arXiv subjects

James S. Bullock

Publications and source records attributed to James S. Bullock.

227 records · Page 13Linked to original sources

arXiv2000

Hierarchical galaxy formation and substructure in the Galaxy's stellar halo

We develop an explicit model for the formation of the stellar halo from tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM) framework, focusing on predictions testable with the Sloan Digital Sky Survey (SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution are calculated using a semi-analytic approach within the Press-Schechter formalism. Motivated by our previous work, we assume that low-mass subhalos (v < 30 km/s) can form significant populations of stars only if they accreted a substantial fraction of their mass before the epoch of reionization. With this assumption, the model reproduces the observed velocity function of galactic satellites in the Local Group, solving the ``dwarf satellite problem'' without modifying the popular LCDM cosmology. The disrupted satellites yield a stellar distribution with a total mass and radial density profile consistent with those observed for the Milky Way stellar halo. Most significantly, the model predicts the presence of many large-scale, coherent substructures in the outer halo. These substructures are remnants of individual, tidally disrupted dwarf satellite galaxies. Substructure is more pronounced at large galactocentric radii because of the smaller number density of tidal streams and the longer orbital times. This model provides a natural explanation for the coherent structures in the outer stellar halo found in the SDSS commissioning data, and it predicts that many more such structures should be found as the survey covers more of the sky. The detection (or non-detection) and characterization of such structures could eventually test variants of the CDM scenario, especially those that aim to solve the dwarf satellite problem by enhancing satellite disruption.

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arXiv2000

Velocity and Mass Functions of Galactic Halos: Evolution and Environmental Dependence

We study the distribution functions of mass and circular velocity for dark matter halos in N-body simulations of the $Λ$CDM cosmology, addressing redshift and environmental dependence. The dynamical range enables us to resolve subhalos and distinguish them from "distinct" halos. The mass function is compared to analytic models, and is used to derive the more observationally relevant circular velocity function. The distribution functions in the velocity range 100--500 km/s are well fit by a power-law with two parameters, slope and amplitude. We present the parameter dependence on redshift and provide useful fitting formulae. The amplitudes of the mass functions decrease with z, but, contrary to naive expectation, the comoving density of halos of a fixed velocity ~200 km/s actually increases out to z=5. This is because high-z halos are denser, so a fixed velocity corresponds to a smaller mass. The slope of the velocity function at z=0 is as steep as ~ -4, and the mass and velocity functions of distinct halos steepen with increasing z, while the functions of subhalos do not steepen with $z$, and become even flatter at z>2. A simple observable prediction is that the slope of the velocity function of isolated galaxies is steeper than that of galaxies in groups by as much as unity, reflecting the density biasing of high-velocity halos. We confirm that the Press-Schechter approximation typically overestimates the halo mass function by a factor of ~2, while modified approximations provide improved predictions.

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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

Reionization and the abundance of galactic satellites

One of the main challenges facing standard hierarchical structure formation models is that the predicted abundance of galactic subhalos with circular velocities of 10-30 km/s is an order of magnitude higher than the number of satellites actually observed within the Local Group. Using a simple model for the formation and evolution of dark halos, based on the extended Press-Schechter formalism and tested against N-body results, we show that the theoretical predictions can be reconciled with observations if gas accretion in low-mass halos is suppressed after the epoch of reionization. In this picture, the observed dwarf satellites correspond to the small fraction of halos that accreted substantial amounts of gas before reionization. The photoionization mechanism naturally explains why the discrepancy between predicted halos and observed satellites sets in at about 30 km/s, and for reasonable choices of the reionization redshift (z_re = 5-12) the model can reproduce both the amplitude and shape of the observed velocity function of galactic satellites. If this explanation is correct, then typical bright galaxy halos contain many low-mass dark matter subhalos. These might be detectable through their gravitational lensing effects, through their influence on stellar disks, or as dwarf satellites with very high mass-to-light ratios. This model also predicts a diffuse stellar component produced by large numbers of tidally disrupted dwarfs, perhaps sufficient to account for most of the Milky Way's stellar halo.

