arXiv subjects

Jeremiah P. Ostriker

Publications and source records attributed to Jeremiah P. Ostriker.

At least 109 records · Page 6Linked to original sources

arXiv2006

Formation of early-type galaxies from cosmological initial conditions

We describe high resolution Smoothed Particle Hydrodynamics (SPH) simulations of three approximately $M_*$ field galaxies starting from \LCDM initial conditions. The simulations are made intentionally simple, and include photoionization, cooling of the intergalactic medium, and star formation but not feedback from AGN or supernovae. All of the galaxies undergo an initial burst of star formation at $z \approx 5$, accompanied by the formation of a bubble of heated gas. Two out of three galaxies show early-type properties at present whereas only one of them experienced a major merger. Heating from shocks and -PdV work dominates over cooling so that for most of the gas the temperature is an increasing function of time. By $z \approx 1$ a significant fraction of the final stellar mass is in place and the spectral energy distribution resembles those of observed massive red galaxies. The galaxies have grown from $z=1 \to 0$ on average by 25% in mass and in size by gas poor (dry) stellar mergers. By the present day, the simulated galaxies are old ($\approx 10 {\rm Gyrs}$), kinematically hot stellar systems surrounded by hot gaseous haloes. Stars dominate the mass of the galaxies up to $\approx 4$ effective radii ($\approx 10$ kpc). Kinematic and most photometric properties are in good agreement with those of observed elliptical galaxies. The galaxy with a major merger develops a counter-rotating core. Our simulations show that realistic intermediate mass giant elliptical galaxies with plausible formation histories can be formed from \LCDM initial conditions even without requiring recent major mergers or feedback from supernovae or AGN.

astro-ph↗

arXiv2006

Topology of structure in the Sloan Digital Sky Survey: model testing

We measure the three-dimensional topology of large-scale structure in the Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured with unprecedented statistical accuracy. The sample size is now sufficiently large to allow the topology to be an important tool for testing galaxy formation models. For comparison, we make mock SDSS samples using several state-of-the-art N-body simulations: the Millennium run of Springel et al. (2005)(10 billion particles), Kim & Park (2006) CDM models (1.1 billion particles), and Cen & Ostriker (2006) hydrodynamic code models (8.6 billion cell hydro mesh). Each of these simulations uses a different method for modeling galaxy formation. The SDSS data show a genus curve that is broadly characteristic of that produced by Gaussian random phase initial conditions. Thus the data strongly support the standard model of inflation where Gaussian random phase initial conditions are produced by random quantum fluctuations in the early universe. But on top of this general shape there are measurable differences produced by non-linear gravitational effects (cf. Matsubara 1994), and biasing connected with galaxy formation. The N-body simulations have been tuned to reproduce the power spectrum and multiplicity function but not topology, so topology is an acid test for these models. The data show a ``meatball'' shift (only partly due to the Sloan Great Wall of Galaxies; this shift also appears in a sub-sample not containing the Wall) which differs at the 2.5σlevel from the results of the Millennium run and the Kim & Park dark halo models, even including the effects of cosmic variance.

astro-ph↗

arXiv2006

Growth of structure seeded by primordial black holes

We discuss the possibilities for primordial black holes (PBHs) to grow via the accretion of dark matter. In agreement with previous works, we find that accretion during the radiation-dominated era does not lead to a significant mass increase. However, during matter-domination, PBHs may grow by up to two orders of magnitude in mass through the acquisition of large dark matter halos. We discuss the possibility of PBHs being an important component in dark matter halos of galaxies as well as their potential to explain the ultra-luminous x-ray sources (ULXs) observed in nearby galactic disks. We point out that although PBHs are ruled out as the dominant component of dark matter, there is still a great deal of parameter space open to them playing a role in the modern-day universe. For example, a primordial halo population of PBHs each at $10^{2.5} M_\odot$ making up 0.1% of the dark matter grow to $10^{4.5} M_\odot$ via the accumulation of dark matter halos to account for $\sim 10%$ of the dark matter mass by a redshift of $z \approx 30$. These intermediate mass black holes may then ``light up'' when passing through molecular clouds, becoming visible as ULXs at the present day, or they may form the seeds for supermassive black holes at the centers of galaxies.

