arXiv subjects

Romeel Davé

Publications and source records attributed to Romeel Davé.

At least 235 records · Page 13Linked to original sources

arXiv2006

From Galaxy-Galaxy Lensing to Cosmological Parameters

Galaxy-galaxy lensing measures the mean excess surface density DS(r) around a sample of lensing galaxies. We develop a method for combining DS(r) with the galaxy correlation function xi_gg(r) to constrain Omega_m and sigma_8, going beyond the linear bias model to reach the level of accuracy demanded by current and future measurements. We adopt the halo occupation distribution (HOD) framework, and we test its applicability to this problem by examining the effects of replacing satellite galaxies in the halos of an SPH simulation with randomly selected dark matter particles from the same halos. The difference between dark matter and satellite galaxy radial profiles has a ~10% effect on DS(r) at r<1 Mpc/h. However, if radial profiles are matched, the remaining impact of individual subhalos around satellite galaxies and environmental dependence of the HOD at fixed halo mass is <5% in DS(r) for 0.1 2 Mpc/h, but it fails at the 30-50% level on smaller scales. The scaling of DS(r) ~ Omega_m^a(r) sigma_8^b(r) approaches the linear bias expectation a=b=1 at r>10 Mpc/h, but a(r) and b(r) vary from 0.8 to 1.6 at smaller r. We calculate a fiducial DS(r) and scaling indices a(r) and b(r) for two SDSS galaxy samples; galaxy-galaxy lensing measurements for these samples can be combined with our predictions to constrain Omega_m and sigma_8.

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arXiv2006

The Physical Properties and Detectability of Reionization-Epoch Galaxies

We present predictions drawn from cosmological hydrodynamic simulations for the physical, photometric and emission line properties of galaxies present during the latter stages of reionization from z=9-6. We find significant numbers of galaxies that have stellar masses exceeding 10^8 Mo during this epoch, with metallicities exceeding one-thirtieth solar. Far from primeval "first-star" objects,these objects are likely to have reionized their infall regions prior to z=9, are dominated by atomic rather than molecular cooling, and are not expected to be forming Population III stars. Galaxies exhibit a slowly evolving comoving autocorrelation length from z=9-6, continuing a trend seen at lower redshifts in which the rapidly dropping bias counteracts the rapidly increasing matter clustering. These sources can be marginally detected using current instruments, but modest increases in sensitivity or survey area would yield significantly increased samples. We compare to current observations of the z~6 rest-UV and Ly-alpha line luminosity functions, and find good agreement. We also compare with the z~7 object observed by Egami et al., and find that such systems are ubiquitous in our simulations. The intrinsic Ly-alpha luminosity function evolves slowly from z=9-6, meaning that it should also be possible to detect these objects with upcoming narrow band surveys such as DAzLE. We make predictions for near-IR surveys with JWST, and show that while a high density of sources will be found, Population III objects may remain elusive. We present and compare simulations with several recipes for superwind feedback, and show that while our broad conclusions are insensitive to this choice, a feedback model based on momentum-driven winds is favored in comparisons with available data. (abridged)

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arXiv2006

Baryon Dynamics, Dark Matter Substructure, and Galaxies

By comparing a collisionless cosmological N-body simulation (DM) to an SPH simulation with the same initial conditions, we investigate the correspondence between the dark matter subhalos produced by collisionless dynamics and the galaxies produced by dissipative gas dynamics in a dark matter background. When galaxies in the SPH simulation become satellites in larger groups, they retain local dark matter concentrations (SPH subhalos) whose mass is typically five times their baryonic mass. The more massive subhalos of the SPH simulation have corresponding subhalos of similar mass and position in the DM simulation; at lower masses, there is fairly good correspondence, but some DM subhalos are in different spatial positions and some suffer tidal stripping or disruption. The halo occupation statistics of DM subhalos -- the mean number of subhalos, pairs, and triples as a function of host halo mass -- are very similar to those of SPH subhalos and SPH galaxies. Gravity of the dissipative baryon component amplifies the density contrast of subhalos in the SPH simulation, making them more resistant to tidal disruption. Relative to SPH galaxies and SPH subhalos, the DM subhalo population is depleted in the densest regions of the most massive halos. The good agreement of halo occupation statistics between the DM subhalo and SPH galaxy populations leads to good agreement of their two-point correlation functions and higher order moments on large scales. The depletion of DM subhalos in dense regions depresses their clustering at R<1 Mpc/h. In these simulations, the "conversation" between dark matter and baryons is mostly one-way, with dark matter dynamics telling galaxies where to form and how to cluster, but the "back talk" of the baryons influences small scale clustering by enhancing the survival of substructure in the densest environments.

