arXiv subjects

Joel R. Primack

Publications and source records attributed to Joel R. Primack.

At least 127 records · Page 7Linked to original sources

arXiv2003

The Origin and Distribution of Angular Momemtum in Galaxies

Cold Dark Matter with a large cosmological constant (LambdaCDM) appears to fit large scale structure observations well. Of the possible small scale problems, the Central Cusps and Too Many Satellites problems now appear to be at least partly solved, so Angular Momentum has become the most serious remaining CDM problem. There are actually at least two different angular momentum problems: A. Too much transfer of angular momentum to the dark halo to make big disks, and B. Wrong distribution of spedific angular momentum to make spiral galaxies, if the baryonic material has the same angular momentum distribution as the dark matter. The angular momentum of dark matter halos, and presumably that of the galaxies they host, appears to arise largely from the orbital angular momentum of the satellites that they accrete. Since the dark and baryonic matter behave very differently in such accretion events, it is possible that the resulting angular momentum distribution of the baryons is different from that of the dark matter, as required to make the sort of galactic disks that are observed. The latest hydrodynamical simulations give some grounds for hope on this score, but much higher resolution simulations are needed.

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arXiv2003

Summary talk: How serious are the problems faced by CDM: cusps, thin disks, and halo substructure

This is a summary of the discussion in Panel I of IAU Symposium 220 on Dark Matter in Galaxies. The panel topics and panellists were as follows -- Cusps Theory: Julio Navarro; Cusps Observations: Erwin de Blok, Rob Swaters; Substructure Observations: Mario Mateo; Substructure Theory: James Taylor, Shude Mao. In addition to the talks by the panellists and the discussion among them, there was a good deal of discussion with the audience. Despite the differences of opinion expressed I think a consensus emerged, and I try to summarize it here.

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arXiv2002

Damped Lyman alpha systems and galaxy formation models - II. High ions and Lyman limit systems

We investigate a model for the high-ionization state gas associated with observed damped Lyman-alpha systems, based on a semi-analytic model of galaxy formation set within the paradigm of hierarchical structure formation. In our model, the hot gas in halos and sub-halos gives rise to CIV absorption, while the low-ionization state gas is associated with the cold gas in galaxies. The model matches the distribution of CIV column densities and leads naturally to kinematic properties that are in good agreement with the data. We examine the contribution of both hot and cold gas to sub-damped systems and suggest that the properties of these systems can be used as an important test of the model. We expect that sub-DLA systems will generally be composed of a single gas disk and thus predict that they should have markedly different kinematics than the damped systems. Finally, we find that hot halo gas produces less than one third of Lyman limit systems at redshift three. We model the contribution of mini-halos (halos with virial velocities < 35 km/s) to Lyman limit systems and find that they may contain as much gas as is observed in these systems. However, if we adopt realistic models of the gas density distribution we find that these systems are not a significant source of Lyman limit absorption. Instead we suggest that uncollapsed gas outside of virialized halos is responsible for most of the Lyman limit systems at high redshift.

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arXiv2002

Status of Cold Dark Matter Cosmology

Cold Dark Matter (CDM) has become the standard modern theory of cosmological structure formation. Its predictions appear to be in good agreement with data on large scales, and it naturally accounts for many properties of galaxies. But despite its many successes, there has been concern about CDM on small scales because of the possible contradiction between the linearly rising rotation curves observed in some dark-matter-dominated galaxies vs. the $1/r$ density cusps at the centers of simulated CDM halos. Other CDM issues on small scales include the very large number of small satellite halos in simulations, far more than the number of small galaxies observed locally, and problems concerning the angular momentum of the baryons in dark matter halos. The latest data and simulations have lessened, although not entirely resolved, these concerns. Meanwhile, the main alternatives to CDM that have been considered to solve these problems, self-interacting dark matter (SIDM) and warm dark matter (WDM), have been found to have serious drawbacks.

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arXiv2002

Probing Galaxy Formation with High Energy Gamma-Rays

I 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, based on the flat LCDM hierarchical structure formation scenario with $Ω_m=0.3$ and Hubble parameter $h=0.65$, to obtain predictions of the EBL from 0.1 to 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. 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 $γ$-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.

