arXiv subjects

Jeremiah P. Ostriker

Publications and source records attributed to Jeremiah P. Ostriker.

At least 127 records · Page 7Linked to original sources

arXiv2004

X-ray Preionisation Powered by Accretion on the First Black Holes. I: a Model for the WMAP Polarisation Measurement

We investigate the possibility that there is a first phase of partial ionisation due to X-rays produced by black hole accretion in small-mass galaxies at redshifts 7<z<20. This is followed by complete reionisation by stellar sources at z~7. This scenario is motivated by the large optical depth to Thompson scattering, tau_e=0.17, measured by WMAP. But it is also consistent with the observed Gunn-Peterson trough in the spectra of quasars at z~5-6. We use a semianalytic code to explore models with different black hole accretion histories and cosmological parameters.We find that ``preionisation'' by X-rays can increase the intergalactic medium (IGM) optical depth from tau_e~0.06 given by stellar sources only, to 0.1<tau_e<0.2, if a fraction of baryons 5x10^-5 is accreted onto seed black holes produced in the collapse of low metallicity, high mass stars before z~15. To be effective, preionisation requires a non-negligible star formation in the first small-mass galaxies in hich seed black holes are formed.The model predicts that dwarf spheroidal galaxies may host a mass in black holes that is 5-40% of their stellar mass. The redshifted X-ray background produced by this early epoch of black hole accretion constitutes about 5-10% of the X-ray background in the 2-50 keV bands and roughtly half of the currently estimated black hole mass density was formed at early times.Moreover, in most models, the photons from the redshifted background are sufficient to fully reionise HeII at redshift z~3 without any additional contribution from quasars at lower redshifts and the temperature of the mean density intergalactic medium remains close to 10^4 K down to redshift z~1.[abridged]

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arXiv2004

Signatures of Galactic Superwinds: Inhomogeneous Metal Enrichment of the Lyman Alpha Forest

We investigate possible signatures of feedback from galactic superwinds on the metallicity of the Lyman alpha forest, using a set of high resolution hydrodynamic simulations of a Lambda cold dark matter model. Simulations produce metals self-consistently, based on one single parameter, the metal yield, which in turn is constrained by metallicity in the intra-cluster gas. We follow metals as a separate density species. For the metallicity of Lyman alpha clouds with column density of N_{HI}~10^{14.5}-10^{15.5}/cm^2 at z=2-4 we find reasonable agreement between simulations, both with and without GSW, and observations (Schaye et al). A unique signature and sensitive test of GSW is, however, provided by lower density regions with gas density of ρ/<ρ> = 0.01-1.0 and a corresponding column density of 10^{12}-10^{14}/cm^2. Without GSW we predict that both the mean and median metallicity of \lya clouds in this column density range at z=2-4 should have Z<10^{3}Zsun, since these small systems support little star formation. GSW contaminate these regions, however, and also there is a significant fraction (~25%) of Lyman alpha clouds in this column density range which have a high metallicity excess of 10^{-2}Zsun, resulting in a mean metallicity of ~10^{-2}Zsun. In addition, we find that there is a minimum in the median metallicity for clouds of N_{HI}~10^{13}-10^{14}/cm^2 in the case with GSW, whereas without GSW the metallicity decreases monotonically and rapidly with decreasing column density. Finally, we predict that the ratio of secondary (e.g., N) to primary metals (e.g., O,C) is expected to be smaller by a factor of 10 in clouds of N_{HI}~10^{14.5}/cm^2 compared to that in large galaxies; this factor increases to >50 for N_{HI}< 10^{13.5}/cm^2.

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arXiv2004

Is There a Missing Galaxy Problem at High Redshift?

We study the evolution of the global stellar mass density in a Lambda cold dark matter universe using two different types of hydrodynamical simulations (Eulerian TVD and SPH) and the analytical model of Hernquist & Springel (2003). We find that the theoretical calculations all predict both a higher stellar mass density at z~3 than indicated by current observations, and that the peak of the cosmic star formation rate history should lie at z~5. Such a star formation history implies that as much as (70%, 30%) of the total stellar mass density today must already have formed by z=(1, 3). Our results suggest that current observations at z~3 are missing as much as 50% of the total stellar mass density in the Universe, perhaps owing to an inadequate allowance for dust obscuration in star-forming galaxies, limited sample sizes, or cosmic variance. We also compare our results with some of the updated semi-analytic models of galaxy formation.

