arXiv subjects

Rachel S. Somerville

Publications and source records attributed to Rachel S. Somerville.

At least 325 records · Page 18Linked to original sources

arXiv2001

Galaxy Halo Occupation at High Redshift

We discuss how current and future data on the clustering and number density of z~3 Lyman-break galaxies (LBGs) can be used to constrain their relationship to dark matter haloes. We explore a three-parameter model in which the number of LBGs per dark halo scales like a power-law in the halo mass: N(M) = (M/M_1)^S for M>M_m. Here, M_m is the minimum mass halo that can host an LBG, M_1 is a normalization parameter, associated with the mass above which haloes host more than one observed LBG, and S determines the strength of the mass dependence. We show how these three parameters are constrained by three observable properties of LBGs: the number density, the large-scale bias, and the fraction of objects in close pairs. Given these three quantities, the three unknown model parameters may be estimated analytically, allowing a full exploration of parameter space. As an example, we assume an LCDM cosmology and consider the observed properties of a recent sample of spectroscopically confirmed LBGs. We find that the favored range for our model parameters is M_m = (0.4-8)x10^10 Msun/h, M_1 = (6-10)x10^12 Msun/h, and 0.9 < S < 1.1. The preferred region in M_m expands by an order of magnitude and slightly shallower slopes are acceptable if the allowed range of bias is permitted to span all recent observational estimates. We also discuss how the observed clustering of LBGs as a function of luminosity can be used to constrain halo occupation, although due to current observational uncertainties we are unable to reach any strong conclusions. Our methods and results can be used to constrain more realistic models that aim to derive the occupation function N(M) from first principles, and offer insight into how basic physical properties affect the observed properties of LBGs.

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arXiv2001

Modeling Angular-Momentum History in Dark-Matter Halos

We model the acquisition of spin by dark-matter halos in semi-analytic merger trees. We explore two different algorithms; one in which halo spin is acquired from the orbital angular momentum of merging satellites, and another in which halo spin is gained via tidal torquing on shells of material while still in the linear regime. We find that both scenarios produce the characteristic spin distribution of halos found in N-body simulations, namely, a log-normal distribution with mean ~0.04 and standard deviation ~0.5 in the log. A perfect match requires fine-tuning of two free parameters. Both algorithms also reproduce the general insensitivity of the spin distribution to halo mass, redshift and cosmology seen in N-body simulations. The spin distribution can be made strictly constant by physically motivated scalings of the free parameters. In addition, both schemes predict that halos which have had recent major mergers have systematically larger spin values. These algorithms can be implemented within semi-analytic models of galaxy formation based on merger trees. They yield detailed predictions of galaxy properties that strongly depend on angular momentum (such as size and surface brightness) as a function of merger history and environment.

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arXiv2001

Galaxy Formation at z~3: Constraints from Spatial Clustering

We use N-body simulations combined with semi-analytic models to compute the clustering properties of modeled galaxies at z~3, and confront these predictions with the clustering properties of the observed population of Lyman-break galaxies (LBGs). Several scenarios for the nature of LBGs are explored, which may be broadly categorized into models in which high-redshift star formation is driven by collisional starbursts and those in which quiescent star formation dominates. For each model, we make predictions for the LBG overdensity distribution, the variance of counts-in-cells, the correlation length, and close pair statistics. Models which assume a one-to-one relationship between massive dark-matter halos and galaxies are disfavored by close pair statistics, as are models in which colliding halos are associated with galaxies in a simplified way. However, when modeling of gas consumption and star formation is included using a semi-analytic treatment, the quiescent and collisional starburst models predict similar clustering properties and none of these models can be ruled out based on the available clustering data. None of the ``realistic'' models predict a strong dependence of clustering amplitude on the luminosity threshold of the sample, in apparent conflict with some observational results.

