Results for "James S. Bullock"

total 157914took 0.16s
Notes on the Missing Satellites ProblemSep 23 2010The 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 ... More
Tilted CDM versus WDM in the Subgalactic ScuffleOct 31 2001Although 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 ... More
Shapes of dark matter halosJun 21 2001I 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 ... More
Beacons In the Dark: Using Novae and Supernovae to Detect Dwarf Galaxies in the Local UniverseApr 15 2015We 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 ... More
Dark matter concentrations and a search for cores in Milky Way dwarf satellitesMar 19 2012We 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 ... More
Faint AGN and the Ionizing BackgroundJul 10 2002Oct 17 2002We 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 ... More
Cosmology and the Stellar HaloJan 30 2004If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted and subsequently tidally destroyed ~100 low-mass galaxies in the past ~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach ... More
Tracing Galaxy Formation with Stellar Halos I: MethodsJun 20 2005If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted ~100-200 luminous satellite galaxies in the past \~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach which distinguishes ... More
On the morphologies, gas fractions, and star formation rates of small galaxiesJun 01 2007Oct 12 2007We use a series of N-body/smoothed particle hydrodynamics simulations and analytic arguments to show that the presence of an effective temperature floor in the interstellar medium at T_F ~ 10^4 K naturally explains the tendency for low-mass galaxies to ... More
Halo Substructure And The Power SpectrumApr 15 2003Jul 27 2003(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 ... More
Dark halo densities, substructure, and the initial power spectrumJul 24 2002Although 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 ... More
Non-Gaussian Fluctuations and Primordial Black Holes from InflationNov 13 1996We 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 ... More
Halo Substructure and the Power SpectrumDec 14 2002In 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. ... More
Comments on non-Gaussian density perturbations and the production of primordial black holesJun 22 1998We 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 ... More
Multi-Phase Galaxy Formation: High Velocity Clouds and the Missing Baryon ProblemJun 28 2004Oct 20 2004The standard treatment of cooling in Cold Dark Matter halos assumes that all of the gas within a ``cooling radius'' cools and contracts monolithically to fuel galaxy formation. Here we take into account the expectation that the hot gas in galactic halos ... More
Inflation, cold dark matter, and the central density problemMay 15 2002Aug 23 2002A problem with high central densities in dark halos has arisen in the context of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is often justified by appealing to the inflation scenario, inflationary models with mild deviations ... More
High-Redshift Galaxy Kinematics: Constraints on Models of Disk FormationAug 07 2008Integral field spectroscopy of galaxies at redshift z~2 has revealed a population of early-forming, rotationally-supported disks. These high-redshift systems provide a potentially important clue to the formation processes that build disk galaxies in the ... More
Galaxy Halo Occupation at High RedshiftJun 15 2001Jun 21 2001We 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 ... More
Observational Gamma-ray CosmologyFeb 09 2005We 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. Semi-analytic models (SAMs) of galaxy formation, based ... More
A Revised LCDM Mass Model For The Andromeda GalaxyDec 08 2006Jul 18 2008We present an updated mass model for M31 that makes use of a Spitzer 3.6 $\mu$m image, a mass-to-light ratio gradient based on the galaxy's B-R colour profile, and observed rotation curve data from a variety of sources. We examine cases where the dark ... More
Dark Matter Properties and Halo Central DensitiesSep 21 2001Using an analytic model calibrated against numerical simulations, we calculate the central densities of dark matter halos in a ``conventional'' cold dark matter model with a cosmological constant (LCDM) and in a ``tilted'' model (TLCDM) with slightly ... More
The Epoch of Reionization in Models with Reduced Small Scale PowerMar 21 2003Reducing the power on small scales relative to the `standard' LCDM model alleviates a number of possible discrepancies with observations, and is favored by the recent analysis of WMAP plus galaxy and Lyman-alpha forest data. Here, we investigate the epoch ... More