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arXiv2004

Gravitational lensing in a concordance LCDM universe: The importance of secondary matter along the line of sight

To date, in almost all strong gravitational lensing analyses for modeling giant arc systems and multiple quasar images, it has been assumed that all the deflecting matter is concentrated in one lens plane at a certain distance - the thin lens approximation. However, in a few observed cases, lenses at more than one redshift have been identified as contributing to the image splitting. Here we report on a quantitative investigation of the importance and frequency of significant multiple lensing agents. We use multi-lens plane simulations to evaluate how frequently two or more lens planes combined are essential for multiple imaging, as compared with the cases where a single lens plane alone provides enough focusing to be supercritical. We find that the fraction of cases for which more than one lens plane contributes significantly to a multi-image lensing situation is a strong function of source redshift. For sources at redshift unity, 95% of lenses involve only a single mass concentration, but for a more typical scenario with, e.g., a source at a redshift of z_s = 3.8, as many as 38% of the strongly lensed quasars/arcs occur because of a significant matter contribution from one or more ADDITIONAL lens planes. In the 30% to 40% of cases when additional planes make a significant contribution, the surface mass density of the primary lens will be overestimated by about 15% to 20%, if the additional contributions are not recognized.

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arXiv2004

Properties of Cold Dark Matter Halos at z>6

We compute the properties of dark matter halos with mass $10^{6.5}-10^9\msun$ at redshift $z=6-11$ in the standard cold dark matter cosmological model, utilizing a very high resolution N-body simulation. We find that dark matter halos in these mass and redshift ranges are significantly biased over matter with a bias factor in the range 2-6. The dark matter halo mass function displays a slope of $2.05\pm 0.15$ at the small mass end. We do not find a universal dark matter density profile. Instead, we find a significant dependence of the central density profile of dark matter halos on halo mass and epoch with $α_0=0.4-1.0$; the high-mass ($M\ge 10^8\msun$) low-redshift ($z\sim 6$) halos occupy the high end of the range and low-mass ($M\sim 10^{7}\msun$) high-redshift ($z\sim 11$) halos occupy the low end. Additionally, for fixed mass and epoch there is a significant dispersion in $α_0$ due to the stochastic assembly of halos. Our results fit a relationship of the form $α_0=0.75((1+z)/7.0)^{-1.25}(M/10^7\msun)^{0.11(1+z)/7.0}$ with a dispersion about this fit of $\pm 0.5$ and no systematic dependence of variance correlated with environment. The median spin parameter of dark matter halos is $0.03-0.04$ but with a large lognormal dispersion of $\sim 0.4$. Various quantities are tabulated or fitted with empirical formulae.

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arXiv2004

Reionisation, chemical enrichment and seed black holes from the first stars: is Population III important?

We investigate the effects of a top-heavy stellar initial mass function on the reionisation history of the intergalactic medium (IGM). We use cosmological simulations that include self-consistently the feedback from ionising radiation, H_2 dissociating radiation and supernova (SN) explosions. We find that it is difficult to reionise the IGM at z_rei>10 with stellar sources even after making extreme assumptions. If star formation in 10^9 M_\odot galaxies is not suppressed by SN explosions, the optical depth to Thomson scattering is tau_e< 0.13. If we allow for the normal energy input from SNe or if pair-instability SNe are dominant, we find taue<0.09. Assuming normal yields for the first stars (popIII), the mean metallicity of the IGM is already Z/Z_\odot=2x10^-3 (10^-3 260 M_\odot, they collapse directly onto black holes without exploding as SNe.If metal-poor stars are initially important and collapse to black holes is the typical outcome, then the secondary emission of ionising radiation from accretion on SN induced seed black holes, might be more important than the primary emission. Surprisingly, including feedback effects, we estimate that a warm dark matter scenario (with particle mass of 1.25 keV) reduces tau_e by only approximately 10%.[abridged]

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arXiv2003

Dependence of Star Formation Rate on Overdensity

We use a large-scale Lambda-CDM hydrodynamical simulation to assess the dependence of the cosmic Star Formation Rate (SFR) on overdensity of luminosity. The `morphology-density relation' is qualitatively captured when the SFR is plotted as a function of redshift, divided into the quartiles of the overdensity distribution.

