arXiv subjects

Jeremiah P. Ostriker

Publications and source records attributed to Jeremiah P. Ostriker.

At least 145 records · Page 8Linked to original sources

arXiv2002

The Cluster Mass Function from Early SDSS Data: Cosmological Implications

The mass function of clusters of galaxies is determined from 400 deg^2 of early commissioning imaging data of the Sloan Digital Sky Survey; ~300 clusters in the redshift range z = 0.1 - 0.2 are used. Clusters are selected using two independent selection methods: a Matched Filter and a red-sequence color magnitude technique. The two methods yield consistent results. The cluster mass function is compared with large-scale cosmological simulations. We find a best-fit cluster normalization relation of sigma_8*omega_m^0.6 = 0.33 +- 0.03 (for 0.1 ~< omega_m ~< 0.4), or equivalently sigma_8 = (0.16/omega_m)^0.6. The amplitude of this relation is significantly lower than the previous canonical value, implying that either omega_m is lower than previously expected (omega_m = 0.16 if sigma_8 = 1) or sigma_8 is lower than expected (sigma_8 = 0.7 if omega_m = 0.3). The best-fit mass function parameters are omega_m = 0.19 (+0.08,-0.07) and sigma_8 = 0.9 (+0.3,-0.2). High values of omega_m (>= 0.4) and low sigma_8 (=< 0.6) are excluded at >~ 2 sigma.

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arXiv2001

Observational Constraints on the Self Interacting Dark Matter Scenario and the Growth of Supermassive Black Holes

We consider the consequences of SIDM for a velocity dependent cross section per unit mass. Accretion of SIDM onto seed black holes can produce supermassive black holes that are too large for certain combinations of parameters,which is used to obtain a new constraint on the dark matter interaction. Constraints due to other considerations are presented and previous ones are generalized. The black hole constraint is extremely sensitive to the slope α, of the inner density profile of dark halos. For the most probable value of α=1.3, there exists a narrow range in parameter space, such that all constraints are satisfied. However, the adiabatic compression of the dark halo by baryons as they cool and contract in normal galaxies yields a steeper cusp, α=1.7. This gives a tighter constraint, which would exclude SIDM as a possible solution to the purported problems with CDM in the absence of other dynamical processes. Nevertheless, SIDM with parameters consistent with this stronger constraint, can explain the ubiquity of supermassive black holes in the centers of galaxies. A ``best fit'' model is presented which reproduces the supermassive black hole masses and their observed correlations with the velocity dispersion of the host bulges. Specifically, the fourth power dependence of black hole mass on velocity dispersion is a direct consequence of the power spectrum having an index of n=-2. Although the dark matter collision rates for this model are too small to directly remedy problems with CDM, mergers between dark halos harboring supermassive black holes at high redshift could ameliorate the cuspy halo problem. This scenario also explains the lack of comparable supermassive black holes in bulgeless galaxies like M33.

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arXiv2001

What does the local black hole mass distribution tell us about the evolution of the quasar luminosity function?

We present a robust method to derive the duty cycle of QSO activity based on the empirical QSO luminosity function and on the present-day linear relation between the masses of supermassive black holes and those of their spheroidal host stellar systems. It is found that the duty cycle is substantially less than unity, with characteristic values in the range $3-6\times 10^{-3}$. Finally, we tested the expectation that the QSO luminosity evolution and the star formation history should be roughly parallel, as a consequence of the above--mentioned relation between BH and galaxy masses.

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arXiv2001

Rayleigh Scattering and Microwave Background Fluctuations

During the recombination epoch, cosmic background photons couple not only to free electrons through Thompson scattering, but also to the neutral hydrogen through Rayleigh scattering. This latter is ~2% effect for photons near the peak of the photon energy distribution at z=800 and a ~0.2% effect at z=1100. Including Rayleigh scattering in the calculation reduces Silk damping at fixed redshift, alters the position of the surface of last scattering and alters the propagation of acoustic waves. We estimate the amplitude of these effects. For the Microwave Anisotropy Probe (MAP), Rayleigh scattering increases the anisotropy spectrum by 0.1% at the most. For the highest frequencies of the Planck Surveyor, the effects of Rayleigh scattering are much more dramatic (decreasing the anisotropy spectrum by 3% at ν~550GHz and l~1000). The relative difference between the spectra of low and high frequencies is imposed by an oscillation with a function of multipole l and the oscillation amplitude is up to 0.5% between 100 and 550GHz. Rayleigh scattering also slows the decoupling between radiation and matter, but the effect is undetectably small.