The Lopsidedness of Satellite Galaxy Systems in $Λ$CDM simulationsOct 20 2017The spatial distribution of satellite galaxies around pairs of galaxies in the Sloan Digital Sky Survey (SDSS) have been found to bulge significantly towards the respective partner. Highly anisotropic, planar distributions of satellite galaxies are in ... More
Too big to fail? The puzzling darkness of massive Milky Way subhaloesFeb 28 2011May 13 2011We show that dissipationless LCDM simulations predict that the majority of the most massive subhaloes of the Milky Way are too dense to host any of its bright satellites (L_V > 10^5 L_sun). These dark subhaloes have circular velocities at infall of 30-70 ... More
Determining the Nature of Dark Matter with AstrometryJan 19 2007We show that measurements of stellar proper motions in dwarf spheroidal galaxies provide a powerful probe of the nature of dark matter. Allowing for general dark matter density profiles and stellar velocity anisotropy profiles, we show that the log-slope ... More
Simulating the Gaseous Halos of GalaxiesJan 28 2008Observations of local X-ray absorbers, high-velocity clouds, and distant quasar absorption line systems suggest that a significant fraction of baryons may reside in multi-phase, low-density, extended, ~100 kpc, gaseous halos around normal galaxies. We ... More
The High-$z$ Universe Confronts Warm Dark Matter: Galaxy Counts, Reionization and the Nature of Dark MatterJan 15 2014May 14 2014We use $N$-body simulations to show that high-redshift galaxy counts provide an interesting constraint on the nature of dark matter, specifically Warm Dark Matter (WDM), owing to the lack of early structure formation these models. Our simulations include ... More
The Metallicity of Diffuse Intrahalo LightMay 19 2008Sep 10 2008We make predictions for the metallicity of diffuse stellar components in systems ranging from small spiral galaxies to rich galaxy clusters. We extend the formalism of Purcell et al. (2007), in which diffuse stellar mass is produced via galaxy disruption, ... More
Cold Dark Matter Substructure and Galactic DisksJul 18 2008We perform a set of high-resolution, dissipationless N-body simulations to investigate the influence of cold dark matter (CDM) substructure on the dynamical evolution of thin galactic disks. Our method combines cosmological simulations of galaxy-sized ... More
Hierarchical galaxy formation and substructure in the Galaxy's stellar haloJul 19 2000We 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 ... More
Angular Momentum Profiles of Warm Dark Matter HalosSep 25 2001We compare the specific angular momentum profiles of virialized dark halos in cold dark matter (CDM) and warm dark matter (WDM) models using high-resolution dissipationless simulations. The simulations were initialized using the same set of modes, except ... More
Mergers and Disk Survival in LCDMNov 06 2008Disk galaxies are common in our universe and this is a source of concern for hierarchical formation models like LCDM. Here we investigate this issue as motivated by raw merger statistics derived for galaxy-size dark matter halos from LCDM simulations. ... More
Shredded Galaxies as the Source of Diffuse Intrahalo Light On Varying ScalesFeb 28 2007May 09 2007We make predictions for diffuse stellar mass fractions in dark matter halos from the scales of small spiral galaxies to those of large galaxy clusters. We use an extensively-tested analytic model for subhalo infall and evolution and empirical constraints ... More
XMM-Newton Survey of Local O VII Absorption Lines in the Spectra of Active Galactic NucleiMar 04 2015Highly ionized, z=0 metal absorption lines detected in the X-ray spectra of background active galactic nuclei (AGNs) provide an effective method to probe the hot ($T\sim10^6$ K) gas and its metal content in and around the Milky Way. We present an all-sky ... More
Reionization and the abundance of galactic satellitesFeb 09 2000One 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 ... More
Infall Times for Milky Way Satellites From Their Present-Day KinematicsOct 03 2011We analyze subhalos in the Via Lactea II (VL2) cosmological simulation to look for correlations among their infall times and z = 0 dynamical properties. We find that the present day orbital energy is tightly correlated with the time at which subhalos ... More
The Milky Way's bright satellites as an apparent failure of LCDMNov 08 2011Mar 21 2012We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf ... More
Heated Disc Stars in the Stellar HaloOct 28 2009Feb 01 2010Minor accretion events with mass ratio M_sat : M_host ~ 1:10 are common in the context of LCDM cosmology. We use high-resolution simulations of Galaxy-analogue systems to show that these mergers can dynamically eject disk stars into a diffuse light component ... More