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arXiv2003

Cosmological Simulations of Elliptical Galaxy Formation in LCDM and LWDM Cosmologies

We present the results of a series of gas dynamical cosmological simulations of the formation of individual massive field galaxies in the standard concordance \LCDM and in a \LWDM cosmology. The galaxies form in an initial burst of star formation followed by accretion of small satellites. The simulated galaxies are old ($\approx$ 10 $Gyrs$) hot stellar systems with masses of $\approx 1.7 \times 10^{11} M_{\odot}$. Baryonic matter dominates the mass in the luminous part of the galaxies up to $\approx 5$ effective radii. The \LCDM galaxy is a slowly rotating spheroidal stellar system (E2) with predominantly disky isophotes. The line-of-sight velocity distributions deviate from Gaussian shape and $h_3$ is anticorrelated with $v_{los}$. The corresponding \LWDM galaxy is more elongated (E3 - E4) and rotates faster. The anisotropy parameter $(v/σ)^*$ is close to unity. Within the uncertainties of $M/L$ both galaxies follow the Fundamental Plane. We conclude that the properties of the two galaxies simulated in the \LCDM and \LWDM cosmology are in good agreement with observations of intermediate mass elliptical or S0 galaxies (abstract abbreviated).

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arXiv2003

Gravitational Lensing by a Compound Population of Halos: Standard Models

Based on observed rotation curves of galaxies and theoretical simulations of dark matter halos, there are reasons for believing that at least three different types of dark matter halos exist in the Universe classified by their masses M and the inner slope of mass density -α: Population A (galaxies): 10^{10} h^{-1} M_\odot < M < 2 \times 10^{13} h^{-1} M_\odot, α= 2; Population B (cluster halos): M > 2 \times 10^{13} h^{-1} M_\odot, α= 1.3; and Population C (dwarf halos): M < 10^{10} h^{-1} M_\odot, α= 1.3. In this paper we calculate the lensing probability produced by such a compound population of dark halos, for both image separation and time delay, assuming that the mass function of halos is given by the Press-Schechter function and the Universe is described by an LCDM, OCDM, or SCDM model. The LCDM model is normalized to the WMAP observations, OCDM and SCDM models are normalized to the abundance of rich clusters. We compare the predictions of the different cosmological models with observational data and show that, both LCDM and OCDM models are marginally consistent with the current available data, but the SCDM model is ruled out. The fit of the compound model to the observed correlation between splitting angle and time delay is excellent but the fit to the number vs splitting angle relation is only adequate using the small number of sources in the objective JVAS/CLASS survey. A larger survey of the same type would have great power in discriminating among cosmological models. Furthermore, population C in an LCDM model has a unique signature in the time domain, an additional peak at ~3 seconds potentially observable in GRBs, which makes it distinguishable from variants of CDM scenarios, such as warm dark matter, repulsive dark matter, or collisional dark matter.

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arXiv2003

Metallicity Evolution of Damped Lyman Alpha Systems In Lambda CDM Cosmology

Utilizing a new, high mass resolution hydrodynamic simulation we compute the metallicity evolution of damped Lyman alpha systems (DLAs) and find a reasonable agreement with observations. In particular, the observed slow evolution of the DLA metallicity occurs naturally in the simulation due to the combined effects of physical and observational selection. The slow metallicity evolution is caused by the steady transformation, with increasing time, of the highest metallicity systems to "galaxies", thus depleting this category, while all the lower metallicity systems show, individually, an increase in metallicity. Although the trend of DLA metallicity with redshift is in good agreement with observations, it appears that the average metallicity of simulated DLAs is higher than observed by 0.3-0.5 dex in the probed redshift range (z=0-5). Our study indicates that this difference may be attributed to observational selection effects due to dust obscuration. If we allow for a dust obscuration effect, our model reproduces the observed metallicity evolution in both amplitude and slope. We find that DLAs are not a simple population but probe a range of different systems and the mix changes with redshift. About 50% of all metals in the gaseous phase is in DLAs at all times from z=5 to z=1, making a rapid downturn at z\le 1 to ~20% by z=0, as metals are swept into the hotter components of the IGM as well as locked up in stars. While not the primary focus of this study, we find that the model provides good matches to observations with respect to column density distribution and evolution of neutral gas content, if the same dust obscuration is taken into account. We find Omega_{DLA,comp}=(1-3)E-3, depending on the effect of dust obscuration.

