant elliptical galaxies formed in a single burst at high redshift. Aging of their stellar populations subsequently caused them to fade and become redder. The Canada-France Redshift Survey provides a sample of about 125 galaxies with the luminosities and colours of passively evolving giant ellipticals and with 0.1 < z < 1. This sample is inconsistent with the standard evolutionary picture with better than 99.9% confidence. The standard Schmidt test gives = 0.398 when restricted to objects with no detected star formation, and = 0.410 when objects with emission lines are also included. A smaller sample of early-type galaxies selected from tha Hawaii Deep Survey gives equally significant results. With increasing redshift a larger and larger fraction of the nearby elliptical and S0 population must drop out of of the sample, either because the galaxies are no longer single units or because star formation alters their colours. If the remaining fraction is modelled as F=(1+z)^{-γ}, the data imply gamma = 1.5 +- 0.3. At z=1 only about one third of bright E and S0 galaxies were already assembled and had the colours of old passively evolving systems. We discuss the sensitivity of these results to the incompleteness corrections and stellar population models we have adopted. We conclude that neither is uncertain enough to reconcile the observations with the standard picture. Hierarchical galaxy formation models suggest that both merging and recent star formation play a role in the strong evolution we have detected.

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arXiv1995

Disk galaxies at Z=0 and at High Redshift: an explanation of the observed evolution of damped Ly-alpha systems

The analysis of disk formation is based on the White & Rees (1978) picture, in which disk galaxies form by continuous cooling and accretion of gas within a merging hierarchy of dark matter halos. A simple Kennicutt law of star formation for disks, based on a single- fluid gravitational instability model is introduced. Since the gas supply in the disk is regulated by infall from the surrounding halo, the gas is always maintained at a critical threshold surface density. Chemical enrichment of the disks occurs when the surrounding hot halo gas is enriched with heavy elements ejected during surpernova explosions. This gas then cools onto the disk producing a new generation of metal-rich stars. I first show that models of this type can reproduce many of the observed properties of a typical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ages and metallicities of solar neighbourhood stars. I then use the models to make inferences about the properties of disk galaxies at high redshift. The total neutral hydrogen density Omega(HI) increases at higher z. The predicted increase is mild, but is roughly consistent with the latest derivation of Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models are also able to account for some of the other trends other trends seen in the high-redshift data, including the increase in the number of high column-density systems at high redshift, as well as the metallicity distribution of damped Lyman-alpha systems at redshifts 2-3.

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arXiv1995

Galaxy Formation and Large-Scale Bias

We outline a simple approach to understanding the physical origin of bias in the distribution of galaxies relative to that of dark matter. The first step is to specify how collapsed, virialized halos of dark matter trace the overall matter distribution. The next step is to make a connection between halos and the luminous galaxies we observe. We appeal to the results of semi-analytic models of galaxy formation that are tuned to fit the observed luminosity functions of local groups and clusters. We have also used a high-resolution N-body simulation of a cold dark matter (CDM) universe to study the bias relation in more detail. The differences between the galaxy and dark matter distributions are quantified using a number of different clustering statistics. We arrive at the following general conclusions: 1) A comparison of the galaxy and dark matter density fields shows that linear biasing is a good description on large scales. 2) The bias factor b depends on the shape and normalization of the power spectrum. The lower the normalization, the larger the bias. More bias is obtained for spectra with more power on large scales. For "realistic" models, b ranges from 1 to 2.5. 3) Galaxies of different luminosity or morphology have different bias factors. 4) The scale dependence of the bias factor is weak.

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arXiv1995

The Ages of Elliptical Galaxies in a Merger Model

The tightness of the observed colour-magnitude and Mg$_{2}$- velocity dispersion relations for elliptical galaxies has often been cited as an argument against a picture in which ellipticals form by the merging of spiral disks. A common view is that merging would mix together stars of disparate ages and produce a large scatter in these relations. Here I use semi-analytic models of galaxy formation to derive the distribution of the mean ages, colours and metallicities of the stars in elliptical galaxies formed by mergers in a flat CDM universe. It is seen that most of the stars in ellipticals form at relatively high redshift (z > 1.9) and that the predicted scatter in the colour-magnitude and Mg_2 - sigma relations falls within observational bounds. I conclude that the apparent homogeneity in the properties of the stellar populations of ellipticals is not inconsistent with a merger scenario for the origin of these systems.