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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

The Velocity Function of Galaxies

We present a galaxy circular velocity function, Psi(log v), derived from existing luminosity functions and luminosity-velocity relations. Such a velocity function is desirable for several reasons. First, it enables an objective comparison of luminosity functions obtained in different bands and for different galaxy morphologies, with a statistical correction for dust extinction. In addition, the velocity function simplifies comparison of observations with predictions from high-resolution cosmological N-body simulations. We derive velocity functions from five different data sets and find rough agreement among them, but about a factor of 2 variation in amplitude. These velocity functions are then compared with N-body simulations of a LCDM model (corrected for baryonic infall) in order to demonstrate both the utility and current limitations of this approach. The number density of dark matter halos and the slope of the velocity function near v_*, the circular velocity corresponding to an ~L_* spiral galaxy, are found to be comparable to that of observed galaxies. The primary sources of uncertainty in construction of Psi(log v) from observations and N-body simulations are discussed and explanations are suggected to account for these discrepancies.

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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

Comments on non-Gaussian density perturbations and the production of primordial black holes

We review the basic arguments for the likelihood of non-Gaussian density perturbations in inflation models with primordial black hole (PBH) production. We discuss our derived distributions of field fluctuations and their implications, specifically commenting on the fine-tuning problem. We also discuss how the derived distributions may be affected when linked to metric perturbations. While linking the metric perturbations to field fluctuations in a nonlinear way may be important for determining exact probability distributions, the correct mapping is not self-evident. The calculation of P. Ivanov, which yields skew positive distribution, is based on an ansatz for the behavior of the nonlinear metric perturbation. We note that the ``natural'' generalization of the gauge-invariant formalism favored by Bond and Salopek yields an effective linear link between the distribution of field fluctuations and metric perturbations during inflation.

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arXiv1998

The Cores of Dark Matter Dominated Galaxies: theory vs. observations

We use the rotation curves of a sample of dark matter dominated dwarf and low-surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shape of the rotation curves is remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp (rho(r)\propto 1/r^gamma, gamma\approx 0.2-0.4), corresponding to a steeply rising velocity curve (v(r)\propto r^g, g\approx 0.9-0.8). We further show that the observed shape of the rotation curves is well matched by the density profiles of DM halos formed in very high resolution simulations of the CDM, LCDM, and CHDM models of structure formation. This is surprising in light of several previous studies which suggested that the structure of simulated DM halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy. We show that although the mass distribution in the hierarchically formed halos is on average consistent with the shape of rotation curves of dark matter dominated galaxies, the scatter of the individual profiles around the average is substantial and should not be neglected in comparisons with the data. Finally, we show that the DM halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales, and increase in their maximum rotation velocities with increasing radii at which their maximum velocity occurs. (Abridged)

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arXiv1996

Non-Gaussian Fluctuations and Primordial Black Holes from Inflation

We explore the role of non-Gaussian fluctuations in primordial black hole (PBH) formation and show that the standard Gaussian assumption, used in all PBH formation papers to date, is not justified. Since large spikes in power are usually associated with flat regions of the inflaton potential, quantum fluctuations become more important in the field dynamics, leading to mode-mode coupling and non-Gaussian statistics. Moreover, PBH production requires several sigma (rare) fluctuations in order to prevent premature matter dominance of the universe, so we are necessarily concerned with distribution tails, where any intrinsic skewness will be especially important. We quantify this argument by using the stochastic slow-roll equation and a relatively simple analytic method to obtain the final distribution of fluctuations. We work out several examples with toy models that produce PBH's, and show that the naive Gaussian assumption can result in errors of many orders of magnitude. For models with spikes in power, our calculations give sharp cut-offs in the probability of large positive fluctuations, meaning that Gaussian distributions would vastly over-produce PBH's. The standard results that link inflation-produced power spectra and PBH number densities must then be reconsidered, since they rely quite heavily on the Gaussian assumption. We point out that since the probability distributions depend on the nature of the potential, it is impossible to obtain results for general models. However, calculating the distribution of fluctuations for any specific model seems to be relatively straightforward, at least in the single inflaton case.

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