astro-ph↗

arXiv2006

The History of Cosmological Star Formation: Three Independent Approaches and a Critical Test Using the Extragalactic Background Light

Taking three independent approaches, we investigate the simultaneous constraints set on the cosmic star formation history from various observations, including stellar mass density and extragalactic background light (EBL). We compare results based on: 1) direct observations of past light-cone, 2) a model using local fossil evidence constrained by SDSS observations at z~0 (the `Fossil' model), and 3) theoretical ab initio models from three calculations of cosmic star formation history: (a) new (1024)^3 Total Variation Diminishing (TVD) cosmological hydrodynamic simulation, (b) analytic expression of Hernquist & Springel based on cosmological Smoothed Particle Hydrodynamics (SPH) simulations, and (c) semi-analytic model of Cole et al. We find good agreement among the three independent approaches up to the order of observational errors, except that all the models predict bolometric EBL of I_tot ~= 37-52 nW m^-2 sr^-1, which is at the lower edge of the the observational estimate by Hauser & Dwek. We emphasize that the Fossil model that consists of two components -- spheroids and disks --, when normalized to the local observations, provides a surprisingly simple but accurate description of the cosmic star formation history and other observable quantities. Our analysis suggests that the consensus global parameters at z=0 are: Omega_* = 0.0023+-0.0004, I_EBL = 43+-7 nW m^-2 sr^-1 rho_SFR=(1.06+-0.22)e-2 Msun yr^-1 Mpc^-3, j_bol = (3.1+-0.2)e8 Lsun Mpc^-3.

astro-ph↗

arXiv2006

Testing a new analytic model for gravitational lensing probabilities

We study gravitational lensing with a multiple lens plane approach, proposing a simple analytical model for the probability distribution function (PDF) of the dark matter convergence, kappa, for the different lens planes in a given cosmology as a function of redshift and smoothing angle, theta. The model is fixed solely by the variance of kappa, which in turn is fixed by the amplitude of the power spectrum, sigma_8. We test the PDF against a high resolution Tree-Particle-Mesh simulation and find that it is far superior to the Gaussian or the lognormal, especially for small values of theta << 1 arcmin and at large values of kappa relevant to strong lensing. With this model, we predict the probabilities of strong lensing by a single plane or by multiple planes. We find that for theta ~ 10 arcsec, a single plane accounts for almost all (~ 98%) of the strong lensing cases for source redshift unity. However, for a more typical source redshift of 4, about 12% of the strong lensing cases will result from the contribution of a secondary clump of matter along the line of sight, introducing a systematic error in the determination of the surface density of clusters, typically overestimating it by about 2-5%. We also find that matter inhomogenieties introduce a dispersion in the value of the angular diameter distance about its cosmological mean. The probable error relative to the mean increases with redshift to a value of about 8% for z ~ 6 and theta ~ 10 arcsec.

astro-ph↗

arXiv2006

Tracing early structure formation with massive starburst galaxies and their implications for reionization

Cosmological hydrodynamic simulations have significantly improved over the past several years, and we have already shown that the observed properties of Lyman-break galaxies (LBGs) at z=3 can be explained well by the massive galaxies in the simulations. Here we extend our study to z=6 and show that we obtain good agreement for the LBGs at the bright-end of the luminosity function (LF). Our simulations also suggest that the cosmic star formation rate density has a peak at z= 5-6, and that the current LBG surveys at z=6 are missing a significant number of faint galaxies that are dimmer than the current magnitude limit. Together, our results suggest that the universe could be reionized at z=6 by the Pop II stars in ordinary galaxies. We also estimate the LF of Lyman-alpha emitters (LAEs) at z=6 by relating the star formation rate in the simulation to the Ly-alpha luminosity. We find that the simulated LAE LFs agree with the observed data provided that the net escape fraction of Ly-alpha photon is f_{Ly-alpha} <= 0.1. We investigate two possible scenarios for this effect: (1) all sources in the simulation are uniformly dimmer by a factor of 10 through attenuation, and (2) one out of ten LAEs randomly lights up at a given moment. We show that the correlation strength of the LAE spatial distribution can possibly distinguish the two scenarios.