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arXiv2001

Evolution of the Luminosity Function and Colors of Galaxies in a Lambda-CDM Universe

The luminosity function of galaxies is derived from a cosmological hydrodynamic simulation of a Lambda cold dark matter (CDM) universe with the aid of a stellar population synthesis model. At z=0, the resulting B band luminosity function has a flat faint end slope of α\approx -1.15 with the characteristic luminosity and the normalization in a fair agreement with observations, while the dark matter halo mass function is steep with a slope of α\approx -2. The colour distribution of galaxies also agrees well with local observations. We also discuss the evolution of the luminosity function, and the colour distribution of galaxies from z=0 to 5. A large evolution of the characteristic mass in the stellar mass function due to number evolution is compensated by luminosity evolution; the characteristic luminosity increases only by 0.8 mag from z=0 to 2, and then declines towards higher redshift, while the B band luminosity density continues to increase from z=0 to 5 (but only slowly at z>3).

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arXiv2001

Revealing the Warm-Hot Intergalactic Medium with OVI Absorption

Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of the total baryons at z=0 may be in a warm-hot intergalactic medium (WHIM) with temperatures ~10^5-10^7K. The O VI λλ1032, 1038 absorption line doublet in the FUV portion of QSO spectra provides an important probe of this gas. Utilizing recent hydrodynamic simulations, it is found that there should be ~5 O VI absorption lines per unit redshift with equivalent widths >= 35 mA, decreasing rapidly to ~0.5 per unit redshift at >= 350 mA. About 10% of the total baryonic matter or 20-30% of the WHIM is expected to be in the O VI absorption line systems with equivalent width >= 20 mA; the remaining WHIM gas may be too hot or have too low metallicity to be detected in O VI. We find that the simulation results agree well with observations with regard to the line abundance and total mass contained in these systems. Some of the O VI systems are collisionally ionized and some are photoionized, but most of the mass is in the collisionally ionized systems. We show that the gas that produces the O VI absorption lines does not reside in virialized regions such as galaxies, groups, or clusters of galaxies, but rather has an overdensity of 10-40 times the average density. These regions form a somewhat connected network of filaments. The typical metallicity of these regions is 0.1-0.3Zsun.

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arXiv2001

Limits on Collisional Dark Matter from Elliptical Galaxies in Clusters

The dynamical evolution of galaxies in clusters is modified if dark matter is self-interacting. Heat conduction from the hot cluster halo leads to evaporation of the relatively cooler galactic halos. The stellar distribution would adiabatically expand as it readjusts to the loss of dark matter, reducing the velocity dispersion and increasing the half-light radius. If the dark matter content within that radius was f_dm = 25-50% of the total, as indicated by current observations, the ellipticals in clusters would be offset from the fundamental plane relation beyond the observational scatter. The requirement that their halos survive for a Hubble time appears to exclude just that range of the dark matter cross-section, 0.3 < sigma/m < 10^4 cm^2/g, thought to be optimal for reducing central halo cusps, unless f_dm < 15%. If the cross-section is allowed to vary with the relative velocity of dark matter particles, sigma \propto v^{-2 delta}, a new problem of evaporation of dark matter arises in the dwarf galaxies with low velocity dispersion. The halos of large galaxies in clusters and dwarf galaxies in the Local Group can both survive only if delta < 1.1 or delta > 1.8. In either case the problem of central density cusps remains.

