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Search Results: 1 - 10 of 342870 matches for " James S. Bullock "
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Shapes of dark matter halos
James S. Bullock
Physics , 2001,
Abstract: I present an analysis of the density shapes of dark matter halos in LCDM and LWDM cosmologies. The main results are derived from a statistical sample of galaxy-mass halos drawn from a high resolution LCDM N-body simulation. Halo shapes show significant trends with mass and redshift: low-mass halos are rounder than high mass halos, and, for a fixed mass, halos are rounder at low z. Contrary to previous expectations, which were based on cluster-mass halos and non-COBE normalized simulations, LCDM galaxy-mass halos at z=0 are not strongly flattened, with short to long axis ratios of s = 0.70 +/- 0.17. I go on to study how the shapes of individual halos change when going from a LCDM simulation to a simulation with a warm dark matter power spectrum (LWDM). Four halos were compared, and, on average, the WDM halos are more spherical than their CDM counterparts (s =0.77 compared to s = 0.71). A larger sample of objects will be needed to test whether the trend is significant.
Notes on the Missing Satellites Problem
James S. Bullock
Physics , 2010,
Abstract: The Missing Satellites Problem (MSP) broadly refers to the overabundance of predicted Cold Dark Matter (CDM) subhalos compared to satellite galaxies known to exist in the Local Group. The most popular interpretation of the MSP is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The question from that standpoint is to identify the feedback source that makes small halos dark and to identify any obvious mass scale where the truncation in the efficiency of galaxy formation occurs. Among the most exciting developments in near-field cosmology in recent years is the discovery of a new population satellite galaxies orbiting the Milky Way and M31. Wide field, resolved star surveys have more than doubled the dwarf satellite count in less than a decade, revealing a population of ultrafaint galaxies that are less luminous that some star clusters. For the first time, there are empirical reasons to believe that there really are missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to have been surveyed. Both kinematic studies and completeness-correction studies seem to point to a characteristic potential well depth for satellite subhalos that is quite close to the mass scale where photoionization and atomic cooling should limit galaxy formation. Among the more pressing problems associated with this interpretation is to understand the selection biases that limit our ability to detect the lowest mass galaxies. The least massive satellite halos are likely to host stealth galaxies with very-low surface brightness and this may be an important limitation in the hunt for low-mass fossils from the epoch of reionization.
Tilted CDM versus WDM in the Subgalactic Scuffle
James S. Bullock
Physics , 2001,
Abstract: Although the currently favored cold dark matter plus cosmological constant model (LCDM) has proven to be remarkably successful on large scales, on subgalactic scales it faces some potentially fatal difficulties; these include over-producing dwarf satellite galaxies and predicting excessive central densities in dark halos. Among the most natural cosmological solutions to these problems is to replace cold dark matter with a warm species (LWDM). The warm component acts to reduce the small-scale power, resulting in fewer galactic subhalos and lower halo central densities. An alternative model with a mild ``tilt'' in the inflationary power spectrum (TLCDM; n =0.9) similarly reduces the central densities of dark halos, although the substructure abundance remains relatively high. Here I argue that because dwarf galaxy formation should be suppressed in the presence of a strong ionizing background, favored LWDM models will generally under-predict the observed abundance of dwarf galaxies. The satellite count for TLCDM fairs much better, as long as the photoionization effect is taken into account. TLCDM provides a more successful alternative to LWDM on subgalactic scales with the added attraction that it relies on only a minor, natural adjustment to the standard framework of CDM.
