Showing papers by "Sloan Fellows published in 2000"

Reionization of the Inhomogeneous Universe

Abstract: A model of the density distribution in the intergalactic medium (IGM), motivated by that found in numerical simulations, is used to demonstrate the effect of a clumpy IGM and discrete sources on the reionization of the universe. In an inhomogeneous universe reionization occurs outside-in, starting in voids and gradually penetrating into overdense regions. Reionization should not be sudden but gradual, with a continuous rise of the photon mean free path over a fair fraction of the Hubble time as the emissivity increases. We show that a hydrogen Gunn-Peterson trough should be present at z 6 unless the emissivity increases with redshift at z > 4. However, the epoch of overlap of cosmological H II regions could have occurred at a higher redshift if sources of low luminosity reionized the IGM; the Gunn-Peterson trough at z ~ 6 would then appear because even the most underdense voids have a large enough neutral fraction in ionization equilibrium to be optically thick to Lyα photons. Cosmological H II regions near the epoch of overlap can produce gaps of transmitted flux only if luminous quasars contributed to the reionization, producing large H II regions. Despite the clumpiness of the matter distribution, recombinations do not increase the required emissivity of ionizing photons by a large factor during the reionization of hydrogen because the high-density gas is not ionized until a late time. We show that the He II reionization was most likely delayed relative to the hydrogen reionization but was probably complete by z ~ 3 (the redshift where observations are available). The reported large optical depth fluctuations of He II are not necessarily due to an incomplete He II reionization but can arise from a combination of IGM density fluctuations and variations in the intensity of the He II ionizing background due to luminous QSOs.

Dark Halo and Disk Galaxy Scaling Laws in Hierarchical Universes

Abstract: We use cosmological N-body/gasdynamical simulations that include star formation and feedback to examine the proposal that scaling laws between the total luminosity, rotation speed, and angular momentum of disk galaxies reflect analogous correlations between the structural parameters of their surrounding dark matter halos. The numerical experiments follow the formation of galaxy-sized halos in two cold dark matter (CDM)-dominated universes: the standard Ω = 1 CDM scenario and the currently popular ΛCDM model. We find that the slope and scatter of the I-band Tully-Fisher relation are well reproduced in the simulations, although not, as proposed in recent work, as a result of the cosmological equivalence between halo mass and circular velocity: large systematic variations in the fraction of baryons that collapse to form galaxies and in the ratio between halo and disk circular velocities are observed in our numerical experiments. The Tully-Fisher slope and scatter are recovered in this model as a direct result of the dynamical response of the halo to the assembly of the luminous component of the galaxy. We conclude that models that neglect the self-gravity of the disk and its influence on the detailed structure of the halo cannot be used to derive meaningful estimates of the scatter or slope of the Tully-Fisher relation. Our models fail, however, to match the zero point of the Tully-Fisher relation, as well as that of the relation linking disk rotation speed and angular momentum. These failures can be traced, respectively, to the excessive central concentration of dark halos formed in the CDM cosmogonies we explore and to the formation of galaxy disks as the final outcome of a sequence of merger events. Disappointingly, our feedback formulation, calibrated to reproduce the empirical correlations linking star formation rate and gas surface density established by Kennicutt, has little influence on these conclusions. Agreement between model and observations appears to demand substantial revision to the CDM scenario or to the manner in which baryons are thought to assemble and evolve into galaxies in hierarchical universes.

The Observed Probability Distribution Function, Power Spectrum, and Correlation Function of the Transmitted Flux in the Lyα Forest*

Abstract: A sample of eight quasars observed at high resolution and signal-to-noise ratio is used to determine the transmitted flux probability distribution function (TFPDF), and the power spectrum and correlation function of the transmitted flux in the Lyα 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 Lyα forest of a ΛCDM model with Ω = 0.4, known to agree with other large-scale structure observational constraints. There is excellent agreement for the TFPDF if the mean transmitted flux is adjusted to match the observations. A small difference between the observed and predicted TFPDF 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(k) = 0.72 ± 0.09 at k = 0.04(km s-1)-1 and z = 3, and the slope of the power spectrum near the same k to be np = -2.55 ± 0.10 (statistical error bars). The results are statistically consistent with those of Croft et al., although our value for the rms fluctuation is lower by a factor of 0.75. For the ΛCDM model we use, the implied primordial slope is n = 0.93 ± 0.10, and the normalization is σ8 = 0.68 + 1.16(0.95 - n) ± 0.04.

