Andrew R. Zentner

The Dependence of Halo Clustering on Halo Formation History, Concentration, and Occupation

We 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 ?CDM cosmology. We confirm results that halo clustering is a function of halo formation time at fixed mass and that this trend depends on halo mass. For the first time, we show unequivocally that halo clustering is a function of halo concentration and show that the dependence of halo bias on concentration, mass, and redshift can be accurately parameterized in a simple way: b(M,c|z) = b(M|z)b(c|M/M). Interestingly, the scaling between bias and concentration changes sign with the value of M/M: high-concentration (early forming) objects cluster more strongly for M M, while low-concentration (late forming) objects cluster more strongly for rare high-mass halos, M M. We show the first explicit demonstration that host dark halo clustering depends on the halo occupation number (of dark matter subhalos) at fixed mass and discuss implications for halo model calculations of dark matter power spectra and galaxy clustering statistics. The effect of these halo properties on clustering is strongest for early-forming dwarf-mass halos, which are significantly more clustered than typical halos of their mass. Our results suggest that isolated low-mass galaxies (e.g., low surface brightness dwarfs) should have more slowly rising rotation curves than their more clustered counterparts and may have consequences for the dearth of dwarf galaxies in voids. They also imply that self-calibrating richness-selected cluster samples with their clustering properties might overestimate cluster masses and bias cosmological parameter estimation.

Formation of z~6 Quasars from Hierarchical Galaxy Mergers

The discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~109 M? when the universe was less than 1 billion years old. This finding presents several challenges for theoretical models because whether such massive objects can form so early in the ?CDM cosmology, the leading theory for cosmic structure formation, is an open question. Furthermore, whether the formation process requires exotic physics such as super-Eddington accretion remains undecided. Here we present the first multiscale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the ?CDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~3 Gpc3 volume and find that this halo of ~8 ? 1012 M? forming at z ~ 6.5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z = 6.42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience vigorous star formation (up to 104 M? yr-1) preceding the major quasar phase such that the stellar mass of the quasar host reaches 1012 M? at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. The merger remnant thus obeys a similar MBH-Mbulge scaling relation observed locally as a consequence of coeval growth and evolution of the SMBH and its host galaxy. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard ?CDM cosmology and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental MBH-Mbulge correlation in a hierarchical universe.

Effects of Baryons and Dissipation on the Matter Power Spectrum

We study the importance of baryonic physics on predictions of the matter power spectrum as it is relevant for forthcoming weak-lensing surveys. We quantify the impact of baryonic physics using a set of cosmological numerical simulations. Each simulation has the same initial density field, but models a different set of physical processes. We find that baryonic processes significantly alter predictions for the matter power spectrum relative to models that include only gravitational interactions. Our results imply that future weak-lensing experiments such as LSST and SNAP will likely be sensitive to the uncertain physics governing the nonlinear evolution of the baryonic component of the universe if these experiments are primarily limited by statistical uncertainties. In particular, this effect could be important for forecasts of the constraining power of future surveys if information from scales l 1000 is included in the analysis. We find that deviations are caused primarily by the rearrangement of matter within individual dark matter halos relative to the gravity-only case, rather than a large-scale rearrangement of matter. Consequently, we propose a simple model, based on the phenomenological halo model of dark matter clustering, for baryonic effects that can be used to aid in the interpretation of forthcoming weak-lensing data.

The Effect of Gas Cooling on the Shapes of Dark Matter Halos

We analyze the effect of dissipation on the shapes of dark matter (DM) halos using high-resolution cosmological gasdynamics simulations of clusters and galaxies in the ΛCDM cosmology. We find that halos formed in simulations with gas cooling are significantly more spherical than corresponding halos formed in adiabatic simulations. Gas cooling results in an average increase of the principle axis ratios of halos by ~0.2-0.4 in the inner regions. The systematic difference decreases slowly with radius but persists almost to the virial radius. We argue that the differences in simulations with and without cooling arise both during periods of quiescent evolution, when gas cools and condenses toward the center, and during major mergers. We perform a series of high-resolution N-body simulations to study the shapes of remnants in major mergers of DM halos and halos with embedded stellar disks. In the DM halo-only mergers, the shape of the remnants depends only on the orbital angular momentum of the encounter and not on the internal structure of the halos. However, significant shape changes in the DM distribution may result if stellar disks are included. In this case the shape of the DM halos is correlated with the morphology of the stellar remnants.