Faint AGN and the Ionizing Background
Michael Schirber,James S. Bullock
Physics , 2002, DOI: 10.1086/345662
Abstract: We determine the evolution of the faint, high-redshift, optical luminosity function (LF) of AGN implied by several observationally-motivated models of the ionizing background. Our results depend crucially on whether we use the total ionizing rate measured by the proximity effect technique or the lower determination from the flux decrement distribution of Ly alpha forest lines. Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end slope and normalization, we find that the LF must break at M_B*=-24.2,-22.3, -20.8 at z=3,4, 5 if we adopt the lower ionization rate and assume no stellar contribution to the background. The break must occur at M_B*=-20.6,-18.7, -18.7 for the proximity effect background estimate. These values brighten by as much as ~2 mag if high-z galaxies contribute to the background with an escape fraction of ionizing photons consistent with recent estimates: f_e=0.16. By comparing to faint AGN searches, we find that the typically-quoted proximity effect estimates of the background imply an over-abundance of faint AGN (even with f_e=1). Even adopting the lower bound on proximity effect measurements, the stellar escape fraction must be high: f_e>0.2. Conversely, the lower flux- decrement-derived background requires a limited stellar contribution: f_e<0.05. Our derived LFs together with the locally-estimated black hole density suggest that the efficiency of converting mass to light in optically-unobscured AGN is somewhat lower than expected, <0.05. Comparison with similar estimates based on X-ray counts suggests that more than half of all AGN are obscured in the UV/optical. We also derive lower limits on typical AGN lifetimes and obtain >10^7 yrs for favored cases.
Dark matter concentrations and a search for cores in Milky Way dwarf satellites
Joe Wolf,James S. Bullock
Physics , 2012,
Abstract: We investigate the mass distributions within eight classical Milky Way dwarf spheroidal galaxies (MW dSphs) using an equilibrium Jeans analysis and we compare our results to the mass distributions predicted for subhalos in dissipationless \Lambda CDM simulations. In order to match the dark matter density concentrations predicted, the stars in these galaxies must have a fairly significant tangential velocity dispersion anisotropy (\beta ~-1.5). For the limiting case of an isotropic velocity dispersion (\beta =0), the classical MW dSphs predominantly prefer to live in halos that are less concentrated than \Lambda CDM predictions. We also investigate whether the dSphs prefer to live in halos with constant density cores in the limit of isotropic velocity dispersion. Interestingly, even in this limit, not all of the dSphs prefer large constant-density cores: the Sculptor dSph prefers a cusp while Carina, Draco and Leo I prefer cores. The other four dSphs do not show a statistically significant preference for either cuspy or cored profiles. Finally, we re-examine the hypothesis that the density profiles of these eight MW dSphs can be quantified by a common dark matter halo.
Beacons In the Dark: Using Novae and Supernovae to Detect Dwarf Galaxies in the Local Universe
Charlie Conroy,James S. Bullock
Physics , 2015, DOI: 10.1088/2041-8205/805/1/L2
Abstract: We propose that luminous transients, including novae and supernovae, can be used to detect the faintest galaxies in the universe. Beyond a few Mpc, dwarf galaxies with stellar masses $<10^6 M_{\odot}$ will likely be too faint and/or too low in surface brightness to be directly detected in upcoming large area ground-based photometric surveys. However, single epoch LSST photometry will be able to detect novae to distances of $\sim30$ Mpc and SNe to Gpc-scale distances. Depending on the form of the stellar mass-halo mass relation and the underlying star formation histories of low mass dwarfs, the expected nova rates will be a few to $\sim100$ yr$^{-1}$ and the expected SN rates (including both type Ia and core-collapse) will be $\sim10^2-10^4$ within the observable ($4\pi$ sr) volume. The transient rate associated with intrahalo stars will be comparably large, but these transients will be located close to bright galaxies, in contrast to the dwarfs, which should trace the underlying large scale structure of the cosmic web. Aggressive follow-up of hostless transients has the potential to uncover the predicted enormous population of low mass field dwarf galaxies.
Halo Substructure And The Power Spectrum
Andrew R. Zentner,James S. Bullock
Physics , 2003, DOI: 10.1086/378797
Abstract: (ABRIDGED) We present a semi-analytic model to explore merger histories, destruction rates, and survival probabilities of substructure in dark matter halos and use it to study the substructure populations of galaxy-sized halos as a function of the power spectrum. We successfully reproduce the subhalo velocity function and radial distribution seen in N-body simulations for standard LCDM. We explore the implications of spectra with normalizations and tilts spanning sigma_8 = 0.65-1 and n = 0.8-1. We also study a running index (RI) model with dn/dlnk=-0.03, as discussed in the first year WMAP report, and several WDM models with masses m_W = 0.75, 1.5, 3.0 keV. The substructure mass fraction is relatively insensitive to the tilt and overall normalization of the power spectrum. All CDM-type models yield projected substructure mass fractions that are consistent with, but on the low side of, estimates from strong lens systems: f = 0.4-1.5% (64 percentile) in systems M_sub < 10^9 Msun. Truncated models produce significantly smaller fractions and are disfavored by lensing results. We compare our predicted subhalo velocity functions to the dwarf satellite population of the Milky Way. Assuming isotropic velocity dispersions, we find the standard n=1 model overpredicts the number of MW satellites as expected. Models with less small-scale power are more successful because there are fewer subhalos of a given circular velocity and the mapping between observed velocity dispersion and halo circular velocity is markedly altered. The RI model, or a fixed tilt with sigma_8=0.75, can account for the MW dwarfs without the need for differential feedback; however, these comparisons depend sensitively on the assumption of isotropic velocities in satellite galaxies.