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arXiv2001

Evolution of the Cluster Mass Function: Gpc^3 Dark Matter Simulations

High-resolution N-body simulations of four popular Cold Dark Matter cosmologies (LCDM, OCDM, QCDM, and tilted SCDM), each containing 10^5 clusters of galaxies in a cubic gigaparsec volume, are used to determine the evolution of the cluster mass function from z=3 to z=0. The large volume and high resolution of these simulations allow an accurate measure of the evolution of cosmologically important (but rare) massive clusters at high redshift. The simulated mass function is presented for cluster masses within several radii typically used observationally (R=0.5, 1.0, and 1.5 Mpc/h, both comoving and physical) in order to enable direct comparison with current and future observations. The simulated evolution is compared with current observations of massive clusters at redshifts 0.3 0.3 and no massive clusters at z>0.5, in stark contradiction with observations. The Omega_m=0.3 models- LCDM, OCDM, and QCDM- all exhibit considerably weaker evolution and are consistent with current data. Among these low density models, OCDM evolves the least. These trends are enhanced at high redshift and can be used to discriminate between flat and open low density models. The simulated mass functions are compared with the Press-Schechter approximation. Standard Press-Schechter predicts too many low mass clusters at z=0, and too few clusters at higher redshift. We modify the approximation by a simple parameterization of the density contrast threshold for collapse, which has a redshift dependence. This modified Press-Schechter approximation provides a good fit to the simulated mass functions.

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arXiv2001

Semi-Analytical Models for Lensing by Dark Halos: I. Splitting Angles

We use the semi-analytical approach to analyze gravitational lensing of quasars by dark halos in various cold dark matter (CDM) cosmologies, in order to determine the sensitivity of the prediction probabilities of images separations to the input assumptions regarding halos and cosmologies. The mass function of dark halos is assumed to be given by the Press-Schechter function. The mass density profile of dark halos is alternatively taken to be the singular isothermal sphere (SIS), the Navarro-Frenk-White (NFW) profile, or the generalized NFW profile. The cosmologies include: the Einstein-de Sitter model (SCDM), the open model (OCDM), and the flat Λ-model (LCDM). As expected, we find that the lensing probability is extremely sensitive to the mass density profile of dark halos, and somewhat less so to the mean mass density in the universe, and the amplitude of primordial fluctuations. NFW halos are very much less effective in producing multiple images than SIS halos. However, none of these models can completely explain the current observations: the SIS models predict too many large splitting lenses, while the NFW models predict too few small splitting lenses. This indicates that there must be at least two populations of halos in the universe. A combination of SIS and NFW halos can reasonably reproduce the current observations if we choose the mass for the transition from SIS to NFW to be ~ 10^{13} solar masses. Additionally, there is a tendency for CDM models to have too much power on small scales, i.e. too much mass concentration; and it appears that the cures proposed for other apparent difficulties of CDM would help here as well, an example being the warm dark matter (WDM) variant which is shown to produce large splitting lenses fewer than the corresponding CDM model by one order of magnitude.

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arXiv2001

Halo Formation in Warm Dark Matter Models

Discrepancies have emerged between the predictions of standard cold dark matter (CDM) theory and observations of clustering on sub-galactic scales. Warm dark matter (WDM) is a simple modification of CDM in which the dark matter particles have initial velocities due either to their having decoupled as thermal relics, or having been formed via non-equilibrium decay. We investigate the nonlinear gravitational clustering of WDM with a high resolution N-body code, and identify a number of distinctive observational signatures. Relative to CDM, halo concentrations and core densities are lowered, core radii are increased, and large halos emerge with far fewer low mass satellites. The number of small halos is suppressed, and those present are formed by `top down' fragmentation of caustics, as part of a `cosmic web' connecting massive halos. Few small halos form outside this web. If we identify small halos with dwarf galaxies, their number, spatial distribution, and formation epoch appear in better agreement with the observations for WDM than they are for CDM.

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arXiv2001

Constraints on Warm Dark Matter from Cosmological Reionization

We study the constraints that high-redshift structure formation in the universe places on warm dark matter (WDM) dominated cosmological models. We modify the extended Press-Schechter formalism to derive the halo mass function in WDM models. We show that our predictions agree with recent numerical simulations at low redshift over the halo masses of interest. Applying our model to galaxy formation at high redshift, we find that the loss of power on small scales, together with the delayed collapse of low-mass objects, results in strong limits on the root-mean-square velocity dispersion v_rms of the WDM particles at z=0. For fermions decoupling while relativistic, these limits are equivalent to constraints on the mass m_X of the particles. The presence of a 4 billion solar mass black hole at z=5.8, believed to power the quasar SDSS 1044-1215, implies m_X > 0.5 keV (or v_rms < 0.10 km/s), assuming that the quasar is unlensed and radiating at or below the Eddington limit. Reionization by z=5.8 also implies a limit on m_X. If high-redshift galaxies produce ionizing photons with an efficiency similar to their z=3 counterparts, we find m_X > 1.2 keV (or v_rms < 0.03 km/s). However, given the uncertainties in current measurements from the proximity effect of the ionizing background at redshift 3, values of m_X as low as 0.75 keV (v_rms = 0.06 km/s) are not ruled out. The limit weakens further if, instead, the ionizing-photon production efficiency is greater at high z, but this limit will tighten considerably if reionization is shown in the future to have occurred at higher redshifts. WDM models with m_X < 1 keV (v_rms > 0.04 km/s) produce a low-luminosity cutoff in the high-redshift galaxy luminosity function which is directly detectable with the Next Generation Space Telescope (abridged).