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arXiv1994

The Observed Properties of High-Redshift Cluster Galaxies

We use the semi-analytic models of galaxy formation developed by Kauffmann, White \& Guiderdoni to generate predictions for the observed properties of cluster and group galaxies at redshifts between 0 and 0.6. We examine four sets of cosmological initial conditions: low-density CDM models with and without cosmological constant, a flat CDM model and a mixed dark matter model. These models were selected because they span a wide range in cluster formation epoch. The semi-analytic models that we employ are able to follow both the evolution of the dark matter component of clusters and the formation and evolution of the stellar populations of the cluster galaxies. We are thus able to generate model predictions that can be compared directly with the observational data. In the low-density CDM models, clusters form at high red- shift and accrete very little mass at recent times. Our models predict that essentially no evolution in the observed properties of clusters will have occurred by a redshift of 0.6, in direct contradiction with the data. In contrast, in the MDM model, both galaxies and clusters form extremely late. This model predicts evolution which appears to be too extreme to be in agreement with the observations. The flat CDM model, which is intermediate in structure formation epoch, is most successful. This model is able to account for the evolution of the blue fraction of rich clusters with redshift, the relationship between blue fraction and cluster richness at different epochs, and the changes in the distribution of the morphologies of cluster galaxies by a redshift of 0.4.

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arXiv1994

Hierarchical Clustering and the Butcher-Oemler Effect

We show that the rapid evolution in the fraction of blue, star-forming galaxies seen in clusters as a function of redshift (the Butcher-Oemler effect) can be explained very simply if structure formation in the universe proceeeds hierarchically. We show that a rich cluster observed at high redshift has had a significantly different evolutionary history to a cluster of the same richness observed today. High redshift clusters take longer to assemble and thus under- go more merging at small lookback times. We have investigated two models of star formation in cluster galaxies: 1) a model in which star formation is induced by galaxy-galaxy mergers and interactions and 2) a model in which star formation is regulated by the infall of galaxies onto larger systems such as groups and clusters. Both models produce trends consistent with the Butcher- Oemler effect. Our models of cluster formation and evolution allow us to make predictions about trends in the observed properties of clusters with redshift. We find that there should be a correlation between the mass of the cluster or group and the strength of the observed Butcher-Oemler effect, with more massive systems exhibiting more evolution than less massive systems. We also predict that both the blue galaxy fraction and the incidence of interacting galaxies in rich clusters should rise continuously with redshift. Finally, we have explored the influence of cosmological parameters on our predictions for cluster evolution. We find that models in which structure formation occurs at very early epochs, such as low $Ω$ models, predict rather little recent star formation and merging activity in clusters at redshifts of around 0.4.

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arXiv1994

Constraints on Models of Galaxy Formation from the Evolution of Damped Ly$α$ Absorption Systems

There is accumulating observational evidence suggesting that damped Ly$α$ absorption systems systems are the progenitors of present-day spiral galaxies. We use the observed properties of these systems to place constraints on the history of star formation in galactic disks, and on cosmological theories of structure formation in the universe. We show that the observed increase in $Ω_{HI}$ contributed by damped Ly$α$ systems at high redshift implies that star formation must have been considerably less efficient in the past. We also show that the data can constrain cosmological models in which structure forms at late epochs. A mixed dark matter (MDM) model with $Ω_ν=0.3$ is unable to reproduce the mass densities of cold gas seen at high redshift, even in the absence of any star formation. We show that at redshifts greater than 3, this model predicts that the total baryonic mass contained in dark matter halos with circular velocities $V_c > 35$ km s$^{-1}$ is less than the observed mass of HI in damped systems. At these redshifts, the photo-ionizing background would prevent gas from dissipating and collapsing to form high column density systems in halos smaller than 35 km s$^{-1}$. MDM models are thus ruled out by the observations.

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