Non-Gaussian Fluctuations and Primordial Black Holes from Inflation
James S. Bullock,Joel R. Primack
Physics , 1996, DOI: 10.1103/PhysRevD.55.7423
Abstract: 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.
Dark halo densities, substructure, and the initial power spectrum
James S. Bullock,Andrew R. Zentner
Physics , 2002, DOI: 10.1016/S0920-5632(03)02091-7
Abstract: Although the currently favored cold dark matter plus cosmological constant model for structure formation assumes an n=1 scale-invariant initial power spectrum, most inflation models produce at least mild deviations from n=1. Because the lever arm from the CMB normalization to galaxy scales is long, even a small ``tilt'' can have important implications for galactic observations. Here we calculate the COBE-normalized power spectra for several well-motivated models of inflation and compute implications for the substructure content and central densities of galaxy halos. Using an analytic model, normalized against N-body simulations, we show that while halos in the standard (n=1) model are overdense by a factor of ~6 compared to observations, several of our example inflation+LCDM models predict halo densities well within the range of observations, which prefer models with n ~ 0.85. We go on to use a semi-analytic model (also normalized against N-body simulations) to follow the merger histories of galaxy-sized halos and track the orbital decay, disruption, and evolution of the merging substructure. Models with n ~0.85 predict a factor of ~3 fewer subhalos at a fixed circular velocity than the standard $n = 1$ case. Although this level of reduction does not resolve the ``dwarf satellite problem'', it does imply that the level of feedback required to match the observed number of dwarfs is sensitive to the initial power spectrum. Finally, the fraction of galaxy-halo mass that is bound up in substructure is consistent with limits imposed by multiply imaged quasars for all models considered: f_sub > 0.01 even for an effective tilt of n ~0.8.We conclude that, at their current level, lensing constraints of this kind do not provide strong limits on the primordial power spectrum.
Halo Substructure and the Power Spectrum
Andrew R. Zentner,James S. Bullock
Physics , 2002, DOI: 10.1063/1.1581785
Abstract: In this proceeding, we present the results of a semi-analytic study of CDM substructure as a function of the primordial power spectrum. We apply our method to several tilted models in the LCDM framework with n=0.85-1.1, sigma_8=0.65-1.2 when COBE normalized. We also study a more extreme, warm dark matter-like spectrum that is sharply truncated below a scale of 10^10 h^-1 Msun. We show that the mass fraction of halo substructure is not a strong function of spectral slope, so it likely will be difficult to constrain tilt using flux ratios of gravitationally lensed quasars. On the positive side, all of our CDM-type models yield projected mass fractions in good agreement with strong lensing estimates: f \sim 1.5% at M \sim 10^8 Msun. The truncated model produces a significantly smaller fraction, f \lsim 0.3%, suggesting that warm dark matter-like spectra may be distinguished from CDM spectra using lensing. We also discuss the issue of dwarf satellite abundances, with emphasis on the cosmological dependence of the map between the observed central velocity dispersion of Milky Way satellites and the maximum circular velocities of their host halos. In agreement with earlier work, we find that standard LCDM over-predicts the estimated count of Milky Way satellites at fixed Vmax by an order of magnitude, but tilted models do better because subhalos are less concentrated. Interestingly, under the assumption that dwarfs have isotropic velocity dispersion tensors, models with significantly tilted spectra (n \lsim 0.85, sigma_8 \lsim 0.7) may under-predict the number of large Milky Way satellites with Vmax \gsim 40 km/s.
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