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arXiv2001

Star Formation History and Stellar Metallicity Distribution in a Cold Dark Matter Universe

We study star formation history and stellar metallicity distribution in galaxies in a Lambda cold dark matter universe using a hydrodynamic cosmological simulation. Our model predicts star formation rate declining in time exponentially from an early epoch to the present with the time-scale of 6 Gyr, which is consistent with the empirical Madau plot with modest dust obscuration. Star formation in L^* galaxies continues intermittently to the present also with an exponentially declining rate of a similar time-scale, whereas in small galaxies star formation ceases at an early epoch. The mean age of the extant stars decreases only slowly with increasing redshift, and exceeds 1 Gyr at z=3. Normal galaxies contain stars with a wide range of metallicity and age: stars formed at z<1 have metallicity of 0.1-1.0 Zsun, while old stars take a wide range of values from 10^{-6} Zsun to 3.0 Zsun. The mean metallicity of normal galaxies is in the range 0.1-1.0 Zsun. Dwarf galaxies that contain only old stars have a wide range of mean metallicity (10^{-4}-1.0 Zsun), but on average they are metal deficient compared with normal galaxies.

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arXiv2001

The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is There a Problem?

It is well known that the mass function for_halos_ in CDM cosmology is a relatively steep power law for low masses, possibly too steep to be consistent with observations. But how steep is the_galaxy_ mass function? We have analyzed the stellar and gas mass functions of the first massive luminous objects formed in a ΛCDM universe, as calculated in the numerical simulation described in Gnedin (2000ab). We found that while the dark matter mass function is steep, the stellar and gas mass functions are flatter for low mass objects. The stellar mass function is consistently flat at the low mass end. Moreover, while the gas mass function follows the dark matter mass function until reionization at z~7, between z=7 and z=4, the gas mass function also flattens considerably at the low mass end. At z=4, the gas and stellar mass functions are fit by a Schechter function with α~ -1.2 +/- 0.1, significantly shallower than the dark matter halo mass function and consistent with some recent observations. The baryonic mass functions are shallower because (a) the dark matter halo mass function is consistent with the Press-Schechter formulation at low masses n(M) M^-2 and (b) heating/cooling and ionization processes appear to cause baryons to collect in halos with the relationship M_b M_d^4 at low masses. Combining (a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus, the well known observational fact that low mass galaxies are underabundant as compared to expectations from numerical dark matter simulations or Press-Schechter modeling of CDM universes emerges naturally from these results, implying that perhaps no ``new physics'' beyond the standard model is needed.

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arXiv2001

Warm Dark Matter, Small Scale Crisis, and the High Redshift Universe

Warm Dark Matter (WDM) models have recently been resurrected to resolve apparent conflicts of Cold Dark Matter (DM) models with observations. Endowing the DM particles with non-negligible velocities causes free-streaming, which suppresses the primordial power spectrum on small scales. The choice of a root-mean-square velocity dispersion v(rms) = 0.05 km/s at redshift z=0 (corresponding to a particle mass of 1 keV if the WDM particles are fermions decoupling while relativistic) helps alleviate most, but probably not all, of the small-scale problems faced by CDM. An important side-effect of the particle velocities is the severe decrease in the number of collapsed halos at high redshift. This is caused both by the loss of small-scale power, and by the delay in the collapse of the smallest individual halos (with masses near the effective Jeans mass of the DM). The presence of early halos is required in order (1) to host either early quasars or galaxies that can reionize the universe by redshift z=5.8, and (2) to allow the growth of the supermassive black hole believed to power the recently discovered quasar SDSS 1044-1215 at this redshift. We quantify these constraints using a modified Press-Schechter formalism, and find v(rms) < 0.04 km/s (or m_X > 1 keV). If future observations uncover massive black holes at z > 10, or reveal that reionization occurred at z > 10, this could conclusively rule out WDM models as the solution to the small-scale crisis of the CDM paradigm.

