arXiv subjects

Jeremiah P. Ostriker

Publications and source records attributed to Jeremiah P. Ostriker.

At least 163 records · Page 9Linked to original sources

arXiv2000

Luminosity Density of Galaxies and Cosmic Star Formation Rate from Lambda-CDM Hydrodynamical Simulations

We compute the cosmic star formation rate (SFR) and the rest-frame comoving luminosity density in various pass-bands as a function of redshift using large-scale Λ-CDM hydrodynamical simulations with the aim of understanding their behavior as a function of redshift. To calculate the luminosity density of galaxies, we use an updated isochrone synthesis model which takes metallicity variations into account. The computed SFR and the UV-luminosity density have a steep rise from z=0 to 1, a moderate plateau between z=1 - 3, and a gradual decrease beyond z=3. The raw calculated results are significantly above the observed luminosity density, which can be explained either by dust extinction or the possibly inappropriate input parameters of the simulation. We model the dust extinction by introducing a parameter f; the fraction of the total stellar luminosity (not galaxy population) that is heavily obscured and thus only appears in the far-infrared to sub-millimeter wavelength range. When we correct our input parameters, and apply dust extinction with f=0.65, the resulting luminosity density fits various observations reasonably well, including the present stellar mass density, the local B-band galaxy luminosity density, and the FIR-to-submm extragalactic background. Our result is consistent with the picture that \sim 2/3 of the total stellar emission is heavily obscured by dust and observed only in the FIR. The rest of the emission is only moderately obscured which can be observed in the optical to near-IR wavelength range. We also argue that the steep falloff of the SFR from z=1 to 0 is partly due to the shock-heating of the universe at late times, which produces gas which is too hot to easily condense into star-forming regions.

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arXiv2000

Collisional Dark Matter and the Origin of Massive Black Holes

If the cosmological dark matter is primarily in the form of an elementary particle which has cross section and mass for self-interaction having a ratio similar to that of ordinary nuclear matter, then seed black holes (formed in stellar collapse) will grow in a Hubble time, due to accretion of the dark matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence of the final black hole mass on the galaxy velocity dispersion will be approximately as observed and the growth rate will show a time dependence consistent with observations. Other astrophysical consequences of collisional dark matter and tests of the idea are noted.

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arXiv2000

Formation of Galactic Bulges

We use cosmological hydrodynamic simulations to investigate formation of galactic bulges within the framework of hierarchical clustering in a representative CDM cosmological model. We show that largest objects forming at cosmological redshifts z~4 resemble observed bulges of spiral galaxies or moderate size ellipticals in their general properties like sizes, shapes, and density profiles. This is consistent with observational data indicating the existence of ``old'' bulges and ellipticals at more moderate redshifts. These bulges are gas dominated at redshift z=3, with high rates of star formation and would appear to be good candidates for small blue galaxies seen in the Hubble Deep Field.

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arXiv2000

Preheated Advection Dominated Accretion Flow

All high temperature accretion solutions including ADAF are physically thick, so outgoing radiation interacts with the incoming flow, sharing as much or more resemblance with classical spherical accretion flows as with disk flows. We examine this interaction for the popular ADAF case. We find that without allowance for Compton preheating, a very restricted domain of ADAF solution is permitted and with Compton preheating included a new high temperature PADAF branch appears in the solution space. In the absence of preheating, high temperature flows do not exist when the mass accretion rate mdot == Mdot c^2 / L_E >~ 10^-1.5. Below this mass accretion rate, a roughly conical region around the hole cannot sustain high temperature ions and electrons for all flows having mdot >~ 10^-4, which may lead to a funnel possibly filled with a tenuous hot outgoing wind. If the flow starts at large radii with the usual equilibrium temperature ~10^4 K, the critical mass accretion rate is much lower, mdot \~10^-3.7 above which level no self-consistent ADAF (without preheating) can exist. However, above this critical mass accretion rate, the flow can be self-consistently maintained at high temperature if Compton preheating is considered. These solutions constitute a new branch of solutions as in spherical accretion flows. High temperature PADAF flows can exist above the critical mass accretion rate in addition to the usual cold thin disk solutions. We also find solutions where the flow near the equatorial plane accretes normally while the flow near the pole is overheated by Compton preheating, possibly becoming, a polar wind, solutions which we designate WADAF.

