Search

Investigations of He II Lya (304 A rest-frame) absorption toward a half-dozen quasars at z [?] 3-4 have demonstrated the great potential of helium studies of the intergalactic medium, but the current critically small sample size of clean sightlines for the He II Gunn-Peterson test limits confidence in cosmological inferences, and a larger sample is required. Although the unobscured quasar sightlines to high redshift are extremely rare, the Sloan Digital Sky Survey (SDSS) DR6 provides thousands of z > 2.8 quasars. We have cross-correlated these SDSS quasars with the Galaxy Evolution Explorer (GALEX) GR2/GR3 to establish a catalog of 200 higher-confidence ([?]70% secure) cases of quasars at z = 2.8-5.1 potentially having surviving far-UV (rest-frame) flux. We also catalog another 112 likely far-UV-bright quasars from GALEX cross-correlation with other (non-SDSS) quasar compilations. Reconnaissance UV prism observations with the Hubble Space Telescope (HST) of 24 of our SDSS/GALEX candidates confirm 12 as detected in the far-UV, with at least nine having flux extending to very near the He II break; with refinements our success rate is even higher. Our SDSS/GALEX selection approach is thereby confirmed to be an order of magnitude more efficient than previous He II quasar searches, more than doubles the number of spectroscopically confirmed clean sightlines to high redshift, and provides a resource list of hundreds of high-confidence sightlines for upcoming He II and other far-UV studies from the HST. Our reconnaissance HST prism spectra suggest some far-UV diversity, confirming the need to obtain a large sample of independent quasar sightlines across a broad redshift range to assess such issues as the epoch(s) of helium reionization, while averaging over individual-object pathology and/or cosmic variance.

We measure the evolution of the luminous red galaxy (LRG) luminosity function in the redshift range 0.1 < z < 0.9 using samples of galaxies from the Sloan Digital Sky Survey as well as new spectroscopy of high-redshift massive red galaxies. Our high-redshift sample of galaxies is largest spectroscopic sample of massive red galaxies at z [?] 0.9 collected to date and covers 7 deg2, minimizing the impact of large-scale structure on our results. We find that the LRG population has evolved little beyond the passive fading of its stellar populations since z [?] 0.9. Based on our luminosity function measurements and assuming a nonevolving Salpeter stellar initial mass function, we find that the most massive ( 3L^{*}"/> L > 3L*) red galaxies have grown by less than 50% (at 99% confidence), since z = 0.9, in stark contrast to the factor of 2-4 growth observed in the L* red galaxy population over the same epoch. We also investigate the evolution of the average LRG spectrum since z [?] 0.9 and find the high-redshift composite to be well described as a passively evolving example of the composite galaxy observed at low redshift. From spectral fits to the composite spectra, we find at most 5% of the stellar mass in massive red galaxies may have formed within 1 Gyr of z = 0.9. While L* red galaxies are clearly assembled at z < 1, 3L* galaxies appear to be largely in place and evolve little beyond the passive evolution of their stellar populations over the last half of cosmic history.

We studied a sample of 415 associated ( zabs [?] zem; relative velocity with respect to QSO in units of c, b < 0.01) Mg II absorption systems with 1.0 [?] zabs [?] 1.86, in the spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in the absorbers. We also compared these properties to those of a similarly selected sample of 809 intervening systems (b > 0.01), so as to understand their origin. Normalized, composite spectra were derived for absorption line measurements, for the full sample and for several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. From these, and from the equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher ionization (higher ratios of the strengths of C IV and Mg II lines), than the intervening absorbers. The ionization decreases with increasing b. Average extinction curves were obtained for the subsamples by comparing their geometric mean QSO spectra with those of matching (in zem and i magnitude) samples of QSOs without absorption lines. There is clear evidence for SMC-like dust attenuation in these systems; the 2175 A absorption feature is absent. The extinction is almost twice that observed in intervening systems. We reconfirm that QSOs with nonzero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in radio-detected QSOs is 1.7 times that in radio-undetected QSOs. The associated absorbers in radio-detected QSOs cause 3 times more reddening than those in radio-undetected QSOs. This excess reddening is correlated with the strength of Mg II absorption, possibly suggesting an intrinsic nature for the associated absorbers in radio-detected QSOs.