astro-ph↗

arXiv2006

Simulations of Strong Gravitational Lensing with Substructure

Galactic sized gravitational lenses are simulated by combining a cosmological N-body simulation and models for the baryonic component of the galaxy. The lens caustics, critical curves, image locations and magnification ratios are calculated by ray-shooting on an adaptive grid. When the source is near a cusp in a smooth lens' caustic the sum of the magnifications of the three closest images should be close to zero. It is found that in the observed cases this sum is generally too large to be consistent with the simulations implying that there is not enough substructure in the simulations. This suggests that other factors play an important role. These may include limited numerical resolution, lensing by structure outside the halo, selection bias and the possibility that a randomly selected galaxy halo may be more irregular, for example due to recent mergers, than the isolated halo used in this study. It is also shown that, with the level of substructure computed from the N-body simulations, the image magnifications of the Einstein cross type lenses are very weak functions of source size up to $\sim 1\kpc$. This is also true for the magnification ratios of widely separated images in the fold and cusp caustic lenses. This means that selected magnification ratios for different the emission regions of a lensed quasar should agree with each other, barring microlensing by stars. The source size dependence of the magnification ratio between the closest pair of images is more sensitive to substructure.

astro-ph↗

arXiv2005

Where Are the Baryons? II: Feedback Effects

Numerical simulations of the intergalactic medium have shown that at the present epoch a significant fraction (40-50%) of the baryonic component should be found in the (T~10^6K) Warm-Hot Intergalactic Medium (WHIM) - with several recent observational lines of evidence indicating the validity of the prediction. We here recompute the evolution of the WHIM with the following major improvements: (1) galactic superwind feedback processes from galaxy/star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a non-equilibrium way; (3) mass and spatial dynamic ranges are larger by a factor of 8 and 2, respectively, than in our previous simulations. Here are the major findings: (1) galactic superwinds have dramatic effects, increasing the WHIM mass fraction by about 20%, primarily through heating up warm gas near galaxies with density 10^{1.5}-10^4 times the mean density. (2) the fraction of baryons in WHIM is increased modestly from the earlier work but is ~40-50%. (3) the gas density of the WHIM is broadly peaked at a density 10-20 times the mean density, ranging from underdense regions to regions that are overdense by 10^3-10^4. (4) the median metallicity of the WHIM is 0.18 Zsun for oxygen with 50% and 90% intervals being (0.040,0.38) and (0.0017,0.83).

astro-ph↗

arXiv2005

A simple model for the evolution of disc galaxies: The Milky Way

A simple model for the evolution of disc galaxies is presented. We adopt three numbers from observations of the Milky Way disc, the local surface mass density, the stellar scale length (of the assumedly exponential disc) and the amplitude of the (assumedly flat) rotation curve, and physically, the (local) dynamical Kennicutt star formation prescription, standard chemical evolution equations assuming and a model for spectral evolution of stellar populations. We can determine the detailed evolution of the model with only the addition of standard cosmological scalings with time of the dimensional parameters. A surprising wealth of detailed specifications follows from this prescription including the gaseous infall rate as a function of radius and time, the distribution of stellar ages and metallicities with time and radius, surface brightness profiles at different wavelengths, colours etc. At the solar neighbourhood stars start to form $\approx 10 Gyrs$ ago at an increasing rate peaking 4 billion years ago and then slowly declining in good agreement with observations. The mean age of long lived stars at the solar neighbourhood is about $4 Gyrs$. The local surface density of the stars and gas are 35 and $15 M_{\odot}pc^{-2}$, respectively. The metallicity distribution of the stars at the solar radius is narrow with a peak at $[Z/Z_{\odot}] = -0.1$.Both a Salpeter IMF and a Chabrier IMF are consistent with observations. Comparisons with the current and local fossil evidence provides support for the model which can then be used to assess other local disc galaxies, the evolution of disc galaxies in deep optical surveys and also for theoretical investigations such as simulations of merging disc galaxies (abbreviated).

