One of the most pernicious theoretical systematics facing upcoming gravitational lensing surveys is the uncertainty introduced by the effects of baryons on the power spectrum of the convergence field. One method that has been proposed to account for
these effects is to allow several additional parameters (that characterize dark matter halos) to vary and to fit lensing data to these halo parameters concurrently with the standard set of cosmological parameters. We test this method. In particular, we use this technique to model convergence power spectrum predictions from a set of cosmological simulations. We estimate biases in dark energy equation of state parameters that would be incurred if one were to fit the spectra predicted by the simulations either with no model for baryons, or with the proposed method. We show that neglecting baryonic effect leads to biases in dark energy parameters that are several times the statistical errors for a survey like the Dark Energy Survey. The proposed method to correct for baryonic effects renders the residual biases in dark energy equation of state parameters smaller than the statistical errors. These results suggest that this mitigation method may be applied to analyze convergence spectra from a survey like the Dark Energy Survey. For significantly larger surveys, such as will be carried out by the Large Synoptic Survey Telescope, the biases introduced by baryonic effects are much more significant. We show that this mitigation technique significantly reduces the biases for such larger surveys, but that a more effective mitigation strategy will need to be developed in order ensure that the residual biases in these surveys fall below the statistical errors.
Recent kinematical constraints on the internal densities of the Milky Ways dwarf satellites have revealed a discrepancy with the subhalo populations of simulated Galaxy-scale halos in the standard CDM model of hierarchical structure formation. This h
as been dubbed the too big to fail problem, with reference to the improbability of large and invisible companions existing in the Galactic environment. In this paper, we argue that both the Milky Way observations and simulated subhalos are consistent with the predictions of the standard model for structure formation. Specifically, we show that there is significant variation in the properties of subhalos among distinct host halos of fixed mass and suggest that this can reasonably account for the deficit of dense satellites in the Milky Way. We exploit well-tested analytic techniques to predict the properties in a large sample of distinct host halos with a variety of masses spanning the range expected of the Galactic halo. The analytic model produces subhalo populations consistent with both Via Lactea II and Aquarius, and our results suggest that natural variation in subhalo properties suffices to explain the discrepancy between Milky Way satellite kinematics and these numerical simulations. At least ~10% of Milky Way-sized halos host subhalo populations for which there is no too big to fail problem, even when the host halo mass is as large as M_host = 10^12.2 h^-1 M_sun. Follow-up studies consisting of high-resolution simulations of a large number of Milky Way-sized hosts are necessary to confirm our predictions. In the absence of such efforts, the too big to fail problem does not appear to be a significant challenge to the standard model of hierarchical formation. [abridged]
We calculate the probability distribution function (PDF) of the expected annihilation luminosities of dark matter subhalos as a function of subhalo mass and distance from the Galactic center using a semi-analytical model of halo evolution. We find th
at the PDF of luminosities is relatively broad, exhibiting a spread of as much as an order of magnitude at fixed subhalo mass and halo-centric distance. The luminosity PDF allows for simple construction of mock samples of gamma-ray luminous subhalos and assessment of the variance in among predicted gamma-ray signals from dark matter annihilation. Other applications include quantifying the variance among the expected luminosities of dwarf spheroidal galaxies, assessing the level at which dark matter annihilation can be a contaminant in the expected gamma-ray signal from other astrophysical sources, as well as estimating the level at which nearby subhalos can contribute to the antimatter flux.
We study the impact of theoretical uncertainty in the dark matter halo mass function and halo bias on dark energy constraints from imminent galaxy cluster surveys. We find that for an optical cluster survey like the Dark Energy Survey, the accuracy r
equired on the predicted halo mass function to make it an insignificant source of error on dark energy parameters is ~ 1%. The analogous requirement on the predicted halo bias is less stringent (~ 5%), particularly if the observable-mass distribution can be well constrained by other means. These requirements depend upon survey area but are relatively insensitive to survey depth. The most stringent requirements are likely to come from a survey over a significant fraction of the sky that aims to observe clusters down to relatively low mass, Mth ~ 10^13.7 Msun/h; for such a survey, the mass function and halo bias must be predicted to accuracies of ~ 0.5% and ~ 1%, respectively. These accuracies represent a limit on the practical need to calibrate ever more accurate halo mass and bias functions. We find that improving predictions for the mass function in the low-redshift and low-mass regimes is the most effective way to improve dark energy constraints.