The Destruction of Thin Stellar Disks Via Cosmologically Common Satellite Accretion EventsOct 15 2008Mar 10 2009Most Galaxy-sized systems (M_host ~ 10^12 M_sun) in the LCDM cosmology are expected to have accreted at least one satellite with a total mass M_sat ~ 10^11 M_sun = 3M_disk in the past 8 Gyr. Analytic and numerical investigations suggest that this is the ... More
The Dark Disk of the Milky WayJun 30 2009Massive satellite accretions onto early galactic disks can lead to the deposition of dark matter in disk-like configurations that co-rotate with the galaxy. This phenomenon has potentially dramatic consequences for dark matter detection experiments. We ... More
Exploring the links between star formation and minor companions around isolated galaxiesOct 10 2011Oct 12 2011Previous studies have shown that galaxies with minor companions exhibit an elevated star formation rate. We reverse this inquiry, constructing a volume-limited sample of \simL\star (Mr \leq -19.5 + 5 log h) galaxies from the Sloan Digital Sky Survey that ... More
The Milky Way's Halo and Subhalos in Self-Interacting Dark MatterMar 04 2019We perform high-resolution simulations of a MW-like galaxy in a self-interacting cold dark matter model with elastic cross section over mass of $1~\rm cm^2/g$ (SIDM) and compare to a model without self-interactions (CDM). We run our simulations with and ... More
Dark Matter Halos with Cores from Hierarchical Structure FormationJun 12 2006Mar 26 2007We show that dark matter emerging from late decays (z < 1000) produces a linear power spectrum identical to that of Cold Dark Matter (CDM) on all observationally relavant scales (> 0.1 Mpc), and simultaneously generates observable constant-density cores ... More
Near-Field Limits on the Role of Faint Galaxies in Cosmic ReionizationMay 05 2014Jul 07 2014Reionizing the Universe with galaxies appears to require significant star formation in low-mass halos at early times, while local dwarf galaxy counts tell us that star formation has been minimal in small halos around us today. Using simple models and ... More
On the Hot Gas Content of the Milky Way HaloNov 05 2012The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea ... More
Scalar Field Dark Matter: Helping or Hurting Small-Scale Problems in Cosmology?Jul 16 2018Building upon results of cosmological simulations of ultra-light scalar field dark matter (SFDM), we present a comprehensive model for the density profiles of SFDM haloes as a function of halo virial mass $M_{\rm h}$ and scalar field mass $m$. The central ... More
M31 Satellite Masses Compared to LCDM SubhaloesMar 25 2014We compare the kinematics of M31's satellite galaxies to the mass profiles of the subhaloes they are expected to inhabit in LCDM. We consider the most massive subhaloes of an approximately M31-sized halo, following the assumption of a monotonic galaxy ... More
Interpreting Debris from Satellite Disruption In External GalaxiesJan 30 2001Apr 12 2001We examine the detectability and interpretation of debris trails caused by satellite disruption in external galaxies using semi-analytic approximations for the dependence of streamer length, width and surface brightness on satellite and primary galaxy ... More
Redistributing hot gas around galaxies: do cool clouds signal a solution to the overcooling problem?Dec 10 2008Apr 06 2009We present a pair of high-resolution smoothed particle hydrodynamics (SPH) simulations that explore the evolution and cooling behavior of hot gas around Milky-Way size galaxies. The simulations contain the same total baryonic mass and are identical other ... More
Probing Galaxy Formation with High Energy Gamma-RaysNov 26 2000We 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, ... More
A Testable Conspiracy: Simulating Baryonic Effects on Self-Interacting Dark Matter HalosSep 27 2016We investigate the response of self-interacting dark matter (SIDM) halos to the growth of galaxy potentials using idealized simulations, each run in tandem with standard collisionless Cold Dark Matter (CDM). We find a greater diversity in the SIDM halo ... More
Chemical Abundance Distributions of Galactic Halos and their Satellite Systems in a Lambda CDM UniverseJul 05 2005We present a cosmologically motivated model for the hierachical formation of the stellar halo which includes a semi-analytic treatment of galactic chemical enrichment coupled to numerical simulations that track the orbital evolution and tidal disruption ... More
Strong Evolution in the Luminosity-Velocity Relation at z>1?May 16 2000We 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 ... More
Phase-Space Distributions of Chemical Abundances in Milky Way-type Galaxy HalosDec 26 2005[Abridged] Motivated by upcoming data from astrometric and spectroscopic surveys of the Galaxy, we explore the chemical abundance properties and phase-space distributions in hierarchically-formed stellar halo simulations set in a LambdaCDM Universe. Our ... More
