Showing papers by "Joel R. Primack published in 2000"

Resolving the Structure of Cold Dark Matter Halos II

Abstract: We study the effects of mass and force resolution on the density profiles of galaxy-size Cold Dark Matter (CDM) halos in a flat, low-density cosmological model with vacuum energy. We show thatalthough increasing the mass and force resolution allows us to probe deeper into the inner halo regions, it does not lead to steeper inner density profiles. Instead, the halo profiles converge at scales larger than four times the formal resolution or the radius containing more than 200 particles, whichever is larger. In the simulations presented in this paper, we are able to probe density profile of a relaxed isolated galaxy-size halo at scales r=(0.005-1)r_vir. We find that the density distribution can be well approximated by the profile suggested by Moore etal (1998): rho= x^{-1.5}(1+x^{1.5})^-1, where x=r/r_s and r_s is the characteristic radius. The analytical profile proposed by Navarro et al. (1996) rho= x^{-1}(1+x)^-2, also provides a good fit, with the same relative errors of about 10% for radii larger than 1% of the virial radius. Both analytical profiles fit equally well because for high-concentration galaxy-size halos the differences between these profiles become significant only at scales well below 0.01r_vir. We also find that halos of similar mass may have somewhat different parameters (characteristic radius, maximum rotation velocity, etc.) and shapes of their density profiles. We associate this scatter in properties with differences in halo merger histories and the amount of substructure present in the analyzed halos.

The Velocity Function of Galaxies

Abstract: We present a galaxy circular velocity function, Ψ(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 ΛCDM model (corrected for baryonic infall) in order to demonstrate both the utility and the 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 those of observed galaxies. The primary sources of uncertainty in construction of Ψ(log v) from observations and N-body simulations are discussed, and explanations to account for discrepancies are suggested.

Breaking the Disk/Halo Degeneracy with Gravitational Lensing

Abstract: The degeneracy between the disk and the dark matter contribution to galaxy rotation curves remains an important uncertainty in our understanding of disk galaxies. Here we discuss a new method for breaking this degeneracy using gravitational lensing by spiral galaxies, and apply this method to the spiral lens B1600+434 as an example. The combined image and lens photometry constraints allow models for B1600+434 with either a nearly singular dark matter halo or a halo with a sizable core. If the dark halo has a core, then the bulge dominates the gravitational potential in the inner part of the galaxy, its mass is between 1.3 and 1.5 × 1011 M☉, and the disk mass is less then 5 × 1010 M☉. If the dark halo is singular, there is a degeneracy between the disk mass and the halo ellipticity. The dark halo flattening (c/a) can be as low as 0.53 if there is no disk mass, while the maximum allowed disk mass, 1.3 × 1011 M☉, is reached with a spherical halo. A maximum disk model is ruled out with high confidence. Further information, such as the circular velocity of this galaxy, will help break the degeneracies. Future studies of spiral galaxy lenses will be able to determine the relative contribution of disk, bulge, and halo to the mass in the inner parts of galaxies.

Arc Statistics in Clusters: Galaxy Contribution

Abstract: The frequency with which background galaxies appear as long arcs as a result of gravitational lensing by foreground clusters of galaxies has recently been found by Bartelmann et al. to be a very sensitive probe of cosmological models. They have found that such arcs would be expected far less frequently than observed (by an order of magnitude) in the currently favored model of the universe, with a large cosmological constant, ΩΛ ~ 0.7. Here we analyze whether including the effect of cluster galaxies on the likelihood of clusters to generate long-arc images of background galaxies can change the statistics. Taking into account a variety of constraints on the properties of cluster galaxies, we find that there are not enough sufficiently massive galaxies in a cluster for them to significantly enhance the cross section of clusters and generate long arcs. We find that cluster galaxies typically enhance the cross section by only 15%.

A universal angular momentum profile for galactic halos

Abstract: [Abridged] We study the angular-momentum profiles of a statistical sample of halos drawn from a high-resolution N-body simulation of the LCDM cosmology. We find that the cumulative mass distribution of specific angular momentum, j, in a halo of mass Mv is well fit by a universal function, M(

Hot Dark Matter in Cosmology

Abstract: Cosmological dark matter in the form of neutrinos with masses of up to a few electron volts is known as hot dark matter. After an historical review of the subject, this article considers constraints on hot dark matter from current data on neutrino oscillations and on cosmology. The atmospheric neutrino oscillation data imply a lower limit on the HDM contribution to the cosmological density $\Omega_ u \gsim 0.001$. The possible improvement of low-$\Omega_m$ flat ($\Lambda$CDM) cosmological models with the addition of light neutrinos appears to be rather limited, but$\Lambda$CDM models with $\Omega_ u \lsim 0.1$ may be consistent with presently available data. Data expected soon may permit detection of such a hot dark matter contribution, or alternatively provide stronger upper limits on $\Omega_ u$ and neutrino masses.