THE ANISOTROPIC DISTRIBUTION OF GALACTIC SATELLITES

We present a study of the spatial distribution of dwarf satellites (or subhalos) in galactic dark matter halos using dissipationless cosmological simulations of the concordance flat cold dark matter (CDM) model with vacuum energy. We find that subhalos are distributed anisotropically and are preferentially located along the major axes of the triaxial mass distributions of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated subhalo sample from an isotropic distribution is PKS 1.5 × 10-4. An isotropic distribution of subhalos is thus not the correct null hypothesis for testing the CDM paradigm. The nearly planar distribution of observed Milky Way (MW) satellites is marginally consistent (probability 0.02) with being drawn randomly from the subhalo distribution in our simulations. Furthermore, if we select the subhalos likely to be luminous, we find a distribution that is consistent with the observed MW satellites. In fact, we show that subsamples of the subhalo population with a centrally concentrated radial distribution that is similar to that of the MW dwarfs typically exhibit a comparable degree of planarity. We explore the origin of the observed subhalo anisotropy and conclude that it is likely due to (1) the preferential accretion of satellites along filaments, often closely aligned with the major axis of the host halo, and (2) evolution of satellite orbits within the prolate, triaxial potentials typical of CDM halos. Agreement between predictions and observations requires the major axis of the outer dark matter halo of the Milky Way to be nearly perpendicular to the disk. We discuss possible observational tests of such disk-halo alignment with current large galaxy surveys.

Merger Histories of Galaxy Halos and Implications for Disk Survival

We study the merger histories of galaxy dark matter halos using a high-resolution ΛCDM N-body simulation. Our merger trees follow ~17,000 halos with masses M0 = 1011–1013 h−1 M☉ at z = 0 and track accretion events involving objects as small as m 1010 h−1 M☉. We find that mass assembly is remarkably self-similar in m/M0 and dominated by mergers that are ~10% of the final halo mass. While very large mergers, m 0.4M0, are quite rare, sizeable accretion events, m ~ 0.1M0, are common. Over the last ~10 Gyr, an overwhelming majority (~95%) of Milky Way-sized halos with M0 = 1012 h−1 M☉ have accreted at least one object with greater total mass than the Milky Way disk (m > 5 × 1010 h−1 M☉), and approximately 70% have accreted an object with more than twice that mass (m > 1011 h−1 M☉). Our results raise serious concerns about the survival of thin-disk-dominated galaxies within the current paradigm for galaxy formation in a ΛCDM universe. In order to achieve a ~70% disk-dominated fraction in Milky Way-sized ΛCDM halos, mergers involving m 2 × 1011 h−1 M☉ objects must not destroy disks. Considering that most thick disks and bulges contain old stellar populations, the situation is even more restrictive: these mergers must not heat disks or drive gas into their centers to create young bulges.

Shredded Galaxies as the Source of Diffuse Intrahalo Light on Varying Scales

We 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 from galaxy survey data to set the stellar mass in each accreted subhalo, which is added to the diffuse light as subhalos become disrupted due to interactions within their hosts. We predict that the stellar mass fraction in diffuse, intrahalo light should rise on average from ~0.5% to ~20% from small galaxy halos (~1011 M☉) to poor groups (~1013 M☉). The trend with mass flattens considerably beyond the group scale, increasing weakly from a fraction of ~20% in poor galaxy clusters (~1014 M☉) to roughly ~30% in massive clusters (~1015 M☉). The mass-dependent diffuse light fraction is governed primarily by the empirical fact that the mass-to-light ratio in galaxy halos must vary as a function of halo mass. Galaxy halos have little diffuse light because they accrete most of their mass in small subhalos that themselves have high mass-to-light ratios; stellar halos around galaxies are built primarily from disrupted dwarf-irregular-type galaxies with M* ~ 108.5 M☉. The diffuse light in group and cluster halos is built from satellite galaxies that form stars efficiently; intracluster light is dominated by material liberated from massive galaxies with M* ~ 1011 M☉. Our results are consistent with existing observations spanning the galaxy, group, and cluster scale; however, they can be tested more rigorously in future deep surveys.