Probing Galaxy Formation with TeV Gamma Ray AbsorptionDec 22 1998We 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 ... More
Push it to the limit: Local Group constraints on high-redshift stellar mass functions for Mstar > 10^5 MsunSep 03 2015We constrain the evolution of the galaxy stellar mass function from 2 < z < 5 for galaxies with stellar masses as low as 10^5 Msun by combining star formation histories of Milky Way satellite galaxies derived from deep Hubble Space Telescope observations ... More
Constraining dark matter halo profiles and galaxy formation models using spiral arm morphology. I. Method outlineMar 22 2006We investigate the use of spiral arm pitch angles as a probe of disk galaxy mass profiles. We confirm our previous result that spiral arm pitch angles (P) are well-correlated with the rate of shear (S) in disk galaxy rotation curves, by using a much larger ... More
Through a Smoother Lens: An expected absence of LCDM substructure detections from hydrodynamic and dark matter only simulationsOct 30 2017A fundamental prediction of the cold dark matter cosmology is the existence of a large number of dark subhalos around galaxies, most of which should be entirely devoid of stars. Confirming the existence of dark substructures stands among the most important ... More
Lambda-Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance PatternJan 18 2005Dec 19 2005(Abridged) The hierarchical formation scenario for the stellar halo requires the accretion and disruption of dwarf galaxies, yet low-metallicity halo stars are enriched in alpha-elements compared to similar, low-metallicity stars in dwarf spheroidal (dSph) ... More
How low does it go? Too few Galactic satellites with standard reionization quenchingAug 10 2018A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than Vpeak ~ 20 km/s, rendering the majority of haloes below this scale completely dark. We use ... More
The Velocity Function of GalaxiesAug 06 1999We 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 ... More
Merger Histories of Galaxy Halos and Implications for Disk SurvivalNov 30 2007Mar 31 2008We study the merger histories of galaxy dark matter halos using a high resolution LCDM N-body simulation. Our merger trees follow ~17,000 halos with masses M_0 = (10^11--10^13) Msun at z=0 and track accretion events involving objects as small as m = 10^10 ... More
The Cores of Dark Matter Dominated Galaxies: theory vs. observationsAug 19 1997May 25 1998We 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 ... More
The Physics of Galaxy Clustering I: A Model for Subhalo PopulationsNov 22 2004Apr 19 2007(Abridged) We present a semi-analytic model for Cold Dark Matter halo substructure that can be used as a framework for studying galaxy formation and an ingredient in halo models of galaxy clustering. We perform a comprehensive comparison of the model ... More
Concentrations and Assembly Histories of Dark Matter HalosNov 05 2001We 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 ... More
The Case Against Warm or Self-Interacting Dark Matter as Explanations for Cores in Low Surface Brightness GalaxiesDec 17 2009Feb 17 2010Warm dark matter (WDM) and self-interacting dark matter (SIDM) are often motivated by the inferred cores in the dark matter halos of low surface brightness (LSB) galaxies. We test thermal WDM, non-thermal WDM, and SIDM using high-resolution rotation curves ... More
On the stark difference in satellite distributions around the Milky Way and AndromedaMay 02 2013We compare spherically-averaged radial number counts of bright (> 10^5 Lsun) dwarf satellite galaxies within 400 kpc of the Milky Way (MW) and M31 and find that the MW satellites are much more centrally concentrated. Remarkably, the two satellite systems ... More
Indirect Dark Matter Detection from Dwarf Satellites: Joint Expectations from Astrophysics and SupersymmetryFeb 27 2009May 12 2009We present a general methodology for determining the gamma-ray flux from annihilation of dark matter particles in Milky Way satellite galaxies, focusing on two promising satellites as examples: Segue 1 and Draco. We use the SuperBayeS code to explore ... More
Stealth Galaxies in the Halo of the Milky WayDec 09 2009May 19 2010We predict that there is a population of low-luminosity dwarf galaxies orbiting within the halo of the Milky Way that have surface brightnesses low enough to have escaped detection in star-count surveys. The overall count of stealth galaxies is sensitive ... More
Cold Dark Matter Substructure and Galactic Disks II: Dynamical Effects of Hierarchical Satellite AccretionFeb 12 2009Jun 04 2009(Abridged) We perform dissipationless N-body simulations to elucidate the dynamical response of thin disks to bombardment by cold dark matter (CDM) substructure. Our method combines (1) cosmological simulations of the formation of Milky Way (MW)-sized ... More