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arXiv2005

Building Galaxies with Simulations

We present an overview of some of the issues surrounding current models of galaxy formation, highlighting recent insights obtained from cosmological hydrodynamic simulations. Detailed examination of gas accretion processes show a hot mode of gas cooling from near the halo's virial temperature, and a previously underappreciated cold mode where gas flows in along filaments on dynamical timescales, emitting its energy in line radiation. Cold mode dominates in systems with halo masses slightly smaller than the Milky Way and below, and hence dominates the global accretion during the heydey of galaxy formation. This rapid accretion path enables prompt assembly of massive galaxies in the early universe, and results in $z\sim 4$ galaxy properties in broad agreement with observations, with the most massive galaxies being the most rapid star formers. Massive galaxies today are forming stars at a much reduced rate, a trend called downsizing. The trend of downsizing is naturally reproduced in simulations, owing to a transition from cold mode accretion in the early growth phase to slower hot mode accretion once their halos grow large. However, massive galaxies at the present epoch are still observed to have considerably redder colors than simulations suggest, suggesting that star formation is not sufficiently truncated in models by the transition to hot mode, and that another process not included in current simulations is required to suppress star formation.

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arXiv2005

Simulations of Early Galaxy Formation

We present the predictions for the photometric and emission line properties of galaxies present during the latter stages of reionization from z=8 to 6. These preliminary predictions are made from cosmological hydrodynamic simulations that include star formation and feedback, but not the effects of radiative transfer. We find significant numbers of galaxies that have stellar masses exceeding 10^8 Mo by z=8, with metallicities in the range of one-tenth solar. These galaxies are just beyond the reach of current near-infrared surveys, but should be found in large numbers by next-generation programs. The Lyman alpha luminosity function does not evolve much from z=6 to z=8, meaning that it should also be possible to detect these objects in significant numbers with upcoming narrow band surveys, unless the escape fraction of Ly-alpha evolves significantly between those epochs.

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arXiv2005

Probing Galaxy Formation with He II Cooling Lines

Using high resolution cosmological simulations, we study hydrogen and helium gravitational cooling radiation. We focus on the HeII cooling lines, which arise from gas with a different temperature history (T_max ~ 10^5K) than HI line emitting gas. We examine whether three major atomic cooling lines, HI 1216A, HeII 1640A and HeII 304A are observable, finding that HI 1216A and HeII 1640A cooling emission at z=2-3 are potentially detectable with deep narrow band (R>100) imaging and/or spectroscopy from the ground. While the expected strength of HI 1216A cooling emission depends strongly on the treatment of the self-shielded phase of the IGM in the simulations, our predictions for the HeII 1640A line are more robust because the HeII 1640A emissivity is negligible below T~10^4.5 K and less sensitive to the UV background. Although HeII 1640A cooling emission is fainter than HI 1216A by at least a factor of 10 and, unlike HI 1216A, might not be resolved spatially with current observational facilities, it is more suitable to study gas accretion in the galaxy formation process because it is optically thin and less contaminated by the recombination lines from star-forming galaxies. The HeII 1640A line can be used to distinguish among mechanisms for powering the so-called "Lyman alpha blobs" -- including gravitational cooling radiation, photoionization by stellar populations, and starburst-driven superwinds -- because (1) HeII 1640A emission is limited to very low metallicity (log(Z/Z_sun) < -5.3) and Population III stars, and (2) the blob's kinematics are probed unambiguously through the HeII 1640A line width, which, for cooling radiation, is narrower (sigma < 400 km/s) than typical wind speeds.