astro-ph↗

arXiv2005

A Simple and Accurate Model for Intra-Cluster Gas

Starting with the well-known NFW dark matter halo distribution, we construct a simple polytropic model for the intracluster medium which is in good agreement with high resolution numerical hydrodynamical simulations, apply this model to a very large scale concordance dark matter simulation, and compare the resulting global properties with recent observations of X-ray clusters, including the mass-temperature and luminosity-temperature relations. We make allowances for a non-negligible surface pressure, removal of low entropy (short cooling time) gas, energy injection due to feedback, and for a relativistic (non-thermal) pressure component. A polytropic index n=5 (Gamma=1.2) provides a good approximation to the internal gas structure of massive clusters (except in the very central regions where cooling becomes important), and allows one to recover the observed M_500-T, L_x-T, and T/n_e^{2/3} ~ T^0.65 relations. Using these concepts and generalizing this method so that it can be applied to fully three-dimensional N-body simulations, one can predict the global X-ray and SZE trends for any specified cosmological model. We find a good fit to observations when assuming that twelve percent of the initial baryonic mass condenses into stars, the fraction of rest mass of this condensed component transferred back to the remaining gas (feedback) is 3.9E-5, and the fraction of total pressure from a nonthermal component is near ten percent.

astro-ph↗

arXiv2005

Massive galaxies & EROs at z=1-3 in cosmological hydrodynamic simulations: near-IR properties

Recent observations have revealed a population of red massive galaxies at high redshift which are challenging to explain in hierarchical galaxy formation models. We analyze this ``massive galaxy problem'' with two different types of hydrodynamic simulations -- Eulerian TVD and SPH -- of a concordance Lambda CDM universe. We consider two separate but connected aspects of the problem posed by these extremely red objects (EROs): (1) the mass-scale, and (2) red colors. We perform spectrophotometric analyses of simulated galaxies in B, z, R, I, Js, Ks, K filters, and compare their near-IR properties with observations at redshift z=1-3. We find that the simulated galaxies brighter than the magnitude limit of K_vega=20 mag have stellar masses Mstar >= 10^{11} Msun/h and a number density of a few \times 10^{-4} h^3 Mpc^{-3} at z~2, in good agreement with the observed number density in the K20 survey. Therefore, our hydrodynamic simulations do not exhibit the ``mass-scale problem''. The answer to the ``redness problem'' is less clear because of our poor knowledge of the amount of dust extinction in EROs and the uncertain fraction of star-forming EROs. However, our simulations can account for the observed comoving number density of ~ 1e-4 Mpc^{-3} at z=1-2 if we assume a uniform extinction of E(B-V)=0.4 for the entire population of simulated galaxies. The critical question of the existence of a significant population of old, passively evolving massive EROs should continue to be tested with future hydrodynamic simulations. Upcoming observations of the thermal emission of dust in 24 micron by the Spitzer Space Telescope will help to better estimate the dust content of EROs at z=1-3, and thus to further constrain the star formation history of the Universe, and theoretical models of galaxy formation. (abridged)

astro-ph↗

arXiv2005

Characterizing the Cluster Lens Population

We present a detailed investigation into which properties of CDM halos make them effective strong gravitational lenses. Strong lensing cross sections of 878 clusters from an N-body simulation are measured by ray tracing through 13,594 unique projections. We measure concentrations, axis ratios, orientations, and the amount of substructure of each cluster, and compare the lensing weighted distribution of each quantity to that of the cluster population as a whole. The concentrations of lensing clusters are on average 34% larger than the typical cluster in the Universe. Despite this bias, the anomalously high concentrations (c >14) recently measured by several groups, appear to be inconsistent with the concentration distribution in our simulations, which predict < 2% of lensing clusters should have concentrations this high. No correlation is found between lensing cross section and the amount of substructure. We introduce several types of simplified dark matter halos, and use them to isolate which properties of CDM clusters make them effective lenses. Projections of halo substructure onto small radii and the large scale mass distribution of clusters do not significantly influence cross sections. The abundance of giant arcs is primarily determined by the mass distribution within an average overdensity of ~ 10,000. A multiple lens plane ray tracing algorithm is used to show that projections of large scale structure increase the giant arc abundance by a modest amount <7%. We revisit the question of whether there is an excess of giant arcs behind high redshift clusters in the RCS survey and find that the number of high redshift (z > 0.6) lenses is in good agreement with LCDM, although our simulations predict more low redshift (z < 0.6) lenses than were observed. (abridged)