COLD DARK MATTER SUBSTRUCTURE AND GALACTIC DISKS. II. DYNAMICAL EFFECTS OF HIERARCHICAL SATELLITE ACCRETION

We perform a set of fully self-consistent, dissipationless N-body simulations to elucidate the dynamical response of thin galactic disks to bombardment by cold dark matter (CDM) substructure. Our method combines (1) cosmological simulations of the formation of Milky Way (MW)-sized CDM halos to derive the properties of substructure, and (2) controlled numerical experiments of consecutive subhalo impacts onto an initially thin, fully formed MW-type disk galaxy. The present study is the first to account for the evolution of satellite populations over cosmic time in such an investigation of disk structure. In contrast to what can be inferred from statistics of the z = 0 surviving substructure, we find that accretions of massive subhalos onto the central regions of host halos, where the galactic disks reside, since z ~ 1 should be common. One host halo accretion history is used to initialize the controlled simulations of satellite-disk encounters. The specific merger history involves six dark matter substructures, with initial masses in the range ~20%-60% of the disk mass and of comparable size to the disk, crossing the central regions of their host in the past ~8 Gyr. We show that these accretion events severely perturb the thin galactic disk and produce a wealth of distinctive dynamical signatures on its structure and kinematics. These include (1) considerable thickening and heating at all radii, with the disk thickness and velocity ellipsoid nearly doubling at the solar radius; (2) prominent flaring associated with an increase in disk thickness greater than a factor of 4 in the disk outskirts; (3) surface density excesses at large radii, beyond ~5 disk scale lengths, resembling those of the observed antitruncated disks; (4) long-lived, lopsidedness at levels similar to those measured in observational samples of disk galaxies; and (5) substantial tilting. The interaction with the most massive subhalo in the simulated accretion history drives the disk response while subsequent bombardment is much less efficient at disturbing the disk. We also explore a variety of disk and satellite properties that influence these responses. We conclude that substructure-disk encounters of the kind expected in the ΛCDM paradigm play a significant role in setting the structure of disk galaxies and driving galaxy evolution.

Galaxy assembly bias: a significant source of systematic error in the galaxy–halo relationship

It is common practice for methods that use galaxy clustering to constrain the galaxy-halo relationship, such as the halo occupation distribution (HOD) and/or conditional luminosity function (CLF), to assume that halo mass alone suffices to determine a halos resident galaxy population. Yet, the clustering strength of cold dark matter halos depends upon halo properties in addition to mass, such as formation time, an effect referred to as assembly bias. If galaxy characteristics are correlated with any of these auxiliary halo properties, the basic assumption of HOD/CLF methods is violated. We estimate the potential for assembly bias to induce systematic errors in inferred halo occupation statistics. We use halo abundance matching and age matching to construct fiducial mock galaxy catalogs that exhibit assembly bias as well as additional mock catalogs with identical HODs, but with assembly bias removed. We fit a parameterized HOD to the projected two-point clustering of mock galaxies in each catalog to assess the systematic errors induced by reasonable levels of assembly bias. In the absence of assembly bias, the inferred HODs generally describe the true underlying HODs well, validating the basic methodology. However, in all of the cases with assembly bias, the inferred HODs have systematic errors that are statistically significant. In most cases, these systematic errors cannot be identified using void statistics as auxiliary observables. We conclude that the galaxy-halo relationship inferred from galaxy clustering should be subject to a non-negligible systematic error induced by assembly bias. Our work suggests that efforts to model and/or constrain assembly bias should be high priorities as it is a threatening source of systematic error in galaxy evolution studies as well as the precision cosmology program.

The Robustness of Dark Matter Density Profiles in Dissipationless Mergers

We present a comprehensive series of dissipationless N-body simulations to investigate the evolution of density distribution in equal-mass mergers between dark matter (DM) halos and multicomponent galaxies. The DM halo models are constructed with various asymptotic power-law indices ranging from steep cusps to corelike profiles and the structural properties of the galaxy models are motivated by the ΛCDM paradigm of structure formation. The adopted force resolution allows robust density profile estimates in the inner ~1% of the virial radii of the simulated systems. We demonstrate that the central slopes and overall shapes of the remnant density profiles are virtually identical to those of the initial systems, suggesting that the remnants retain a remarkable memory of the density structure of their progenitors, despite the relaxation that accompanies merger activity. We also find that halo concentrations remain approximately constant through hierarchical merging involving identical systems and show that remnants contain significant fractions of their bound mass well beyond their formal virial radii. These conclusions hold for a wide variety of initial asymptotic density slopes, orbital energies, and encounter configurations, including sequences of consecutive merger events, simultaneous mergers of several systems, and mergers of halos with embedded cold baryonic components in the form of disks, spheroids, or both. As an immediate consequence, the net effect of gas cooling, which contracts and steepens the inner density profiles of DM halos, should be preserved through a period of dissipationless major merging. Our results imply that the characteristic universal shape of DM density profiles may be set early in the evolution of halos.