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arXiv2000

Physical Bias of Galaxies From Large-Scale Hydrodynamic Simulations

We analyze a new large-scale ($100h^{-1}$Mpc) numerical hydrodynamic simulation of the popular $Λ$CDM cosmological model, including in our treatment dark matter, gas and star-formation, on the basis of standard physical processes. The method, applied with a numerical resolution of $<200h^{-1}$kpc (which is still quite coarse for following individual galaxies, especially in dense regions), attempts to estimate where and when galaxies form. We then compare the smoothed galaxy distribution with the smoothed mass distribution to determine the "bias" defined as $b\equiv (δM/M)_{gal}/(δM/M)_{total}$ on scales large compared with the code numerical resolution (on the basis of resolution tests given in the appendix of this paper). We find that (holding all variables constant except the quoted one) bias increases with decreasing scale, with increasing galactic age or metallicity and with increasing redshift of observations. At the $8h^{-1}$Mpc fiducial comoving scale bias (for bright regions) is 1.35 at $z=0$ reaching to 3.6 at $z=3$, both numbers being consistent with extant observations. We also find that $(10-20)h^{-1}$Mpc voids in the distribution of luminous objects are as observed (i.e., observed voids are not an argument against CDM-like models) and finally that the younger systems should show a colder Hubble flow than do the early type galaxies (a testable proposition). Surprisingly, little evolution is found in the amplitude of the smoothed galaxy-galaxy correlation function (as a function of {\it comoving} separation). Testing this prediction vs observations will allow a comparison between this work and that of Kauffmann et al which is based on a different physical modelingmethod.

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arXiv1999

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyman-alpha Forest

A sample of eight quasars observed at high resolution and signal-to-noise is used to determine the probability distribution function (PDF), the power spectrum, and the correlation function of the transmitted flux in the \lya forest, in three redshift bins centered at z=2.41, 3.00, and 3.89. All the results are presented in tabular form, with full error covariance matrices to allow for comparisons with any numerical simulations and with other data sets. The observations are compared with a numerical simulation of the \lya forest of a Lambda-CDM model with Omega=0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the PDF, if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted PDF is found at high fluxes and low redshift, which may be due to the uncertain effects of fitting the spectral continuum. Using the numerical simulation, we show how the flux power spectrum can be used to recover the initial power spectrum of density fluctuations. From our sample of eight quasars, we measure the amplitude of the mass power spectrum to correspond to a linear variance per unit ln(k) of $Δ^2_ρ(k)=0.72\pm0.09$ at k=0.04(km/s)^{-1} and z=3, and the slope of the power spectrum near the same k to be $n_p=-2.55\pm0.10$ (statistical error bars). The results are statistically consistent with Croft et. al. (1999), although our value for the rms fluctuation is lower by a factor 0.75. For the Lambda-CDM model we use, the implied primordial slope is $n=0.93\pm0.10$, and the normalization is $σ_8=0.68+1.16(0.95-n)\pm0.04$.

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arXiv1999

The structure of dark matter halos in hierarchical clustering theories

During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small mass halo could be dense enough to resist disruption and survive undigested, when it is incorporated into a bigger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo, which have survived the hierarchical, inhomogeneous collapse to form larger and larger objects, determines the halo structure in the inner regions. For a flat universe with $P(k) \propto k^n$, scaling arguments then suggest that the core density profile is, $ρ\propto r^{-α}$ with $α= (9+3n)/(5+n)$. But whether such behaviour obtains depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than $1/r^2$ in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot obtain, in self-similar collapse. We then briefly look at the profiles of the outer halos in low density cosmological models where the total halo mass is convergent. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n= 0, -1 and -2. We find that the core-density profiles of dark halos, show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. (Abridged)

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arXiv1999

A Semi-Analytic Model for Cosmological Reheating and Reionization Due to the Gravitational Collapse of Structure