astro-ph↗

arXiv2005

Galactic Wind Effects on the Lyman-alpha Absorption in the Vicinity of Galaxies

We present predictions of Ly-alpha forest-galaxy correlations at z=3 from Eulerian simulations that include the effects of galactic winds, driven primarily by supernova explosions. Galactic winds produce expanding bubbles of shock-heated gas within ~0.5 comoving Mpc/h of luminous galaxies in the simulation, which have space density similar to that of observed Lyman break galaxies (LBGs). However, most of the low-density intergalactic gas that determines the observed properties of the Ly-alpha forest is unaffected by winds. The impact of winds on the Ly-alpha optical depth near galaxies is less dramatic than their impact on gas temperature because winds heat only a small fraction of the gas present in the turnaround regions surrounding galaxies. Hence, Ly-alpha absorption from gas outside the wind bubbles is spread out over the same velocity range occupied by the wind-heated gas. In general, Ly-alpha absorption is expected to be stronger than average near galaxies because of the high gas density. Winds result in a modest reduction of this expected increase of Ly-alpha absorption. Our predictions can be compared to future observations to detect the wind effects and infer their strength, although with the caveat that the results are still dependent on the correspondence of simulated galaxies and observed LBGs. We find that wind effects in our simulations are not strong enough to reproduce the high Ly-alpha transmission within 0.5 Mpc/h comoving of galaxies that has been suggested by recent observations; powerful galactic explosions or ejecta with hyper-escape velocities would be required, but these are unlikely to be produced by ordinary star formation and supernovae alone

astro-ph↗

arXiv2005

Massive Black Holes in Dwarf Spheroidal Galaxy Haloes?

It is now established that several of the Local Group dwarf Spheroidal galaxies (dSphs) have large mass-to-light ratios. We consider the possibility that the dark matter in the halos of dSphs is composed of massive black holes with masses in the range 10^5 to 10^7 solar masses. We use direct N-body simulations to determine the long term evolution of a two-component dSph composed of a pressure-supported stellar population in a black hole halo. The black holes are initially distributed according to a Navarro, Frenk & White profile. For black hole masses between 10^5 and 10^6 solar masses, the dark matter halo evolves towards a shallower inner profile in less than a Hubble time. This suggests that black holes in this mass range might provide an explanation for the origin of the dark matter cores inferred from observations of Low Surface Brightness galaxy rotation curves. We compare the simulated evolution of the stellar population with observed data for the Draco dSph. The dependence of the heating rate on the black hole mass is determined, and an upper limit of 10^5 solar masses is placed on the individual black holes comprising the dark matter halo of Draco, if its present stellar distribution is representative of the initial one. We also present a simple scaling argument which demonstrates that the dynamical heating of an initially compact, though not self-gravitating, stellar distribution might produce a remnant distribution similar in extent to Draco after 10 Gyr, even for black hole masses somewhat in excess of 10^5 solar masses.

astro-ph↗

arXiv2004

X-ray Preionisation Powered by Accretion on the First Black Holes. II: Cosmological Simulations and Observational Signatures