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arXiv2001

The Masses of Lyman Break Galaxies

Data on galaxies at high redshift, identified by the Lyman-break photometric technique, can teach us about how galaxies form and evolve. The stellar masses and other properties of such Lyman break galaxies (LBGs) depend sensitively on the details of star formation. In this paper we consider three different star formation prescriptions, and use semi-analytic methods applied to the now-standard $Λ$CDM theory of hierarchical structure formation to show how these assumptions about star formation affect the predicted masses of the stars in these galaxies and the masses of the dark matter halos that host them. We find that, within the rather large uncertainties, recent estimates of the stellar masses of LBGs from multi-color photometry are consistent with the predictions of all three models. However, the estimated stellar masses are more consistent with the predictions of two of the models in which star formation is accelerated at high redshifts $z\gsim3$, and of these models the one in which many of the LBGs are merger-driven starbursts is also more consistent with indications that many high redshift galaxies are gas rich. The clustering properties of LBGs have put some constraints on the masses of their host halos, but due to similarities in the halo occupation of the three models we consider and degeneracies between model parameters, current constraints are not yet sufficient to distinguish between realistic models.

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arXiv2001

The Nature of Dark Matter

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arXiv2001

Whatever Happened to Hot Dark Matter?

This article summarizes the possible roles of neutrinos in cosmology, from the first three minutes onward. The fact that primordial neutrinos are about as numerous as the photons of the cosmological background radiation means that neutrino (or ``hot'') dark matter can have a significant impact upon how matter is distributed in the universe. Over the past two decades, the liklihood of this has risen and fallen as more and more data has become available from laboratory experiments and the latest telescopes. Hot dark matter models were studied in the late 1970s and early 1980s, and there was considerable interest in cold plus hot dark matter models for a few years in the mid-1990s. We now know from Super-Kamiokande and Sudbury Neutrino Observatory data that neutrinos have mass, and that the total neutrino contribution to the cosmological density must be at least as large as $Ω_ν\sim 0.001$, a little less than that of the visible stars in the universe. But the upper limit on the hot dark matter cosmological density is $Ω_ν< 0.1$; correspondingly, the sum of the masses of all three active neutrino species is not more than about 5 eV.

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arXiv2001

Concentrations of Dark Halos from their Assembly Histories

(abridged) We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify its merger-history tree, and determine concentration parameters c_vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a_c, defined when the log mass accretion rate dlogM/dloga falls below a critical value S. We find that late forming galaxies tend to be less concentrated, such that c_vir ``observed'' at any epoch a_o is strongly correlated with a_c via c_vir=c_1*a_o/a_c. Scatter about this relation is mostly due to measurement errors in c_v and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. We demonstrate that this relation can also be used to predict the mass and redshift dependence of c_v, and the scatter about the median c_vir(M,z), using accretion histories derived from the Extended Press-Schechter (EPS) formalism, after adjusting for a constant offset between the formation times as predicted by EPS and as measured in the simulations;this new ingredient can thus be easily incorporated into semi-analytic models of galaxy formation. The correlation found between halo concentration and mass accretion rate suggests a physical interpretation: for high mass infall rates the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant and the halo concentration just grows as R_vir. The tight correlation demonstrated here provides an essential new ingredient for galaxy formation modeling.

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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

A universal angular momentum profile for galactic halos

[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(<j) = Mv μj/(j_0+j). This profile is defined by one shape parameter (μor j_0) in addition to the global spin parameter λ. It follows a power-law over most of the mass, and flattens at large j, with the flattening more pronounced for small values of μ. Compared to a uniform sphere in solid-body rotation, most halos have a higher fraction of their mass in the low- and high-j tails of the distribution. The spatial distribution of angular momentum in halos tends to be cylindrical and is well-aligned within each halo for ~80% of the halos. We investigate two ideas for the origin of this profile. The first is based on a revised version of linear tidal-torque theory combined with extended Press-Schechter mass accretion, and the second focuses on j transport in minor mergers. Finally, we briefly explore implications of the M(<j) profile on the formation of galactic disks assuming that j is conserved during an adiabatic baryonic infall. The implied gas density profile deviates from an exponential disk, with a higher density at small radii and a tail extending to large radii. The steep central density profiles may imply disk scale lengths that are smaller than observed. This is reminiscent of the "angular-momentum problem" seen in hydrodynamic simulations, even though we have assumed perfect j conservation. A possible solution is to associate the central excesses with bulge components and the outer regions with extended gaseous disks.