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arXiv2000

Probing Galaxy Formation with High Energy Gamma-Rays

We discuss how measurements of the absorption of $γ$-rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. We use semi-analytic models (SAMs) of galaxy formation, set within the hierarchical structure formation scenario, to obtain predictions of the EBL for 0.1-1000$μ$m. SAMs incorporate simplified physical treatments of the key processes of galaxy formation --- including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production --- and have been shown to reproduce key observations at low and high redshift. Here we also introduce improved modelling of the spectral energy distributions in the mid-to-far-IR arising from emission by dust grains. Assuming a flat \lcdm cosmology with $Ω_m=0.3$ and Hubble parameter $h=0.65$, we investigate the consequences of variations in input assumptions such as the stellar initial mass function (IMF) and the efficiency of converting cold gas into stars. We conclude that observational studies of the absorption of $γ$-rays with energies from 10s of Gev to 10s of TeV will help to determine the EBL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the IMF, the history of star formation, and the reprocessing of light by dust.

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arXiv2000

Disks at High Redshift: Interactions, Mergers, and Starbursts

Do disk galaxies exist at redshifts much greater than unity, and how might they look different from local disks? How does the morphological mix of galaxies change with redshift? What can we learn from current observations about the properties of high redshift galactic disks? I present theoretical predictions based on semi-analytic hierarchical models, focussing on the role played by interactions, mergers, and starbursts in determining the observable properties of disk galaxies at high redshift, and discuss the interpretation of high redshift observations of possible proto-disks (damped Lyman-$α$ systems and Lyman-break galaxies) in light of these predictions.

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arXiv2000

Profiles of dark haloes: evolution, scatter, and environment

We study dark-matter halo density profiles in a high-resolution N-body simulation of an LCDM cosmology. Our statistical sample contains ~5000 haloes in the range 10^{11}-10^{14} M_sun. The profiles are parameterized by an NFW form with two parameters, an inner radius r_s and a virial radius r_v, and we define the halo concentration c_v = r_v/r_s. First, we find that, for a given halo mass, the redshift dependence of the median concentration is c_v ~ 1/(1+z), corresponding to a roughly constant r_s with redshift. We present an improved analytic treatment of halo formation that fits the measured relations between halo parameters and their redshift dependence. The implications are that high-redshift galaxies are predicted to be more extended and dimmer than expected before. Second, we find that the scatter in log(c_v) is ~0.18, corresponding to a scatter in maximum rotation velocities of dV/V ~ 0.12. We discuss implications for modelling the Tully-Fisher relation, which has a smaller reported intrinsic scatter. Third, haloes in dense environments tend to be more concentrated than isolated haloes. These results suggest that c_v is an essential parameter for the theory of galaxy modelling, and we briefly discuss implications for the universality of the Tully-Fisher relation, the formation of low surface brightness galaxies, and the origin of the Hubble sequence.

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arXiv2000

The Nature of High-Redshift Galaxies

This paper has been withdrawn because the models on which it was based have undergone significant changes and improvements. A new paper with the same title, based on the improved models, is accepted for publication in MNRAS and is available at astro-ph/0006364. We have withdrawn this paper to avoid confusion.

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arXiv2000

Strong Evolution in the Luminosity-Velocity Relation at z>1?

We present a method for constraining the evolution of the galaxy luminosity-velocity (LV) relation in hierarchical scenarios of structure formation. The comoving number density of dark-matter halos with circular velocity of 200 km/s is predicted in favored CDM cosmologies to be nearly constant over the redshift range 0<z<5. Any observed evolution in the density of bright galaxies implies in turn a corresponding evolution in the LV relation. We consider several possible forms of evolution for the zero-point of the LV relation and predict the corresponding evolution in galaxy number density. The Hubble Deep Field suggests a large deficit of bright (M_V < -19) galaxies at 1.4 < z < 2. If taken at face value, this implies a dimming of the LV zero-point by roughly 2 magnitudes. Deep, wide-field, near-IR selected surveys will provide more secure measurements to compare with our predictions.