We present a semi-analytic model for the thermal and ionization history of the universe at 1000 >~ z >~ 3. This model incorporates much of the essential physics included in full-scale hydrodynamical simulations, such as (1) gravitational collapse and virialization; (2) star/quasar formation and subsequent ionizing radiation; (3) heating and cooling; (4) atomic and molecular physics of hydrogen; and (5) the feedback relationships between these processes. In addition, we model the process of reheating and reionization using two separate phases, self-consistently calculating the filling factor of each phase. Thus radiative transfer is treated more accurately than simulations to date have done: we allow to lowest order for both the inhomogeneity of the sources and the sinks of radiation. After calibrating and checking the results of this model against a hydrodynamical simulation, we apply our model to a variety of Gaussian and non-Gaussian CDM-dominated cosmological models. Our major conclusions include: (1) the epoch of reheating depends most strongly on the power spectrum of density fluctuations at small scales; (2) because of the effects of gas clumping, full reionization occurs at z ~ 10 in all models; (3) the CMBR polarization and the stars and quasars to baryons ratio are strong potential discrimants between different assumed power spectra; (4) the formation of galactic spheroids may be regulated by the evolution of reheating through feedback, so that the Jeams mass tracks the non-linear mass scale; and (5) the evolution of the bias of luminous objects can potentially discriminate strongly between Gaussian and non-Gaussian PDFs.

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arXiv1999

The Physical Origin of Scale Dependent Bias in Cosmological Simulations

Using large-scale hydrodynamic simulations with heuristic criteria for galaxy formation, we investigate how the galaxy field is related to the mass density and the gas temperature. We find that the bias b(R) = sigma_g(R)/sigma(R), where sigma_g(R) is the variance of galaxy counts in spheres of radius R and sigma(R) is the same for mass, varies from 2.6 at 1 h^{-1} Mpc to 1.2 at 30 h^{-1} Mpc. Including the dependence of the galaxy density on local gas temperature as well as on local mass density can fully account for this scale dependence. Galaxy density depends on temperature because gas which is too hot cannot cool to form galaxies; this causes scale dependence of b(R) because local gas temperature is related to the gravitational potential, and thus contains information about the large scale density field. We show that temperature dependence generally causes b(R) to vary on quasilinear and nonlinear scales, indicating that scale dependence of bias may be a generic effect in realistic galaxy formation scenarios. We find that the relationship between the galaxy and mass density fields is also a function of galaxy age. On large scales, the older galaxies are highly biased (b ~ 1.7) and highly correlated (r ~ 1.0) with the mass density field; younger galaxies are not biased (b ~ 0.8) and are poorly correlated (r ~ 0.5) with the mass. We argue that linear bias is inadequate to describe the relationship between galaxies and mass. (Abridged)

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arXiv1999

Effects of Tidal Shocks on the Evolution of Globular Clusters

We present new Fokker-Planck models of the evolution of globular clusters, including gravitational tidal shocks. We extend our calculations beyond the core collapse by adopting three-body binary heating. Effects of the shocks are included by adding the tidal shock diffusion coefficients to the ordinary Fokker-Planck equation: the first order heating term, , and the second order energy dispersion term, . As an example, we investigate the evolution of models for the globular cluster NGC 6254. Using the Hipparcos proper motions, we are now able to construct orbits of this cluster in the Galaxy. Tidal shocks accelerate significantly both core collapse and the evaporation of the cluster and shorten the destruction time from 24 Gyr to 18 Gyr. We examine various types of adiabatic corrections and find that they are critical for accurate calculation of the evolution. Without adiabatic corrections, the destruction time of the cluster is twice as short. We examine cluster evolution for a wide range of the concentration and tidal shock parameters, and determine the region of the parameter space where tidal shocks dominate the evolution. We present fitting formulae for the core collapse time and the destruction time, covering all reasonable initial conditions. In the limit of strong shocks, the typical value of the core collapse time decreases from 10 t_{rh} to 3 t_{rh} or less, while the destruction time is just twice that number. The effects of tidal shocks are rapidly self-limiting: as clusters lose mass and become more compact, the importance of the shocks diminishes. This implies that tidal shocks were more important in the past.

