Zoltan Haiman

A Survey of z > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z ∼ 6* **

We present the results from a survey of i-dropout objects selected from ~1550 deg2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at z 5.8. Objects with i*-z* > 2.2 and z* 0.90. The ARC 3.5 m spectrum of SDSSp J103027.10+052455.0 shows that over a range of ~300 A immediately blueward of the Lyα emission, the average transmitted flux is only 0.003 ± 0.020 times that of the continuum level, consistent with zero flux over a ~300 A range of the Lyα forest region and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines in the quasar spectra suggests early metal enrichment in the quasar environment. The three new objects, together with the previously published z = 5.8 quasar SDSSp J104433.04-012502.2, form a complete color-selected flux-limited sample at z 5.8. We estimate the selection function of this sample, taking into account the estimated variations in the quasar spectral energy distribution, as well as observational photometric errors. We find that at z = 6, the comoving density of luminous quasars at M1450 < -26.8 (H0 = 50 km s-1 Mpc-1, Ω = 1) is 1.1 × 10-9 Mpc-3. This is a factor of ~2 lower than that at z ~ 5 and is consistent with an extrapolation of the observed quasar evolution at z < 5. Using the current sample, we discuss the constraint on the shape of the quasar luminosity function and the implications for the contribution of quasars to the ionizing background at z ~ 6. The luminous quasars discussed in the paper have central black hole masses of several times 109 M⊙ by the Eddington argument, with likely dark halo masses on the order of 1013 M⊙. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift.

A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey. II. Discovery of Three Additional Quasars at z > 6*

We present the discovery of three new quasars at z > 6 in ~ 1300 deg2 of Sloan Digital Sky Survey imaging data, J114816.64+525150.3 (z = 6.43), J104845.05+463718.3 (z = 6.23), and J163033.90+401209.6 (z = 6.05). The first two objects have weak Lyα emission lines; their redshifts are determined from the positions of the Lyman break. They are only accurate to ~0.05 and could be affected by the presence of broad absorption line systems. The last object has a Lyα strength more typical of lower redshift quasars. Based on a sample of six quasars at z > 5.7 that cover 2870 deg2 presented in this paper and in Paper I, we estimate the comoving density of luminous quasars at z ~ 6 and M1450 5.7 quasars and high-resolution, ground-based images (seeing ~04) of three additional z > 5.7 quasars show that none of them is gravitationally lensed. The luminosity distribution of the high-redshift quasar sample suggests the bright-end slope of the quasar luminosity function at z ~ 6 is shallower than Ψ ∝ L-3.5 (2 σ), consistent with the absence of strongly lensed objects.

Constraints on Cosmological Parameters from Future Galaxy Cluster Surveys

We study the expected redshift evolution of galaxy cluster abundance between 0 z 3 in different cosmologies, including the effects of the cosmic equation of state parameter w ? p/?. Using the halo mass function obtained in recent large-scale numerical simulations, we model the expected cluster yields in a 12 deg2 Sunyaev-Zeldovich effect (SZE) survey and a deep 104 deg2 X-ray survey over a wide range of cosmological parameters. We quantify the statistical differences among cosmologies using both the total number and redshift distribution of clusters. Provided that the local cluster abundance is known to a few percent accuracy, we find only mild degeneracies between w and either ?m or h. As a result, both surveys will provide improved constraints on ?m and w. The ?m-w degeneracy from both surveys is complementary to those found either in studies of cosmic microwave background (CMB) anisotropies or of high-redshift supernovae (SNe). As a result, combining these surveys together with either CMB or SNe studies can reduce the statistical uncertainty on both w and ?m to levels below what could be obtained by combining only the latter two data sets. Our results indicate a formal statistical uncertainty of ?3% (68% confidence) on both ?m and w when the SZE survey is combined with either the CMB or SN data; the large number of clusters in the X-ray survey further suppresses the degeneracy between w and both ?m and h. Systematics and internal evolution of cluster structure at the present pose uncertainties above these levels. We briefly discuss and quantify the relevant systematic errors. By focusing on clusters with measured temperatures in the X-ray survey, we reduce our sensitivity to systematics such as nonstandard evolution of internal cluster structure.

The Radiative Feedback of the First Cosmological Objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small K) halos that collapse before reionization. The UVB at energies below 13.6 eV sup- (T vir ( 104 presses the formation of stars or black holes inside small halos by photodissociating their only cooling agent, molecular We self-consistently compute the buildup of the early UVB in Press-Schechter H 2 . models, coupled with photodissociation both in the intergalactic medium (IGM) and inside virialized H 2 halos. We —nd that the intergalactic has a negligible eUect on the UVB, both because its initial H 2 optical depth is small and because it is photodissociated at an early stage. If the UV sources in ((0.1) the —rst collapsed halos are stars, then their UV —ux suppresses further star formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (T vir Z 104 K) collapse. If the small halos host miniquasars with hard spectra extending to D1 keV, then their X-rays balance the eUects of the UVB, the negative feedback does not occur, and reionization could be caused by the small halos. Subject headings: cosmology: theorydiUuse radiationearly universegalaxies: formation ¨ molecular processesradiative transfer

Destruction of Molecular Hydrogen during Cosmological Reionization

We investigate the ability of primordial gas clouds to retain molecular hydrogen (H2) during the initial phase of the reionization epoch. We find that before the Str?mgren spheres of the individual ionizing sources overlap, the UV background below the ionization threshold is able to penetrate large clouds and suppress their H2 abundance. The consequent lack of H2 cooling could prevent the collapse and fragmentation of clouds with virial temperatures Tvir 104 K (or masses 108 M?[(1 + zvir)/10]-3/2). This negative feedback on structure formation arises from the very first ionizing sources and precedes the feedback due to the photoionization heating.