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arXiv2000

What Damped Ly-alpha Systems Tell Us About the Radial Distribution of Cold Gas at High Redshift

We investigate the properties of damped Lyman-alpha systems (DLAS) in semi-analytic models, focusing on whether the models can reproduce the kinematic properties of low-ionization metal lines described by Prochaska & Wolfe (1997b, 1998). We explore a variety of approaches for modelling the radial distribution of the cold neutral gas associated with the galaxies in our models, and find that our results are very sensitive to this ingredient. If we use an approach based on Fall & Efstathiou (1980), in which the sizes of the discs are determined by conservation of angular momentum, we find that the majority of the DLAS correspond to a single galactic disc. These models generically fail to reproduce the observed distribution of velocity widths. In alternative models in which the gas discs are considerably more extended, a significant fraction of DLAS arise from lines of sight intersecting multiple gas discs in a common halo. These models produce kinematics that fit the observational data, and also seem to agree well with the results of recent hydrodynamical simulations. Thus we conclude that Cold Dark Matter based models of galaxy formation can be reconciled with the kinematic data, but only at the expense of the standard assumption that DLAS are produced by rotationally supported gas discs whose sizes are determined by conservation of angular momentum. We suggest that the distribution of cold gas at high redshift may be dominated by another process, such as tidal streaming due to mergers.

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arXiv2000

Optical Counterparts to Damped Lyman Alpha Systems

Previously we have shown (Maller et al, 1998) that the kinematics of Damped Lyman Alpha Systems (DLAS) as measured by Prochaska and Wolfe (1998) can be reproduced in a multiple disk model (MDM) if the gaseous disks are of sufficient radial extent. Here we discuss this model's predictions for the relationship between DLAS and Lyman break galaxies (LBGs), which we here take to be objects at z~3 brighter than R=25.5. We expect that future observations of the correlations between DLAS and LBGs will provide a new data set able to discriminate between different theoretical models of the DLAS. Djorgovski (1997) has already detected a few optical counterparts and more studies are underway.

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arXiv2000

Damped Ly-alpha Systems in Semi-Analytic Models: Sensitivity to dynamics, disk properties, and cosmology

Previously we have shown that it is possible to account for the kinematic properties of damped Lyman alpha systems (DLAS) in the context of semi-analytic models. In these models, hierarchical structure formation is approximated by constructing a merger tree for each dark matter halo. A natural consequence is that every virialized halo may contain not only a central galaxy, but also a number of satellite galaxies as determined by its merging history. Thus the kinematics of the DLAS arise from the combined effects of the internal rotation of gas disks and the motions between gas disks within a common halo. Here we investigate the sensitivity of this model to some of the assumptions made previously, including the modeling of satellite dynamics, the scale height of the gas, and the cosmology.

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arXiv1999

Clustering of high-redshift galaxies: relating LBGs to dark matter halos

We investigate the clustering properties of high-redshift galaxies within three competing scenarios for assigning luminous galaxies to dark matter halos from N-body simulations: a one galaxy per massive halo model, a quiescent star formation model, and a collisional starburst model. We compare these models to observations of Lyman-Break galaxies at z~3$ With current data and the simple statistic used here, one cannot rule out any of these models, but we see potential for finding distinguishing features using statistics that are sensitive to the tails of the distribution, and statistics based on the number of multiple galaxies per halo, which we explore in an ongoing study.

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arXiv1999

Semi-Analytic Modelling of Galaxy Formation: The Local Universe

Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully-Fisher relation, the B and K-band luminosity functions, cold gas contents, sizes, metallicities, and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment in Kauffmann, White & Guiderdoni (1993) and Cole et al. (1994) as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B and K-band luminosity functions and the I-band Tully-Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction, and an improved Press-Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of $Ω_0 \simeq 0.3$--0.5 give the best agreement with observations.

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arXiv1999

Young galaxies: what turns them on?

Lyman break galaxies (LBGs) at $z \sim 3$ exhibit number densities and clustering similar to local $L_*$ galaxies; however, their star formation rates (SFRs) are much higher. We explore the scenario in which LBGs are starburst galaxies triggered by collisions, and thus provide an explanation for these key properties. The relative paucity of starburst galaxies at low redshift can be attributed to a much higher collision rate in the past. We use high-resolution cosmological $N$-body simulations and a hierarchical halo finder to estimate the galaxy collision rate as a function of time in the popular $Λ$CDM cosmological model. We find that bright collisional starbursts are frequent enough to account for most of the high-$z$ ($2.5-4.5$) LBGs. Moreover, many of the objects are of relatively small mass, but they cluster about large-mass halos. They therefore exhibit strong clustering, similar to that observed and stronger than that of the relevant massive halos.