Tracking Evolutionary Processes with Large Samples of Galaxy PairsSep 29 2009Modern redshift surveys enable the identification of large samples of galaxies in pairs, taken from many different environments. Meanwhile, cosmological simulations allow a detailed understanding of the statistical properties of the selected pair samples. ... More
Cosmological Simulations with Self-Interacting Dark Matter II: Halo Shapes vs. ObservationsAug 15 2012If dark matter has a large self-interaction scattering cross section, then interactions among dark-matter particles will drive galaxy and cluster halos to become spherical in their centers. Work in the past has used this effect to rule out velocity-independent, ... More
Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxiesMar 15 2016Nov 01 2016We use Local Group galaxy counts together with the ELVIS N-body simulations to explore the relationship between the scatter and slope in the stellar mass vs. halo mass relation at low masses, $M_\star \simeq 10^5 - 10^8 M_\odot$. Assuming models with ... More
The Dependence of Halo Clustering on Halo Formation History, Concentration, and OccupationDec 16 2005Jun 23 2006We investigate the dependence of dark matter halo clustering on halo formation time, density profile concentration, and subhalo occupation number, using high-resolution numerical simulations of a LCDM cosmology. We confirm results that halo clustering ... More
The Local Group: The Ultimate Deep FieldMar 08 2016Jul 05 2016Near-field cosmology -- using detailed observations of the Local Group and its environs to study wide-ranging questions in galaxy formation and dark matter physics -- has become a mature and rich field over the past decade. There are lingering concerns, ... More
Mass distribution of spiral galaxies from characteristics of spiral structure: Constraints on galaxy formation modelsAug 15 2005Recently, it has been shown that a correlation exists between the rate of shear and the spiral arm pitch angle in disk galaxies. The rate of shear depends upon the shape of the rotation curve, which is dependent upon the mass distribution in spiral galaxies. ... More
How to Zoom: Bias, Contamination, and Lagrange Volumes in Multimass Cosmological SimulationsMay 29 2013Oct 19 2013We perform a suite of multimass cosmological zoom simulations of individual dark matter halos and explore how to best select Lagrangian regions for resimulation without contaminating the halo of interest with low-resolution particles. Such contamination ... More
Lyman break galaxy close and interacting pairs at z ~ 3Dec 08 2009To date, the identification of interactions at z ~ 3 and above has relied on morphological analysis. Here, we present five serendipitous spectroscopic z ~ 3 Lyman break galaxy (LBG) pairs with projected proper separations < 15 h^-1 kpc in our survey of ... More
The Mass Dependance of Satellite Quenching in Milky Way-like HalosJul 11 2014Using the Sloan Digital Sky Survey, we examine the quenching of satellite galaxies around isolated Milky Way-like hosts in the local Universe. We find that the efficiency of satellite quenching around isolated galaxies is low and roughly constant over ... More
Concentrations of Dark Halos from their Assembly HistoriesAug 09 2001Nov 05 2001(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 ... More
The Local Group as a time machine: studying the high-redshift Universe with nearby galaxiesApr 24 2015Aug 27 2015We infer the UV luminosities of Local Group galaxies at early cosmic times ($z \sim 2$ and $z \sim 7$) by combining stellar population synthesis modeling with star formation histories derived from deep color-magnitude diagrams constructed from Hubble ... More
Small-Scale Structure in the SDSS and LCDM: Isolated L* Galaxies with Bright SatellitesMar 09 2011May 23 2011We use a volume-limited spectroscopic sample of isolated galaxies in the Sloan Digital Sky Survey (SDSS) to investigate the frequency and radial distribution of luminous (M_r <~ -18.3) satellites like the Large Magellanic Cloud (LMC) around ~L* Milky ... More
A Candidate Brightest Proto-Cluster Galaxy at z = 3.03Mar 26 2008We report the discovery of a very bright (m_R = 22.2) Lyman break galaxy at z = 3.03 that appears to be a massive system in a late stage of merging. Deep imaging reveals multiple peaks in the brightness profile with angular separations of ~0.''8 (~25 ... More
The Outer Limits of the M31 System: Kinematics of the Dwarf Galaxy Satellites And XXVIII and And XXIXFeb 04 2013Feb 27 2013We present Keck/DEIMOS spectroscopy of resolved stars in the M31 satellites And XXVIII & And XXIX. We show that these are likely self-bound galaxies based on 18 and 24 members in And XXVIII & And XXIX, respectively. And XXVIII has a systemic velocity ... More