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arXiv1999

Cosmic Chemical Evolution

Numerical simulations of standard cosmological scenarios have now reached the degree of sophistication required to provide tentative answers to the fundamental question: Where and when were the heavy elements formed? Averaging globally, these simulations give a metallicity that increases from 1% of the solar value at $z=3$ to 20% at present. This conclusion is, in fact, misleading, as it masks the very strong dependency of metallicity on local density. At every epoch higher density regions have much higher metallicity than lower density regions. Moreover, the highest density regions quickly approach near solar metallicity and then saturate, while more typical regions slowly catch up. These results are much more consistent with observational data than the simpler picture (adopted by many) of gradual, quasi-uniform increase of metallicity with time.

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arXiv1999

Time Evolution of Galaxy Formation and Bias in Cosmological Simulations

The clustering of galaxies relative to the mass distribution declines with time because: first, nonlinear peaks become less rare events; second, the densest regions stop forming new galaxies because gas there becomes too hot to cool and collapse; third, after galaxies form, they are gravitationally ``debiased'' because their velocity field is the same as the dark matter. To show these effects, we perform a hydrodynamic cosmological simulation and examine the density field of recently formed galaxies as a function of redshift. We find the bias b_* of recently formed galaxies (the ratio of the rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around 1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_* between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at z=0. As gas in the universe heats up and prevents star formation, star-forming galaxies become poorer tracers of the mass density field. After galaxies form, the linear continuity equation is a good approximation to the gravitational debiasing, even on nonlinear scales. The most interesting observational consequence of the simulations is that the linear regression of the star-formation density field on the galaxy density field evolves from about 0.9 at z=1 to 0.35 at z=0. These effects also provide a possible explanation for the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega, finding that while Omega(z) increases with z, one's estimate Omega_est(z) decreases. (Abridged)

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arXiv1999

On the Self-Consistent Response of Stellar Systems to Gravitational Shocks

We study the reaction of a globular star cluster to a time-varying tidal perturbation (gravitational shock) using self-consistent N-body simulations and address two questions. First, to what extent is the cluster interior protected by adiabatic invariants. Second, how much further energy change does the postshock evolution of the cluster potential produce and how much does it affect the dispersion of stellar energies. We introduce the adiabatic correction} as ratio of the energy change, , to its value in the impulse approximation. When the potential is kept fixed, the numerical results for the adiabatic correction for stars with orbital frequency ωcan be approximated as (1 + ω^2 τ^2)^{-γ}. For shocks with the characteristic duration of the order the half-mass dynamical time of the cluster, τ< t_{dyn,h}, the exponent γ= 5/2. For more prolonged shocks, τ> 4 t_{dyn,h}, the adiabatic correction is shallower, γ= 3/2. When we allow for self-gravity and potential oscillations which follow the shock, the energy of stars in the core changes significantly, while the total energy of the system is conserved. Paradoxically, the postshock potential fluctuations reduce the total amount of energy dispersion, . The effect is small but real and is due to the postshock energy change being statistically anti-correlated with the shock induced heating. These results are to be applied to Fokker-Planck models of the evolution of globular clusters.

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arXiv1999

Thermal Properties of Two-Dimensional Advection Dominated Accretion Flow

We study the thermal structure of the widely adopted two-dimensional advection dominated accretion flow (ADAF) of Narayan & Yi (1995a). The critical radius for a given mass accretion rate, outside of which the optically thin hot solutions do not exist in the equatorial plane, agrees with one-dimensional study. However, we find that, even within the critical radius, there always exists a conical region of the flow, around the pole, which cannot maintain the assumed high electron temperature, regardless of the mass accretion rate, in the absence of radiative heating. This could lead to torus-like advection inflow shape since, in general, the ions too will cool down. We also find that Compton preheating is generally important and, if the radiative efficiency, defined as the luminosity output divided by the mass accretion rate times the velocity of light squared, is above sim 4x10^-3, the polar region of the flow is preheated above the virial temperature by Compton heating and it may result in time-dependent behaviour or outflow while accretion continues in the equatorial plane. Thus, under most relevant circumstances, ADAF solutions may be expected to be accompanied by polar outflow winds. While preheating instabilities exist in ADAF, as for spherical flows, the former are to some extent protected by their characteristically higher densities and higher cooling rates, which reduce their susceptibility to Compton driven overheating.