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arXiv2003

The Probability Distribution Function of Light in the Universe: Results from Hydrodynamic Simulations

While second and higher order correlations of the light distribution have received extensive study, the lowest order probability distribution function (PDF) -- the probability that a unit volume of space will emit a given amount of light -- has received very little attention. We estimate this function with the aid of hydrodynamic simulations of the Lambda CDM model, finding it significantly different from the mass density PDF, and not simply related to it by linear bias or any of the other prescriptions commonly adopted. If the optical light PDF is, in reality, similar to what we find in the simulations, then some measures of Omega_M based on mass-to-light ratio and the cosmic virial theorem will have significantly underestimated Omega_M. Basically, the problem is one of selection bias, with galaxy forming regions being unrepresentative of the dark matter distribution in a way not described by linear bias. Knowledge of the optical PDF and the plausible assumption of a log-normal distribution for the matter PDF will allow one to correct for these selection biases. We find that this correction (which amounts to 20-30%) brings the values of Omega_M estimated by using the mass-to-light ratio and the cosmic virial theorem to the range 0.2-0.3, in better agreement with the WMAP result than the uncorrected estimates. In addition, the relation between mass and light PDFs gives us insight concerning the nature of the void phenomenon. In particular our simulation indicates that 20% of mass is distributed in voids, which occupy 85% of volume in the universe.

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arXiv2003

New Light on Dark Matter

Dark matter, proposed decades ago as a speculative component of the universe, is now known to be the vital ingredient in the cosmos, eight times more abundant than ordinary matter, one quarter of the total energy density and the component which has controlled the growth of structure in the universe. Its nature remains a mystery, but, assuming it is comprised of weakly interacting sub-atomic particles, is consistent with large scale cosmic structure. However, recent analyses of structure on galactic and sub-galactic scales have suggested discrepancies and stimulated numerous alternative proposals. We discuss how studies of the density, demography, history and environment of smaller scale structures may distinguish among these possibilities and shed new light on the nature of dark matter.

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arXiv2003

Giant Arc Statistics In Concordance With A Concordance LCDM Universe

The frequency of giant arcs - highly distorted and strongly gravitationally lensed background galaxies - is a powerful test for cosmological models. Previous comparisons of arc statistics for the currently favored concordance cosmological model (a flat lambda-dominated universe) with observations have shown an apparently large discrepancy. We present here new ray-shooting results, based on a high resolution (1024^3 particles in a 320 Mpc/h box) large-scale structure simulation normalized to the WMAP observations. We follow light rays through a pseudo-3D matter distribution approximated by up to 38 lens planes, and evaluate the occurrence of arcs for various source redshifts. We find that the frequency of strongly lensed background sources is a steep function of source redshift: the optical depth for giant arcs increases by a factor of five when the background sources are moved from redshift z_s = 1.0 to z_s = 1.5. This is a consequence of a moderate decrease of the critical surface mass density for lensing, combined with the very steep cluster mass function at the high mass end. Our results are consistent with those of Bartelmann et al. (1998) if we - as they did - restrict all sources to be exactly at z_s = 1. But if we allow for a more realistic distribution of source redshifts extending to or beyond z_s > 1.5, the apparent discrepancy vanishes: the frequency of arcs is increased by about a factor of ten as compared to previous estimates, and results in roughly one arc per 20 square degrees over the sky. This prediction for an LCDM model is then in good agreement with the observed frequency of arcs. Hence we consider the ``missing arc'' problem for a concordance LCDM cosmology to be solved.

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arXiv2003

Combining WMAP and SDSS Quasar Data on Reionization Constrains Cosmological Parameters and the Star Formation Efficiency

We present constraints on cosmological and star formation parameters based on combining observations of the Wilkinson Microwave Anisotropy Probe (WMAP) and high-redshift quasars from the Sloan Digital Sky Survey (SDSS). We use a semi-analytic model for reionization (Chiu and Ostriker 2000) that takes into account a number of important physical processes both within collapsing halos and in the intergalactic medium. Assuming that the efficiency of producing UV photons per baryon is constant, we derive a constraint of the form sigma_8 Omega_0^0.5~0.33 in a flat, Lambda-dominated universe with h=0.72, n=0.99, and Omega_b h^2=0.024. However, the calculated optical depth to electron scattering of tau_es~0.06 is well below the value found by WMAP of 0.17+/-(0.04~0.07) (Spergel et al 2003). Since the WMAP constraints on tau_es are somewhat degenerate with the value of the spectral index n, we then permit the primordial spectral index n to float and fix Omega_0 h^2=0.14, while normalizing the power spectrum using WMAP. In addition, we allow the UV-efficiency to have time-dependence. Combining the WMAP constraints with the quasar transmission data, our analysis then favors a model with tau_es=0.11^{+0.02}_{-0.03}, n=0.96^{+0.02}_{-0.03}$, implying sigma_8=0.83^{+0.03}_{-0.05} (95% confidence), and an effective UV-efficiency that was at least ~10x greater at z >> 6. These results indicate that the quasar and WMAP observations are consistent. If future observations confirm an optical depth to electron scattering tau_es~0.1, then it would appear that no more "exotic" sources of UV-photons, such as mini-quasars or AGNs, are necessary; but our analysis indicates that a determination of tau_es>~0.17 would require a more radical solution.