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arXiv2005

Time dependent non-LTE calculations of ionisation in the early universe

We present a new implicit numerical algorithm for the calculation of the time dependent non-Local Thermodynamic Equilibrium of a gas in an external radiation field that is accurate, fast and unconditionally stable for all spatial and temporal increments. The method is presented as a backward difference scheme in 1-D but can be readily generalised to 3-D. We apply the method for calculating the evolution of ionisation domains in a hydrogen plasma with plane-parallel Gaussian density enhancements illuminated by sources of UV radiation. We calculate the speed of propagation of ionising fronts through different ambient densities and the interaction of such ionising fronts with density enhancements. We show that for a typical UV source that may be present in the early universe, the introduction of a density enhancement of a factor ~10 above an ambient density 10^{-4} atoms/cm^3 could delay the outward propagation of an ionisation front by millions of years. Our calculations show that within the lifetime of a single source (~a few million years), and for ambient intergalactic densities appropriate to redshifts z~6-20, neutral fractions of of ~10^{-3}-10^{-5} can be achieved within its zone of influence. We also present calculations which demonstrate that once started, ionisation will proceed very efficiently as multiple sources are subsequently introduced, even if the time between the appearence of such sources may be much longer than their lifetimes.

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arXiv2004

Interpreting the Relationship Between Galaxy Luminosity, Color and Environment

We study the relationship between galaxy luminosity, color, and environment in a cosmological simulation of galaxy formation. We compare the predicted relationship with that found for SDSS galaxies and find that the model successfully predicts most of the qualitative features seen in the data, but also shows some interesting differences. Specifically, the simulation predicts that the local density around bright red galaxies is a strong increasing function of luminosity, but does not depend much on color at fixed luminosity. Moreover, we show that this is due to central galaxies in dark matter halos whose baryonic masses correlate strongly with halo mass. The simulation also predicts that the local density around blue galaxies is a strong increasing function of color, but does not depend much on luminosity at fixed color. We show that this is due to satellite galaxies in halos whose stellar ages correlate with halo mass. Finally, the simulation fails to predict the luminosity dependence of environment observed around low luminosity red galaxies. However, we show that this is most likely due to the simulation's limited resolution. A study of a higher resolution, smaller volume simulation suggests that this dependence is caused by the fact that all low luminosity red galaxies are satellites in massive halos, whereas intermediate luminosity red galaxies are a mixture of satellites in massive halos and central galaxies in less massive halos.

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arXiv2003

Simulations of the Intergalactic Medium

I present a review by epoch of baryons in the intergalactic medium (IGM), from the first star until today. Recent observations indicate a protracted period of reionization, suggesting multiple populations of reionizers; detection of these z>6 sources is a key goal that is now coming within reach. The optical Lyman alpha forest (2<z<4) is well-described by the Fluctuating Gunn-Peterson Approximation, but puzzling observations of associated metal lines and nearby galaxies may provide insights into galactic feedback processes. IGM studies at z<1.5 are progressing rapidly thanks to ultraviolet absorption line studies of both Ly-alpha absorbers and the warm-hot intergalactic medium. Cosmological hydrodynamic simulations have played an integral part in these advances, and have helped to shape our understanding of the IGM at each epoch. Working jointly, observations and theory continue to expand our knowledge of the IGM as the earliest stages of galaxy formation and the dominant reservoir of baryons at all redshifts.

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arXiv2003

The Halo Occupation Distribution and the Physics of Galaxy Formation

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative distributions of galaxies and dark matter within halos. We calculate predicted HODs for a Lambda-CDM cosmological model using an SPH hydrodynamic simulation and a semi-analytic (SA) galaxy formation model. Although the two methods predict different galaxy mass functions, their HOD predictions agree remarkably well. For mass-selected samples, the mean occupation exhibits a sharp cutoff at low halo masses, a slowly rising plateau for ~1-2, and a more steeply rising high occupancy regime. At low , the mean pair and triple counts are well below Poisson expectations, with important consequences for small scale behavior of 2- and 3-point correlation functions. The HOD depends strongly on galaxy age, with high mass halos populated mainly by old galaxies and low mass halos by young galaxies. The SPH simulation supports several simplifying assumptions about HOD bias: the most massive galaxy in a halo usually lies close to the center and moves near the halo's mean velocity; satellite galaxies have the same radial profile and velocity dispersion as the dark matter; and the mean occupation at fixed halo mass is independent of the halo's larger scale environment. By applying the SPH and SA HODs to a large volume N-body simulation, we show that both methods predict slight, observable departures from a power-law galaxy correlation function. The predicted HODs are closely tied to the underlying galaxy formation physics, they offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. (Shortened)