(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)-size
d 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. 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. 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 observed antitruncated disks; (4) lopsidedness at levels similar to those measured in observational samples of disk galaxies; and (5) substantial tilting. The interaction with the most massive subhalo drives the disk response while subsequent bombardment is much less efficient at disturbing the disk. We conclude that substructure-disk encounters of the kind expected in the LCDM paradigm play a significant role in setting the structure of disk galaxies and driving galaxy evolution.
We 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 disrup
tion, and we convolve this result with the observed mass-metallicity relation for galaxies in order to analyze the chemical abundance of intrahalo light (IHL) in host halos with virial mass 10^10.5 M_sun < M_host < 10^15 M_sun. We predict a steep rise of roughly two dex in IHL metallicity from the scales of small to large spiral galaxies. In terms of the total dynamical mass M_host of the host systems under consideration, we predict diffuse light metallicities ranging from Z_IHL < -2.5 for M_host ~ 10^11 M_sun, to Z_IHL ~ -1.0 for M_host ~ 10^12 M_sun. In larger systems, we predict a gradual flattening of this trend with Z_IHL ~ -0.4 for M_host ~ 10^13 M_sun, increasing to Z_IHL ~ 0.1 for M_host ~ 10^15 M_sun. This behavior is coincident with a narrowing of the intrahalo metallicity distribution as host mass increases. The observable distinction in surface brightness between old, metal-poor IHL stars and more metal-rich, dynamically-younger tidal streams is of crucial importance when estimating the chemical abundance of an intrahalo population with multiple origins.
We 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-size
d CDM halos to derive the properties of substructure populations and controlled numerical experiments of consecutive subhalo impacts onto initially-thin, fully-formed disk galaxies. We demonstrate that close encounters between massive subhalos and galactic disks since z~1 should be common occurrences in LCDM models. In contrast, extremely few satellites in present-day CDM halos are likely to have a significant impact on the disk structure. One typical host halo merger history is used to seed controlled N-body experiments of subhalo-disk encounters. As a result of these accretion events, the disk thickens considerably at all radii with the disk scale height increasing in excess of a factor of 2 in the solar neighborhood. We show that interactions with the subhalo population produce a wealth of distinctive morphological signatures in the disk stars including: conspicuous flares; bars; low-lived, ring-like features in the outskirts; and low-density, filamentary structures above the disk plane. We compare a resulting dynamically-cold, ring-like feature in our simulations to the Monoceros ring stellar structure in the MW. The comparison shows quantitative agreement in both spatial distribution and kinematics, suggesting that such observed complex stellar components may arise naturally as disk stars are excited by encounters with subhalos. These findings highlight the significant role of CDM substructure in setting the structure of disk galaxies and driving galaxy evolution.
(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 camp
aign is based on a hybrid approach. Substructure properties are culled directly from cosmological simulations of galaxy-sized cold dark matter (CDM) halos. In contrast to what can be inferred from statistics of the present-day substructure populations, accretions of massive subhalos onto the central regions of host halos, where the galactic disk resides, since z~1 should be common occurrences. One host halo merger history is subsequently used to seed controlled numerical experiments of repeated satellite impacts on an initially-thin Milky Way-type disk galaxy. We show that these accretion events produce several distinctive observational signatures in the stellar disk including: a ring-like feature in the outskirts; a significant flare; a central bar; and faint filamentary structures that (spuriously) resemble tidal streams. The final distribution of disk stars exhibits a complex vertical structure that is well-described by a standard ``thin-thick disk decomposition. We conclude that satellite-disk encounters of the kind expected in LCDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way, M31, and in other disk galaxies. These results highlight the significant role of CDM substructure in setting the structure of disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys and astrometric satellites may be able to distinguish between competing cosmological models by testing whether the detailed structure of galactic disks is as excited as predicted by the CDM paradigm.