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arXiv1998

Probing Galaxy Formation with TeV Gamma Ray Absorption

We present here the extragalactic background light (EBL) predicted by semi-analytic models of galaxy formation, and show how measurements of the absorption of gamma rays of $\sim$ TeV energies via pair production on the EBL can probe cosmology and the formation of galaxies. Semi-analytic models permit a physical treatment of the key processes of galaxy formation -- including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production -- and have been shown to reproduce key observations at low and high redshift. Using this approach, we investigate the consequences of variations in input assumptions such as the stellar initial mass function and the underlying cosmology. We conclude that observational studies of the absorption of $\sim 10^{-2}-10^{2}$ TeV gamma rays will help to constrain the star formation history of the universe, and the nature and extent of the extinction of starlight due to dust and reradiation of the absorbed energy at infrared wavelengths.

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arXiv1998

Cosmological Implications of Lyman-Break Galaxy Clustering

We review our analysis of the clustering properties of ``Lyman-break'' galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998). We examine the likelihood of spikes found by Steidel et al (1998) in the redshift distribution of LBGs, within a suite of models for the evolution of structure in the Universe. Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBGs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. The LBG correlation functions are less steep than galaxies today (gamma~1.4), but show similar or slightly longer correlation lengths. We have extened this analysis and include a preliminary comparison to the new data presented in Adelberger et al (1998). We also discuss work in progress, in which we use semi-analytic models to identify Lyman-break galaxies within dark-matter halos.

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arXiv1998

Making Damped Lyman Alpha Systems in Semi-Analytic Models

The velocity profiles of weak metal absorption lines can be used to observationally probe the kinematic state of gas in damped Lyman-alpha systems. Prochaska and Wolfe have argued that the flat distribution of velocity widths combined with the asymmetric line profiles indicate that the DLAS are disks with large rotation velocities. An alternative explanation has been proposed by Haehnelt, Steinmetz, and Rauch, in which the observed large velocity widths and asymmetric profiles can be produced by lines of sight passing through two or more clumps each having relatively small internal velocity dispersions. We investigate the plausibility of this scenario in the context of semi-analytic models based on hierarchical merging trees and including simple treatments of gas dynamics, star formation, supernova feedback, and chemical evolution. We find that all the observed properties of the metal-line systems including the distribution of velocities and the asymmetric profiles, can be reproduced by lines of sight passing through sub-clumps that are bound within larger virialized dark matter halos. In order to produce enough multiple hits, we find that the cold gas must be considerably more extended than the optical radius of the proto-galaxies, perhaps even beyond the tidal radius of the sub-halo. This could occur due to tidal stripping or supernova-driven outflows.

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arXiv1998

The Star Formation History in a Hierarchical Universe

Observations now probe the star formation history of the Universe back to a redshift of $z\sim5$. We investigate whether the predictions of semi-analytic models of galaxy formation based on hierarchical Cold Dark Matter (CDM) type models are in agreement with these direct observations and also with the ``fossil'' evidence contained in constraints on the ages of present day early-type galaxies. Previous models predicted that the star formation rate density falls off rather steeply at $z\ga 2$, and correspondingly that the majority of the stars in the Universe formed at relatively low redshift. We investigate the effect of including a bursting mode of star formation, assuming that galaxy-galaxy mergers trigger starbursts and using the merger rate that arises naturally in the CDM merging hierarchy. The resulting starbursts substantially increase the global star formation rate at high redshift, leading to predictions that are in good agreement with the star formation rate density at $z\sim3$ obtained from sub-millimeter observations (SCUBA) and optical/UV estimates after correction for dust extinction. The mass of stars formed at $z \ge 3$ is correspondingly in better agreement with the fossil evidence. We also investigate complementary global quantities such as the mass of cold gas and the average metallicity of cold gas as a function of redshift, and the integrated extra-galactic background light. We argue that these observations, taken together, provide strong constraints on the star formation history of the Universe, and that hierarchical models of the CDM type are in reasonable agreement with these observations when starbursts are included.

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