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arXiv2002

Lyman Alpha Absorber Correlations and the Bias of the Lyman Alpha Forest

Lyman alpha absorber correlations contain information about the underlying density distribution associated with a particular class of absorbers. As such, they provide an opportunity to independently measure the "bias" of the Lyman alpha forest, i.e. the relationship between HI column density and underlying dark matter density. In these proceedings we use hydrodynamic simulations to investigate whether the evolution of this bias is measurable from observable correlations. Unfortunately, the increasingly complex physics in the IGM at z<1 makes a direct measurement of the bias difficult. Nevertheless, current simulations do make predictions for HI absorber correlations that are in broad agreement with observations at both high and low redshift, thus reinforcing the bias evolution predictions given by these models.

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arXiv2002

X-Ray Scaling Relations of Galaxy Groups in a Hydrodynamic Cosmological Simulation

We examine the scalings of X-ray luminosity, temperature, and dark matter or galaxy velocity dispersion for galaxy groups in an LCDM cosmological simulation, which incorporates gravity, gas dynamics, radiative cooling, and star formation, but no substantial non-gravitational heating. In agreement with observations, the simulated L_X-sigma and L_X-T_X relations are steeper than those predicted by adiabatic simulations or self-similar models, with L_X\propto sigma^{4.4} and L_X\propto T_X^{2.6} for massive groups and significantly steeper relations below a break at sigma~180 km/s (T_X~0.7 keV), but T_X-sigma is fairly close to self-similar, with T_X\propto sigma^{1.75}. The entropy of hot gas in low mass groups is higher than predicted by self-similar scaling, and agrees with observations that suggest an "entropy floor". The steeper scalings of the luminosity relations are driven by radiative cooling, which reduces the hot gas fraction from 50% of the total baryons at sigma \approx 500 km/s to 20% at sigma \approx 100 km/s. A secondary effect is that hot gas in smaller systems is less clumpy, further driving down L_X. A smaller volume simulation with eight times higher mass resolution predicts nearly identical X-ray luminosities at a given group mass, demonstrating the insensitivity of the predicted scaling relations to numerical resolution. There remain some quantitative discrepancies: the predicted mass scale of the L_X-T_X and L_X-sigma breaks is somewhat too low, and the luminosity-weighted temperatures are too high at a given sigma. We conclude that radiative cooling has an important quantitative impact on group X-ray properties and can account for many of the observed trends that have been interpreted as evidence for non-gravitational heating. (abridged)

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arXiv2001

The Metallicity of the Local IGM from the HST/STIS Spectrum of 3C273

We present a preliminary study of the metallicity of low-redshift Lyman alpha absorbers from the HST/STIS spectrum of 3C273. Using a pixel-based shift-and-coadd technique, we compare observations to carefully-constructed mock quasar spectra from a cosmological hydrodynamic simulation. We place an upper limit of [C/H]<-1.5 at >90% confidence using two Ly-alpha absorbers with N_HI>10^14 cm^-2, from the fact that we see little absorption near the expected CIV positions, whereas the artificial spectra predict significant absorption. We assume a Haardt & Madau (quasar-based) ionizing background spectrum, but a softer spectrum results in a stronger constraint on the metallicity. With only two absorbers and various modeling uncertainties, it is too early to make a definitive determination of the diffuse IGM metallicity, but upcoming observations should improve constraints.

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arXiv2001

Group Scaling Relations From a Cosmological Hydrodynamic Simulation: No Pre-heating Required?