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

Abstract: 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

Cosmological Parameters 2000

Abstract: The cosmological parameters that I emphasize are the age of the universe $t_0$, the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the average matter density $\Omega_m$, the baryonic matter density $\Omega_b$, the neutrino density $\Omega_ u$, and the cosmological constant $\Omega_\Lambda$. The evidence currently favors $t_0 \approx 13$ Gyr, $h \approx 0.65$, $\Omega_m \approx 0.4\pm0.1$, $\Omega_b = 0.02h^{-2}$, $0.001 < \Omega_ u < 0.1$, and $\Omega_\Lambda \approx 0.7$.

Cosmological parameters 2000

Abstract: The cosmological parameters that I emphasize are the age of the universe $t_0$, the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the average matter density $\Omega_m$, the baryonic matter density $\Omega_b$, the neutrino density $\Omega_ u$, and the cosmological constant $\Omega_\Lambda$. The evidence currently favors $t_0 \approx 13$ Gyr, $h \approx 0.65$, $\Omega_m \approx 0.4\pm0.1$, $\Omega_b = 0.02h^{-2}$, $0.001 < \Omega_ u < 0.1$, and $\Omega_\Lambda \approx 0.7$.

Probing Galaxy Formation with High Energy Gamma-Rays

Abstract: I discuss how measurements of the absorption of $\gamma$-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, based on the flat LCDM hierarchical structure formation scenario with $\Omega_m=0.3$ and Hubble parameter $h=0.65$, to obtain predictions of the EBL from 0.1 to 1000$\mu$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. 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 also discuss recent attempts to determine the emitted spectrum of high energy gamma rays from blazars such as Mrk 501 from the observed X-rays using the synchrotron self-Compton model, and note that our favorite SAM EBL plus the observed spectrum of Mrk 501 do not imply unphysical upturns in the high energy emitted spectrum - thus undermining recent claims of a crisis with drastic possible consequences such as breaking of Lorentz invariance. We conclude that observational studies of the absorption of $\gamma$-rays 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.

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

Abstract: We study the distribution functions of mass and circular velocity for dark matter halos in N-body simulations of the $\Lambda$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

The Nature of High-Redshift Galaxies

Abstract: Using semi-analytic models of galaxy formation set within the Cold Dark Matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift ($z \ga 2$) Lyman-break galaxies (LBGs). We consider a ``collisional starburst'' model in which bursts of star formation are triggered by galaxy-galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at $z\sim 3$ and $z\sim 4$, with a reasonable amount of dust extinction. Model galaxies at $z \sim 3$ have star formation rates, half-light radii, I-K colours, and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two ``quiescent'' models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out as it does not produce enough high redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints.

Spin Profile of Galactic Halos and Disk Formation

Abstract: We summarize recent developments in the study of the origin of halo spin profiles and preliminary implications on disk formation. The specific angular-momentum distributions within halos in N-body simulations match a universal shape, M(

Origin and Destiny of Dark Matter Halos: Cosmological Matter Exchange and Metal Enrichment

Abstract: [Abridged] We analyze the exchange of dark matter between halos, subhalos, and their environments in a high-resolution cosmological N-body simulation of a Lambda CDM cosmology At each analyzed redshift z we divide the dark matter particles into 4 components: (i) isolated galactic halos, (ii) subhalos, (iii) the diffuse medium of group and cluster halos, and (iv) the background outside of virialized halos We follow the time evolution of the mass distribution and flows between these components and provide fitting functions for the exchange rates We use our derived exchange rates to gauge the importance of metal redistribution in the universe due solely to gravity-induced interactions The diffuse metallicity in clusters is predicted to be ~40% that in isolated galaxies (~55% of groups) at z=0, and should be lower only slightly by z=1, consistent with observations The metallicity of the diffuse media in poor groups is expected to be lower by a factor of 5 by z~2, in agreement with the observed metallicity of damped Ly$\alpha$ systems The metallicity of the background IGM is predicted to be (1-3)x10^{-4} that of z=0 clusters, also consistent with observations The agreement of predicted and observed trends indicates that gravitational interaction alone may play an important role in metal enrichment of the intra-cluster and intergalactic media

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

Abstract: 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.

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

Abstract: 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

Optical Counterparts to Damped Lyman Alpha Systems

Abstract: 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.