Cosmological formation of low-mass objects

We investigate the early formation of bound objects with masses comparable to the cosmological Jeans mass (10^5 solar masses). We follow the growth of isolated spherically symmetric density peaks starting from the linear perturbative regime. The initial parameters correspond to density peaks of various widths and heights in a Cold Dark Matter cosmology. We use a one-dimensional spherical Lagrangian hydrodynamics code to follow the dynamical, thermal, and non-equilibrium chemical evolution of the gas. The system includes a collisionless dark matter component and a baryonic component composed of the nine species H, H^-, H^+, He, He^+, He^{++}, H_2, H_2^+, and e^-. All relevant chemical reactions between these species and their cooling mechanisms are included in the calculations. We find that radiative cooling by H_2 affects the collapse dynamics of the gas only after it has already virialized and become part of the bound object. Therefore, radiative cooling is unlikely to have triggered the initial collapse of perturbations at redshifts z>10. Nevertheless, objects with baryonic masses well below the linear-theory Jeans mass (<10^3 solar masses) collapse due to shell crossing by the dark matter. Such objects could be the progenitors of a primordial population of high-mass stars in the intergalactic medium.

Second-Generation Objects in the Universe: Radiative Cooling and Collapse of Halos with Virial Temperatures above 104 K

The first generation of stars is thought to have formed in low-mass halos with Tvir 104 K, i.e., those that can cool in the absence of H2 via neutral atomic lines. The evolution of these halos differs from their less massive counterparts. Efficient atomic line radiation allows rapid cooling to ~8000 K; subsequently the gas can contract nearly isothermally at this temperature. In the absence of H2 molecules, the gas would likely settle into a locally stable disk, and only disks with unusually low spin would be unstable. However, we find that the initial atomic line cooling leaves a large, out-of-equilibrium residual free electron fraction. This allows the molecular fraction to build up to a universal value of x ≈ 10-3, almost independently of initial density and temperature. We show that this is a nonequilibrium freeze-out value that can be understood in terms of timescale arguments. Unlike in less massive halos, H2 formation and cooling is largely impervious to feedback from external UV fields, due to the high initial densities achieved by atomic cooling. The newly formed molecules cool the gas further to ~100 K and allow the gas to fragment on scales of a few times 100 M☉. We investigate the importance of various feedback effects such as H2 photodissociation from internal UV fields and radiation pressure due to Lyα photon trapping, which are likely to regulate the efficiency of star formation.

Observational Signatures of the First Quasars

We study the observational signatures of a potential population of low-luminosity quasars at high redshifts in a ΛCDM cosmology. We derive the evolution of the quasar luminosity function at fainter luminosities and higher redshifts than currently detected based on three assumptions: (1) the formation of dark matter halos follows the Press-Schechter theory, (2) the ratio of central black hole mass to halo mass is the same for all halos, and (3) the light curve of quasars, in Eddington units, is universal. We show that a universal light curve provides an excellent fit to the observed quasar luminosity function at redshifts 2.6 10 with its proposed 1 nJy sensitivity at 1-3.5 μm. Absorption spectra of several such quasars would reveal the reionization history of the universe.

Photoionization Feedback in Low-Mass Galaxies at High Redshift

The cosmic ultraviolet ionizing background impacts the formation of dwarf galaxies in the low-redshift universe (z 3) by suppressing gas infall into galactic halos with circular velocities up to vcirc ~ 75 km s-1. Using a one-dimensional, spherically symmetric hydrodynamics code (Thoul & Weinberg), we examine the effect of an ionizing background on low-mass galaxies forming at high redshifts (z 10). We find that the importance of photoionization feedback is greatly reduced because (1) at high redshift, dwarf galaxy-sized objects can self-shield against the ionizing background, (2) collisional cooling processes at high redshift are more efficient, (3) the amplitude of the ionizing background at high redshift is lower, and (4) the ionizing radiation turns on when the perturbation that will become the dwarf galaxy has already grown to a substantial overdensity. We find that because of these reasons, gas can collect inside halos with circular velocities as low as vcirc ~ 10 km s-1 at z > 10. This result has important implications for the reionization history of the universe.

Signatures of Stellar Reionization of the Universe

The high ionization level and nonzero metallicity (~1% Z☉) of the intergalactic gas at redshifts z 5 implies that nonlinear structure had started to form in the universe at earlier times than we currently probe. In cold dark matter (CDM) cosmologies, the first generation of baryonic objects emerges at redshifts z ~ 10-50. Here we examine the observable consequences of the possibility that an early generation of stars reionized the universe and resulted in the observed metallicity of the Lyα forest. Forthcoming microwave anisotropy experiments will be sensitive to the damping of anisotropies caused by scattering off free electrons from the reionization epoch. For a large range of CDM models with a Scalo stellar mass function, we find that reionization occurs at a redshift z 10 and damps the amplitude of anisotropies on angular scales 10° by a detectable amount, ~10%-25%. However, reionization is substantially delayed if the initial stellar mass function transformed most of the baryons into low-mass stars. In this case, the mass fraction of pregalactic stars could be constrained from the statistics of microlensing events in galactic halos or along lines of sight to quasars. Deep infrared imaging with future space telescopes (such as the Space Infrared Telescope Facility or the Next Generation Space Telescope) will be able to detect bright star clusters at z 5. The cumulative bremsstrahlung emission from these star clusters yields a measurable distortion to the spectrum of the microwave background.