We investigate the X-ray vs. optical scaling relations of poor groups to small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic simulation of a Lambda-CDM universe, with cooling and star formation but no pre-heating. We find that the scaling relations between X-ray luminosity, X-ray temperature, and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at \~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting) gas fraction varies substantially with halo mass in this regime. Our predictions roughly agree with observations. Thus radiative cooling is a critical physical process in modeling galaxy groups, and may present an alternative to ad hoc models such as pre-heating or entropy floors for explaining X-ray group scaling relations.

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arXiv2001

Sub-mm Galaxies in Cosmological Simulations

We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM universe, using hydrodynamic cosmological simulations. Assuming that most of the emission from newly formed stars is absorbed and reradiated in the rest-frame far-IR, we calculate the number of galaxies that would be detected in sub-mm surveys conducted with SCUBA. The predicted number counts are strongly dependent on the assumed dust temperature and emissivity law. With plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation predictions reproduce the observed number counts above ~ 1 mJy. The sources have a broad redshift distribution with median z ~ 2, in reasonable agreement with observational constraints. However, the predicted count distribution may be too steep at the faint end, and the fraction of low redshift objects may be larger than observed. In this physical model of the sub-mm galaxy population, the objects detected in existing surveys consist mainly of massive galaxies (several M_*) forming stars fairly steadily over timescales ~ 10^8-10^9 years, at moderate rates ~100 Msun/yr. The typical descendants of these sub-mm sources are even more massive galaxies, with old stellar populations, found primarily in dense environments. While the resolution of our simulations is not sufficient to determine galaxy morphologies, these properties support the proposed identification of sub-mm sources with massive ellipticals in the process of formation. The most robust and distinctive prediction of this model, stemming directly from the long timescale and correspondingly moderate rate of star formation, is that the far-IR SEDs of SCUBA sources have a relative high 850 micron luminosity for a given bolometric luminosity. [Abridged]

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arXiv2001

The Statistical and Physical Properties of the Low Redshift Lyman Alpha Forest Observed with HST/STIS

We examine the Ly-alpha absorber population at z<0.3 detected in spectra of the QSOs PG0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. We compare their statistical properties to those in carefully-constructed mock quasar spectra drawn from a cosmological hydrodynamic simulation of a Lambda-CDM universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with N_HI\approx 10^{12.3} cm^-2. The observed absorber population is complete for N_HI\ga 10^{13} cm^-2, with a column density distribution slope of β=2.04\pm 0.23 and a median b-parameter of 21 km/s above this limit. The intergalactic gas giving rise to these weak absorbers is analogous to that at high redshift, located in diffuse large-scale structures that are highly photoionized by the metagalactic UV flux, though a greater number arise within shock-heated warm gas. The density, temperature, and column density of these absorbers follow similar relationships to those at high redshift, though with substantially larger scatter due to the shock-heated gas. The b-parameters typically have a significant contribution from thermal broadening, which facilitates a measurement of the low-z IGM temperature as traced by Ly-alpha absorbers. From our simulation we estimate T_IGM\sim 5000 K, with an upper limit of 10^4 K, at the mean density. The agreement in predicted and observed amplitude of the column density distributions allows us to measure the HI photoionization rate at z=0.17 to be 10^{-13.3\pm 0.7} s^{-1} (estimated modeling uncertainty), close to predictions based on quasar properties.

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arXiv2001

The Evolution of the Lyman Alpha Forest From Redshift 3 to 0

I review results obtained from studies of the high-redshift Lyman alpha forest, and present new results from HST/STIS spectra of low-redshift quasars in comparison with cosmological hydrodynamic simulations. The evolution of the Lyman alpha forest from redshift 3 to 0 is well-described by current structure formation models, in which Ly-a forest absorbing gas at all redshifts traces moderate-overdensity large-scale structures. I describe some of the insights provided by hydrodynamic simulations into the observed statistical trends of Ly-a absorbers and the physical state of the absorbing gas.

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arXiv2000

Baryons in the Warm-Hot Intergalactic Medium

Approximately 30-40% of all baryons in the present day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures between 10^5<T<10^7 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, non-equilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas is consistent with observations of the 0.25 keV X-ray background without being significantly heated by non-gravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Dave' et al.

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