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite AccretionAug 15 2007Jul 17 2008(Abridged) We conduct a series of high-resolution, dissipationless N-body simulations to investigate the cumulative effect of substructure mergers onto thin disk galaxies in the context of the LCDM paradigm of structure formation. Our simulation campaign ... More
Resonant Sterile Neutrino Dark Matter in the Local and High-z UniverseDec 14 2015Sterile neutrinos comprise an entire class of dark matter models that, depending on their production mechanism, can be hot, warm, or cold dark matter. We simulate the Local Group and representative volumes of the Universe in a variety of sterile neutrino ... More
Redefining the Missing Satellites ProblemApr 13 2007Oct 24 2007Numerical simulations of Milky-Way size Cold Dark Matter (CDM) halos predict a steeply rising mass function of small dark matter subhalos and a substructure count that greatly outnumbers the observed satellites of the Milky Way. Several proposed explanations ... More
Gas-Rich Mergers in LCDM: Disk Survivability and the Baryonic Assembly of GalaxiesJan 28 2009Nov 14 2009We use N-body simulations and observationally-normalized relations between dark matter halo mass, stellar mass, and cold gas mass to derive robust, arguably inevitable expectations about the baryonic content of major mergers out to redshift z~2. First, ... More
The Sagittarius impact as an architect of spirality and outer rings in the Milky WaySep 13 2011Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has ... More
Isolating Triggered Star FormationAug 21 2007Galaxy pairs provide a potentially powerful means of studying triggered star formation from galaxy interactions. We use a large cosmological N-body simulation coupled with a well-tested semi-analytic substructure model to demonstrate that the majority ... More
Organized Chaos: Scatter in the relation between stellar mass and halo mass in small galaxiesMar 15 2016We use Local Group galaxy counts together with the ELVIS N-body simulations to jointly constrain the scatter and slope in the stellar mass vs. halo mass relation at low masses, $M_\star \simeq 10^5 - 10^8 M_\odot$. Assuming log-normal scatter about a ... More
Cold dark matter: controversies on small scalesJun 04 2013The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos ... More
The dynamics of isolated Local Group galaxiesJan 06 2014Jan 23 2014We measured velocities of 862 individual red giant stars in seven isolated dwarf galaxies in the Local Group: NGC 6822, IC 1613, VV 124 (UGC 4879), the Pegasus dwarf irregular galaxy (DDO 216), Leo A, Cetus, and Aquarius (DDO 210). We also computed velocity ... More
Galaxy Mergers and Dark Matter Halo Mergers in LCDM: Mass, Redshift, and Mass-Ratio DependenceNov 10 2008Nov 14 2009We employ a high-resolution LCDM N-body simulation to present merger rate predictions for dark matter halos and investigate how common merger-related observables for galaxies--such as close pair counts, starburst counts, and the morphologically disturbed ... More
Are rotating planes of satellite galaxies ubiquitous?May 21 2015Aug 20 2015We compare the dynamics of satellite galaxies in the Sloan Digital Sky Survey to simple models in order to test the hypothesis that a large fraction of satellites co-rotate in coherent planes. We confirm the previously-reported excess of co-rotating satellite ... More
The Effect of Substructure on Mass Estimates of GalaxiesApr 04 2006Large galaxies are thought to form hierarchically, from the accretion and disruption of many smaller galaxies. Such a scenario should naturally lead to galactic phase-space distributions containing some degree of substructure. We examine the errors in ... More
The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf GalaxiesOct 02 2013Oct 16 2013We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass-metallicity relation as the dwarf spheroidal (dSph) satellites of both the Milky Way and M31: Z_* ... More
Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion HistoriesJul 24 2008This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The ... More
Segue 2: The Least Massive GalaxyApr 22 2013May 11 2013Segue 2, discovered by Belokurov et al. (2009), is a galaxy with a luminosity of only 900 L_sun. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2--a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be ... More
Too Big to Fail in the Local GroupApr 21 2014We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to the predicted masses of halos in the ELVIS suite of $\Lambda$CDM simulations, a sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar to the LG. We ... More