Foregrounds and Forecasts for the Cosmic Microwave Background

Abstract: One of the main challenges facing upcoming cosmic microwave background (CMB) experiments will be to distinguish the cosmological signal from foreground contamination. We present a comprehensive treatment of this problem and study how foregrounds degrade the accuracy with which the Boomerang, MAP, and Planck experiments can measure cosmological parameters. Our foreground model includes not only the normalization, frequency dependence, and scale dependence for each physical component, but also variations in frequency dependence across the sky. When estimating how accurately cosmo- logical parameters can be measured, we include the important complication that foreground model parameters (we use about 500) must be simultaneously measured from the data as well. Our results are quite encouraging: despite all these complications, precision measurements of most cosmological param- eters are degraded by less than a factor of 2 for our main foreground model and by less than a factor of 5 in our most pessimistic scenario. Parameters measured though large-angle polarization signals suUer more degradation: up to 5 in the main model and 25 in the pessimistic case. The foregrounds that are potentially most damaging and therefore most in need of further study are vibrating dust emission and point sources, especially those in the radio frequencies. It is well known that E and B polarization contain valuable information about reionization and gravity waves, respectively. However, the cross- correlation between polarized and unpolarized foregrounds also deserves further study, as we —nd that it carries the bulk of the polarization information about most other cosmological parameters. Subject headings: cosmic microwave backgrounddiUuse radiationmethods: numerical ¨ polarization

A cluster of black holes at the galactic center

Abstract: If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M 30 M☉, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii) or after losing orbital energy through gravitational waves. The overall depletion timescale is ~30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster as a result of relaxation, implying a core in the profile of solar-mass red giants with a radius of ~2 pc (i.e., 1'). The observed central density cusp (which has a core radius of only a few arcseconds) should be composed primarily of young (1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (2 Gyr) stars found at distances ~3-10 pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution K-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary ("planet-like") perturbations on the main event.

Constraints on the Mass of Warm Dark Matter Particles and the Shape of the Linear Power Spectrum from the Lyα Forest

Abstract: High-resolution N-body simulations of cold dark matter (CDM) models predict that galaxies and clusters have cuspy halos with considerable substructure. Observations reveal smooth halos with central density cores. One possible resolution of the substructure problem is that the dark matter is warm; this will suppress the power spectrum on small scales. The Lyα forest is a powerful probe of the linear power spectrum on these scales. We use collisionless N-body simulations to follow the evolution of structure in warm dark matter (WDM) models and analyze artificial Lyα forest spectra extracted from them. By requiring that there be enough small-scale power in the linear power spectrum to reproduce the observed properties of the Lyα forest in quasar spectra, we derive a lower limit to the mass of the WDM particle of 750 eV. This limit is robust to reasonable uncertainties in our assumption about the temperature of the gas at mean density (T0) at z = 3. We argue that any model that suppresses the CDM linear theory power spectrum more severely than a 750 eV WDM particle cannot produce the Lyα forest.

The Core Density of Dark Matter Halos: A Critical Challenge to the ΛCDM Paradigm?

Abstract: We compare the central mass concentration of cold dark matter (CDM) halos found in cosmological N-body simulations with constraints derived from the Milky Way disk dynamics and from the Tully-Fisher relation. For currently favored values of the cosmological parameters (Ω0 ~ 0.3; Λ0 = 1 - Ω0 ~ 0.7; h ~ 0.7; COBE- and cluster abundance-normalized σ8; big bang nucleosynthesis Ωb), we find that halos with circular velocities comparable to the rotation speed of the Galaxy have typically 3 times more dark matter inside the solar circle than inferred from observations of Galactic dynamics. Such high central concentrations of dark matter on the scale of galaxy disks also imply that stellar mass-to-light ratios much lower than expected from population synthesis models must be assumed in order to reproduce the zero point of the Tully-Fisher relation. Indeed, even under the extreme assumption that all baryons in a dark halo are turned into stars, disks with conventional I-band stellar mass-to-light ratios [M/LI ~ 2 ± 1(M/LI)☉] are about 2 mag fainter than observed at a given rotation speed. We examine several modifications to the ΛCDM model that may account for these discrepancies and conclude that agreement can only be accomplished at the expense of renouncing other major successes of the model. Reproducing the observed properties of disk galaxies thus appears to demand substantial revision to the currently most successful model of structure formation.

Damped Lyα Absorber and the Faint End of the Galaxy Luminosity Function at High Redshift

Abstract: We combine predictions for several hierarchical cosmogonies with observational evidence on damped Lya systems (DLASs) to establish a correspondence between the high-redshift galaxy population and the properties of DLASs. We assume that high-redshift galaxies and damped Lya systems are hosted by the same dark matter halos and require consistency between the predicted halo space density, the rate of incidence and the velocity width distribution of damped Lya systems, and the observed galaxy lumi- nosity function at the bright end. We arrive at the following results: (1) predicted impact parameters between the damped absorption system and the luminous parts of the absorbing galaxy are expected to be very small for most galaxies; (2) luminosities of galaxies causing damped absorption are gen- (0A.3¨1A) erally fainter than and damped Lya systems are predicted to sample preferentially the outer m R \ 25, regions of galaxies at the faint end of the galaxy luminosity function at high redshift. Therefore, DLASs should currently provide the best probe of the progenitors of normal present-day galaxies. Subject headings: galaxies: kinematics and dynamicsgalaxies: structureintergalactic medium ¨ quasars: absorption lines

Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach.