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arXiv2000

Evolution of Mass Distribution in CDM Halos

On the basis of a new convergence study of high-resolution N-body simulations, my colleagues and I now agree that the Navarro, Frenk, & White (1996) density profile $ρ_{NFW}(r) \propto r^{-1} (r+r_s)^{-2}$ is a good representation of typical dark matter halos of galactic mass. Comparing simulations of the same halo with numbers of particles ranging from $\sim10^3$ to $\sim10^6$, we have also shown that $r_s$, the radius where the log-slope is -2, can be determined accurately for halos with as few as $\sim10^3$ particles. Based on a study of thousands of halos at many redshifts in an Adaptive Refinement Tree (ART) simulation of a cosmological volume in a $Λ$CDM cosmology, we have found that the concentration $\cvir \equiv \Rvir/\rs$ has a log-normal distribution, with $1σ$ $Δ(\log \cvir) = 0.18$ at a given mass, corresponding to a scatter in maximum rotation velocities of $Δ\Vmax/\Vmax = 0.12$. The average concentration declines with redshift at fixed mass as $\cvir(z) \propto (1+z)^{-1}$. This may have important implications for galaxy rotation curves. Finally, we have found that the velocity function determined from galaxy luminosity functions plus luminosity-velocity relations agrees with the predictions from our $Λ$CDM simulations. But we also note that the very limited evolution with redshift of the velocity function predicted by $Λ$CDM conflicts with the data that is becoming available on the number density of bright galaxies unless there is significant evolution of the luminosity-velocity relation at $z>1$.

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arXiv2000

Interaction rates of dark-matter halos and subhalos

[Abridged] The interaction rates of dark-matter halos and subhalos, are computed using high-resolution cosmological N-body simulations of the Lambda-CDM model. Although the number fraction of subhalos of mass $>2x10^{11}\hsolmass$ is only ~10%, we find that the interaction rate of such subhalos is relatively high because they reside in high density environments. At low redshift, the subhalo collisions dominate the total collision rate, and even at z=3 they are involved in more than 30% of all collisions. About 40% of the "major" mergers are between subhalos. Therefore subhalo interactions must be incorporated in models of structure formation. We find that the collision rate between halos in physical density units, is $\propto (1+z)^δ$, with $δ= 3-4$, in agreement with earlier simulations and most observational data. We test previous analytic estimates of the interaction rates of subhalos based on statistical models, which could be very inaccurate because of the small number of subhalos and the variation of conditions within small host halos. We find that, while such statistical estimates may severely overestimate the rate within hosts $< 10^{13} \hsolmass$, typical of high redshifts, they are valid for larger hosts regardless of the number of subhalos in them. We find the Makino & Hut (1997) estimate of the subhalo merger rate to be valid for hosts $ > 6x10^{11}\hsolmass$ at all redshifts. The collision rate between subhalos and the central object of their host halo is approximated relatively well using the timescale for dynamical friction in circular orbits. This approximation fails in ~40% of the cases, partly because of deviations from the assumption of circular orbits and partly because of the invalidity of the assumption that the subhalo mass is negligible.

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arXiv2000

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

[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$α$ 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.

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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

Cosmological Parameters 2000

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 $Ω_m$, the baryonic matter density $Ω_b$, the neutrino density $Ω_ν$, and the cosmological constant $Ω_Λ$. The evidence currently favors $t_0 \approx 13$ Gyr, $h \approx 0.65$, $Ω_m \approx 0.4\pm0.1$, $Ω_b = 0.02h^{-2}$, $0.001 < Ω_ν< 0.1$, and $Ω_Λ\approx 0.7$.

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arXiv2000

Hot Dark Matter in Cosmology

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 $Ω_ν\gsim 0.001$. The possible improvement of low-$Ω_m$ flat ($Λ$CDM) cosmological models with the addition of light neutrinos appears to be rather limited, but$Λ$CDM models with $Ω_ν\lsim 0.1$ 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 $Ω_ν$ and neutrino masses.

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