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arXiv1999

Hydrodynamics of Accretion onto Black Holes

Spherical and axisymmetric accretion onto black holes is discussed. Physical processes in various families of solutions are explained and their characteristics are summarized. Recently discovered solutions of axisymmetric flow provide us with various radiation efficiency and spectrum, which may successfully model diverse accretion systems. Possible role of preheating is also speculated. The various families of solutions can be plotted as trajectories on the plane of (l,e) [or (l,dot m)] where l is the luminosity in units of the Eddington luminosity, dot m is the similarly defined mass accretion rate and e is the efficiency defined by e equiv L/dot M c^2 = l/dot m. We discuss the domains on these planes where solutions are known or expected to be unstable to either spherical or on-spherical perturbations. Preliminary analysis indicates that a preheating instability will occur along the polar direction of the advection dominated flow for e \gsim 10^{-2}.

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arXiv1998

Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections

We perform a variety of tests to determine the numerical resolution of the cosmological TVD eulerian code developed by Ryu et al (1993). Tests include 512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for self-similarity and comparison of results with those from higher resolution SPH and grid-based calculations (Frenk et al 1998). We conclude that in regions where density gradients are not produced by shocks the code degrades resolution with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused gradients (for which the code was designed) the smoothing length is 1.1 cells. Finally, for βmodel fit clusters, we can approximately correct numerical resolution by the transformation R^2_{core}\to R^2_{core}-(CΔl)^2, where Δl is the cell size and C=1.1-1.7. When we use these corrections on our previously published computations for the SCDM and ΛCDM models we find luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86, 280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected core radii, the COBE normalized SCDM model predicts the number of bright L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the ΛCDM model is consistent with observations.

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arXiv1998

The Effect of Cooling on the Density Profile of Hot Gas in Clusters of Galaxies: Is Additional Physics Needed?

We use high-resolution hydrodynamic simulations to investigate the density profile of hot gas in clusters of galaxies, adopting a variant of cold dark matter cosmologies and employing a cosmological N-body/smoothed particle hydrodynamics code to follow the evolution of dark matter and gas. In addition to gravitational interactions, gas pressure, and shock heating, we include bremsstrahlung cooling in the computation. Dynamical time, two-body relaxation time, and cooling time in the simulations are examined to demonstrate that the results are free from artificial relaxation effects and that the time step is short enough to accurately follow the evolution of the system. In the simulation with nominal resolution of 66h^{-1} kpc the computed cluster appears normal, but in a higher (by a factor 2) resolution run, cooling is so efficient that the final gas density profile shows a steep rise toward the cluster center that is not observed in real clusters. Also, the X-ray luminosity of 7\times10^{45} ergs s^{-1} far exceeds that for any cluster of the computed temperature. The most reasonable explanation for this discrepancy is that there are some physical processes still missing in the simulations that actually mitigate the cooling effect and play a crucial role in the thermal and dynamical evolution of the gas near the center. Among the promising candidate processes are heat conduction and heat input from supernovae. We discuss the extent to which these processes can alter the evolution of gas.

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arXiv1998

Where are the Baryons?

New, high resolution, large-scale, cosmological hydrodynamic galaxy formation simulations of a standard cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and at moderate redshift. It is found that the average temperature of baryons is an increasing function of time, with most of the baryons at the present time having a temperature in the range 10^{5-7} K. Thus, not only is the universe dominated by dark matter, but more than one half of the normal matter is yet to be detected. Detection of this warm/hot gas poses an observational challenge, requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto clusters along the line of sight, absorption lines in X-ray and UV quasar spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density $\lya$ absorption lines. We estimate that approximately 1/4 of the extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the warm/hot gas, half of it coming from $z<0.65$ and three-quarters from $z<1.00$, so the source regions should be identifiable on deep optical images.

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