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arXiv2003

Reasoning From Fossils: Learning From the Local Black Hole Population About the Evolution of Quasars

We discuss a simple model for the growth of supermassive black holes (BHs) at the center of spheroidal stellar systems. In particular, we assess the hypotheses that (1) star formation in spheroids and BH fueling are proportional to one another, and (2) the BH accretion luminosity stays near the Eddington limit during luminous quasar phases. With the aid of this simple model, we are able to interpret many properties of the QSO luminosity function, including the puzzling steep decline of the characteristic luminosity from redshift z=2 to to z=0: indeed the residual star formation in spheroidal systems is today limited to a small number of bulges, characterized by stellar velocity dispersions a factor of 2-3 smaller those of the elliptical galaxies hosting QSOs at z > 2. A simple consequence of our hypotheses is that the redshift evolution of the QSO emissivity and of the star formation history in spheroids should be roughly parallel. We find this result to be broadly consistent with our knowledge of the evolution of both the global star formation rate, and of the evolution of the QSO emissivity, but we identify interesting discrepancies at both low and high redshifts, to which we offer tentative solutions. Finally, our hypotheses allow us to present a robust method to derive the duty cycle of QSO activity, based on the observed QSO luminosity function, and on the present-day relation between the masses of supermassive BHs and those of their spheroidal host stellar systems. The duty cycle is found to be substantially less than unity, with characteristic values in the range (3-6)x10^(-3), and we compute that the average bolometric radiative efficiency is epsilon=0.07. Finally, we find that the growth in mass of individual black holes at high redshift (z>2) can be dominated by mergers, and is therefore not necessarily limited by accretion.

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arXiv2003

Precision Cosmology? Not Just Yet

The recent announcement from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite experiment combined with other recent advances in observational cosmology verifies key components of the standard cosmological model. However, we argue that there remain some significant open issues regarding the basic history and composition of the Universe and uncertainties in some of the most important parameters.

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arXiv2003

Tidal Effects in Clusters of Galaxies

High-redshift clusters of galaxies show an over-abundance of spirals by a factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative to the nearby clusters. This morphological evolution can be explained by tidal interactions with neighboring galaxies and with the hierarchically growing cluster halo. The efficiency of tidal interactions depends on the size and structure of the cluster, as well as on the epoch of its formation. I simulate the formation and evolution of Virgo-type clusters in three cosmologies: a critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model Omega_0=0.4 with a cosmological constant. The orbits of identified halos are traced with a high temporal resolution (~10^7 yr). Halos with low relative velocities merge only shortly after entering the cluster; after virialization mergers are suppressed. The dynamical evolution of galaxies is determined by the tidal field along their trajectories. The maxima of the tidal force do not always correspond to closest approach to the cluster center. They are produced to a large extent by the local density structures, such as the massive galaxies and the unvirialized remnants of infalling groups of galaxies. Collisions of galaxies are intensified by the substructure, with about 10 encounters within 10 kpc per galaxy in the Hubble time. These very close encounters add an important amount (10-50%) of the total heating rate. The integrated effect of tidal interactions is insufficient to transform a spiral galaxy into an elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in the low Omega_0 clusters.

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arXiv2003

Tree-Particle-Mesh: an adaptive, efficient, and parallel code for collisionless cosmological simulation

An improved implementation of an N-body code for simulating collisionless cosmological dynamics is presented. TPM (Tree-Particle-Mesh) combines the PM method on large scales with a tree code to handle particle-particle interactions at small separations. After the global PM forces are calculated, spatially distinct regions above a given density contrast are located; the tree code calculates the gravitational interactions inside these denser objects at higher spatial and temporal resolution. The new implementation includes individual particle time steps within trees, an improved treatment of tidal forces on trees, new criteria for higher force resolution and choice of time step, and parallel treatment of large trees. TPM is compared to P^3M and a tree code (GADGET) and is found to give equivalent results in significantly less time. The implementation is highly portable (requiring a Fortran compiler and MPI) and efficient on parallel machines. The source code can be found at http://astro.princeton.edu/~bode/TPM/

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