Abstract: Weak gravitational lensing observations probe the spectrum and evolution of density fluctuations and the cosmological parameters that govern them, but they are currently limited to small fields and subject to selection biases. We show how the expected signal from large-scale structure arises from the contributions from and correlations between individual halos. We determine the convergence power spectrum as a function of the maximum halo mass and so provide the means to interpret results from surveys that lack high-mass halos either through selection criteria or small fields. Since shot noise from rare massive halos is mainly responsible for the sample variance below 10a, our method should aid our ability to extract cosmological information from small fields.

Lopsided Galaxies, Weak Interactions, and Boosting the Star Formation Rate

Abstract: To investigate the link between weak tidal interactions in disk galaxies and the boosting of their recent star formation, we obtain images and spatially integrated spectra (3615 for 40 Ae " j " 5315 Ae ) late-type spiral galaxies (SabSbc) with varying degrees of lopsidedness (a dynamical indicator of weak interactions). We quantify lopsidedness as the amplitude of the m \ 1 Fourier component of the SA3 1 T azimuthal surface brightness distribution averaged over a range of radii. The median spectrum of the most lopsided galaxies shows strong evidence for a more prominent young stellar population (i.e., strong Balmer absorption, strong nebular emission, a weak 4000 break, and a blue continuum) when com- " pared to the median spectrum of the most symmetric galaxies. We compare the young stellar content, quanti—ed by and the strength of the 4000 break with lopsidedness and —nd a 3¨4 EW(Hd abs ) " (D 4000 ), p correlation between the two. We also —nd a 3.2 p correlation between and lopsidedness. EW(Hb emission ) Using the evolutionary population synthesis code of Bruzual & Charlot we model the spectra as an ii underlying population ˇˇ and a superimposed ii boost population ˇˇ with the aim of constraining the frac- tional boost in the SFR averaged over the past 0.5 Gyr (the characteristic lifetime of lopsidedness). From the diUerence in both and the strength of the 4000 break between the most and EW(Hd abs )

A Cluster of Black Holes at the Galactic Center

Abstract: If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M \ga 30 M_{\odot}, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii), or after losing orbital energy through gravitational waves. The overall depletion timescale is ~ 30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster due to relaxation, implying a core in the profile of solar-mass red giants with a radius of ~ 2 pc (i.e., 1'). The observed central density cusp (which has a core radius of only a few arc seconds) should be composed primarily of young (\la 1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (\la 2 Gyr) stars found at distances ~ 3 - 10 pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution $K$-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary (``planet-like'') perturbations on the main event.

Soft X-Ray Absorption by High-Redshift Intergalactic Helium

Abstract: The Lyα absorption from intergalactic, once-ionized helium (He II) has been measured with the Hubble Space Telescope in four quasars over the last few years in the redshift range 24 < z < 32 These observations have indicated that the He II reionization may not have been completed until z 28 and that large fluctuations in the intensity of the He II-ionizing background were present before this epoch The detailed history of He II reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, since the intergalactic medium (IGM) can be completely optically thick to Lyα photons when only a small fraction of the helium remains as He II In addition, finding quasars in which the He II Lyα absorption can be observed becomes increasingly difficult at higher redshift owing to the large abundance of hydrogen Lyman limit systems It is pointed out here that He II in the IGM should also cause detectable continuum absorption in the soft X-rays The spectrum of a high-redshift source seen behind the IGM when most of the helium was He II should recover from the He II Lyman continuum absorption at an observed energy of ~01 keV Galactic absorption will generally be stronger, but not by a large factor; the intergalactic He II absorption can be detected as an excess over the expected Galactic absorption from the 21 cm H I column density In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift if the measurement is done on a large sample of high-redshift sources over a range of redshifts

Lopsided Galaxies, Weak Interactions and Boosting the Star Formation Rate

Abstract: To investigate the link between weak tidal interactions in disk galaxies and the boosting of their recent star formation, we obtain images and spatially integrated spectra (3615A of the m=1 Fourier component of the azimuthal surface brightness distribution, averaged over a range of radii. We compare the young stellar content, quantified by EW(H\delta_abs) and the strength of the 4000 Angstrom break (D_4000), with lopsidedness and find a 3-4 sigma correlation between the two. We also find a 3.2 sigma correlation between EW(H\beta_emission) and lopsidedness. Using the evolutionary population synthesis code of Bruzual & Charlot we model the spectra as an ``underlying population'' and a superimposed ``boost population'' with the aim of constraining the fractional boost in the SFR averaged over the past 0.5 Gyr (the characteristic lifetime of lopsidedness). From the difference in both EW(H\delta_abs) and D_4000 between the most and least symmetric thirds of our sample, we infer that ~ 1x10^9 M_solar of stars are formed over the duration of a lopsided event in addition to the ``underlying'' SFH (assuming a final galactic stellar mass of 10^10 M_solar). This corresponds to a factor of 8 increase in the SFR over the past 5x10^8 years. For the nuclear spectra, all of the above correlations except D_4000 vs. are weaker than for the disk, indicating that in lopsided galaxies, the SF boost is not dominated by the nucleus.