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arXiv2001

The Tree-Particle-Mesh N-body Gravity Solver

The Tree-Particle-Mesh (TPM) N-body algorithm couples the tree algorithm for directly computing forces on particles in an hierarchical grouping scheme with the extremely efficient mesh based PM structured approach. The combined TPM algorithm takes advantage of the fact that gravitational forces are linear functions of the density field. Thus one can use domain decomposition to break down the density field into many separate high density regions containing a significant fraction of the mass but residing in a very small fraction of the total volume. In each of these high density regions the gravitational potential is computed via the tree algorithm supplemented by tidal forces from the external density distribution. For the bulk of the volume, forces are computed via the PM algorithm; timesteps in this PM component are large compared to individually determined timesteps in the tree regions. Since each tree region can be treated independently, the algorithm lends itself to very efficient parallelization using message passing. We have tested the new TPM algorithm (a refinement of that originated by Xu 1995) by comparison with results from Ferrell & Bertschinger's P^3M code and find that, except in small clusters, the TPM results are at least as accurate as those obtained with the well-established P^3M algorithm, while taking significantly less computing time. Production runs of 10^9 particles indicate that the new code has great scientific potential when used with distributed computing resources.

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arXiv2001

Cosmic Mach Number as a Function of Overdensity and Galaxy Age

We carry out an extensive study of the cosmic Mach number (\mach) on scales of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We particularly put emphasis on the environmental dependence of \mach on overdensity, galaxy mass, and galaxy age. We start by discussing the difference in the resulting \mach according to different definitions of \mach and different methods of calculation. The simulated Mach numbers are slightly lower than the linear theory predictions even when a non-linear power spectrum was used in the calculation, reflecting the non-linear evolution in the simulation. We find that the observed \mach is higher than the simulated mean by more than 2-standard deviations, which suggests either that the Local Group is in a relatively low-density region or that the true value of Ω_m is ~ 0.2, significantly lower than the simulated value of 0.37. We show from our simulation that the Mach number is a weakly decreasing function of overdensity. We also investigate the correlations between galaxy age, overdensity and \mach for two different samples of galaxies --- DWARFs and GIANTs. Older systems cluster in higher density regions with lower \mach, while younger ones tend to reside in lower density regions with larger \mach, as expected from the hierarchical structure formation scenario. However, for DWARFs, the correlation is weakened by the fact that some of the oldest DWARFs are left over in low-density regions during the structure formation history. For giant systems, one expects blue-selected samples to have higher \mach than red-selected ones. We briefly comment on the effect of the warm dark matter on the expected Mach number.

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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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arXiv2000

Properties of Cosmic Shock Waves in Large Scale Structure Formation

We have examined the properties of shock waves in simulations of large scale structure formation for two cosmological scenarios (a SCDM and a LCDM with Omega =1). Large-scale shocks result from accretion onto sheets, filaments and Galaxy Clusters (GCs) on a scale of circa 5 Mpc/h in both cases. Energetic motions, both residual of past accretion history and due to current asymmetric inflow along filaments, generate additional, common shocks on a scale of about 1 Mpc/h, which penetrate deep inside GCs. Also collisions between substructures inside GCs form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving shock surface density decreases, reflecting the ongoing structure merger process in both scenarios. Accretion shocks have very high Mach numbers (10-10^3), when photo-heating of the pre-shock gas is not included. The typical shock speed is of order v_{sh}(z) =H(z)lambda_{NL}(z), with lambda_{NL}(z) the wavelength scale of the nonlinear perturbation at the given epoch. However, the Mach number for shocks occuring within clusters is usually smaller (3-10), due to the fact that the intracluster gas is already hot. Statistical fits of shock speed around GCs as a function of GCs temperature give power-law's in accord with 1-D predictions. However, a very different result is obtained for fits of the shock radius, reflecting the very complex shock structures forming in 3-D simulations. The in-flowing kinetic energy across such shocks, giving the power available for cosmic-ray acceleration, is comparable to the cluster X-ray luminosity emitted from a central region of radius 0.5 Mpc/h. Considering their large size and long lifetimes, those shocks are potentially interesting sites for cosmic-ray acceleration, if modest magnetic fields exist within them.

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