We use cosmological simulations to study the X-ray ionisation and heating of the intergalactic medium by an early population of accreting black holes. By considering observational constraints from the X-ray background, we find an upper limit for the optical depth to Thompson scattering tau_e~0.17. The redshifted soft X-ray background from these early sources produces:(i) fully ionised atomic hydrogen in the low density intergalactic medium before redshift z~7 (consequently stellar reionisation is characterised by an instantaneous overlap phase of HII regions),(ii) a second HeII reionisation at z~3 and (iii) heats the intergalactic medium to near 10000 K at low redshifts. The typical luminosity in the soft X-ray band of the galaxies hosting the black holes is about one order of magnitude below the sensitivity limit of the Chandra deep field. About a thousand of these sources may be present per square arcmin of the sky, producing detectable fluctuations. Few rarer objects could be luminous enough to be visible in the Chandra deep field. XEUS and Con-X satellites will be able to detect more of these sources that, if radio loud, could be used to study the 21cm forest in absorption.A signature of an early X-ray preionisation is the production of secondary CMB anisotropies on small angular scales. We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (dT~16 muK at 1arcsec scales), while for stellar reionisation scenarios the power decreases on smaller scales. We also show that the redshifted 21 cm radiation from neutral hydrogen can be marginally detected in emission at redshifts 7<z<12. At a redshift of about z~30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models,could be detectable.[abridged]

astro-ph↗

arXiv2004

Massive galaxies at redshift 2 in cosmological hydrodynamic simulations

We study the properties of galaxies at z=2 in a Lambda cold dark matter universe, using two different types of hydrodynamic simulation methods -- Eulerian TVD and smoothed particle hydrodynamics (SPH) -- and a spectrophotometric analysis in the U_n, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) and Steidel et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 1e-2 h^3 Mpc^-3 for E(B-V)=0.15, which is roughly twice that of the number density found by Erb et al. (2004) for the UV bright sample. This suggests that roughly half of the massive galaxies with M*>10^{10} Msun/h at z=2 are UV bright population, and the other half is bright in the infra-red wavelengths. The most massive galaxies at z=2 have stellar masses >= 10^{11-12} Msun. They typically have been continuously forming stars with a rate exceeding 30 Msun/yr over a few Gyrs from z=10 to z=2, together with significant contribution by starbursts reaching up to 1000 Msun/yr which lie on top of the continuous component. TVD simulations indicate a more sporadic star formation history than the SPH simulations. Our results do not imply that hierarchical galaxy formation fails to account for the observed massive galaxies at z>=1. The global star formation rate density in our simulations peaks at z>=5, a much higher redshift than predicted by the semianalytic models. This star formation history suggests early build-up of the stellar mass density, and predicts that 70 (50, 30)% of the total stellar mass at z=0 had already been formed by z=1 (2, 3). Upcoming observations by Spitzer and Swift might help to better constrain the star formation history at high redshift.

astro-ph↗

arXiv2004

The current status of observational cosmology

Observational cosmology has indeed made very rapid progress in recent years. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation. The transition to precision cosmology has been spearheaded by measurements of the anisotropy in the cosmic microwave background (CMB) over the past decade. Observations of the large scale structure in the distribution of galaxies, high red-shift supernova, have provided the required complementary information. We review the current status of cosmological parameter estimates from joint analysis of CMB anisotropy and large scale structure (LSS) data. We also sound a note of caution on overstating the successes achieved thus far.

astro-ph↗

arXiv2004

Massive galaxies in cosmological simulations: UV-selected sample at redshift z=2

We study the properties of galaxies at z=2 in a Lambda CDM universe, using two different types of hydrodynamic simulation methods (Eulerian TVD and SPH) and a spectrophotometric analysis in the Un, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 2e-2 h^3/Mpc^3, roughly one order of magnitude larger than that of Lyman break galaxies at z=3. The most massive galaxies at z=2 have stellar masses >~1e11 Msun, and their observed-frame G-R colors lie in the range 0.0 = 1.0 and Mstar>1e10 Msun/h finished the build-up of their stellar mass by z~3. Interestingly, our study suggests that the majority of the most massive galaxies at z=2 should be detectable at rest-frame UV wavelengths, contrary to some recent claims made on the basis of near-IR studies of galaxies at the same epoch, provided the median extinction is less than E(B-V)<0.3. However, our results also suggest that the fraction of stellar mass contained in galaxies that pass the color-selection criteria could be as low as 50% of the total stellar mass in the Universe at z=2. Our simulations suggest that the missing stellar mass is contained in fainter (R>25.5) and intrinsically redder galaxies. Our results do not suggest that hierarchical galaxy formation fails to account for the massive galaxies at z>=1. (abridged)

astro-ph↗