We study the luminosity and color dependence of the galaxy two-point correlation function in the Sloan Digital Sky Survey, starting from a sample of ~200,000 galaxies over 2500 deg2. We concentrate our analysis on volume-limited subsamples of specified luminosity ranges, for which we measure the projected correlation function wp(rp), which is directly related to the real-space correlation function x(r). The amplitude of wp(rp) rises continuously with luminosity from Mr [?] -17.5 to Mr [?] -22.5, with the most rapid increase occurring above the characteristic luminosity L* (Mr [?] -20.5). Over the scales 0.1 h-1 Mpc < rp < 10 h-1 Mpc, the measurements for samples with Mr > -22 can be approximated, imperfectly, by power-law three-dimensional correlation functions x(r) = (r/r0)-g with g [?] 1.8 and r0(L*) [?] 5.0 h-1 Mpc. The brightest subsample, -23 < Mr < -22, has a significantly steeper x(r). When we divide samples by color, redder galaxies exhibit a higher amplitude and steeper correlation function at all luminosities. The correlation amplitude of blue galaxies increases continuously with luminosity, but the luminosity dependence for red galaxies is less regular, with bright red galaxies exhibiting the strongest clustering at large scales and faint red galaxies exhibiting the strongest clustering at small scales. We interpret these results using halo occupation distribution (HOD) models assuming concordance cosmological parameters. For most samples, an HOD model with two adjustable parameters fits the wp(rp) data better than a power law, explaining inflections at rp ~ 1-3 h-1 Mpc as the transition between the one-halo and two-halo regimes of x(r). The implied minimum mass for a halo hosting a central galaxy more luminous than L grows steadily, with Mmin [?] L at low luminosities and a steeper dependence above L*. The mass at which a halo has, on average, one satellite galaxy brighter than L is M1 [?] 23Mmin(L), at all luminosities. These results imply a conditional luminosity function (at fixed halo mass) in which central galaxies lie far above a Schechter function extrapolation of the satellite population. The HOD model fits nicely explain the color dependence of wp(rp) and the cross correlation between red and blue galaxies. For galaxies with Mr < -21, halos slightly above Mmin have blue central galaxies, while more massive halos have red central galaxies and predominantly red satellite populations. The fraction of blue central galaxies increases steadily with decreasing luminosity and host halo mass. The strong clustering of faint red galaxies follows from the fact that nearly all of them are satellite systems in high-mass halos. The HOD fitting results are in good qualitative agreement with the predictions of numerical and semianalytic models of galaxy formation.

We identify a concordant model for the intergalactic medium (IGM) at redshift z = 1.9 that uses popular values for cosmological and astrophysical parameters and accounts for all baryons with an uncertainty of 5%. The amount of absorption by H I in the IGM provides the best evidence on the physical conditions in the IGM, especially the combination of the mean gas density, the density fluctuations, the intensity of the ionizing flux, and the level of ionization. We have measured the amount of absorption, known as the flux decrement (DA), in the Lya forest at redshift 1.9. We used spectra of 77 QSOs that we obtained with 250 km s-1 resolution from the Kast spectrograph on the Lick observatory 3 m telescope. We fitted unabsorbed continua to these spectra using b-splines. We also fitted equivalent continua to 77 artificial spectra that we made to match the real spectra in most obvious ways: redshift, resolution, signal-to-noise ratio (S/N), emission lines and absorption lines. The typical relative error in our continuum fits to the artificial spectra is 3.5%. Averaged over all 77 QSOs, the mean level is within 1%-2% of the correct value, except at S/N < 6, where we systematically placed the continuum too high. We then adjusted the continua on the real spectra to remove this bias as a function of S/N and a second smaller bias. Absorption from all lines in the Lya forest at z = 1.9 removes DA(z = 1.9) = 15.1% +- 0.7% of the flux at rest-frame wavelengths 1070 < lr < 1170 A. This is the first measurement using many QSOs at this z, and the first calibrated measurement at any redshift. Using similar methods on 1225 < lr < 1500 A, we find metal lines absorb an average 1.6% the flux, increasing slightly as the rest-frame wavelength lr decreases because more types of spectral lines contribute and there is more C IV at lower redshifts. We estimate that the metal lines absorb 2.3% +- 0.5% of the flux in the Lya forest at z = 1.9. The absorption from Lya alone then has DA = 12.8% +- 0.9%. The Lya lines of Lyman limit systems with H I column densities log Nimg1.gif > 17.2 cm-2 are responsible for a DA = 1.0% +- 0.4% at z = 1.9. These lines arise in higher density regions than the bulk of the IGM Lya absorption, and hence they are harder to simulate in the huge boxes required to represent the large-scale variations in the IGM. If we subtract these lines, for comparison with simulations of the lower density bulk of the IGM, we are left with DA = 11.8% +- 1.0%. The mean DA in segments of individual spectra with Dz = 0.1, or 153 Mpc comoving at z = 1.9, has a large dispersion, s = 6.1% +- 0.3% including Lyman limit systems (LLSs) and metal lines, and s(Dz = 0.1) = 3.9img2.gif% for the Lya from the lower density IGM alone, excluding LLSs and metal lines. This is consistent with the usual description of large-scale structure and accounts for the large variations from QSO to QSO. Although the absorption at z = 1.9 is mostly from the lower density IGM, the Lya of LLSs and the metal lines are both major contributors to the variation in the mean flux on 153 Mpc scales at z = 1.9, and they make the flux field significantly different from a random Gaussian field with an enhanced probability of a large amount of absorption. We find that a hydrodynamic simulation on a 10243 grid in a 75.7 Mpc box reproduces the observed DA from the low-density IGM alone when we use popular parameters values H0 = 71 km s-1 Mpc-1, Ob = 0.044, Om = 0.27, OL = 0.73, s8 = 0.9, and an ultraviolet background (UVB) that has an ionization rate per H I atom of G912 = (1.44 +- 0.11) x 10-12 s-1. This is 1.08 +- 0.08 times the prediction by Madau et al. with 61% from QSOs and 39% from stars. Our measurement of the DA gives a new joint constraint on these parameters, and the DA is very sensitive to each parameter. Given fixed values for all other parameters, and assuming the simulation has insignificant errors, the error of our DA measurement gives an error on H0 of 10%, OL of 6%, Ob of 5%, and s8 of 4%, comparable to the best measurements by other methods.

We measure the projected correlation function wp img1.gif from the Sloan Digital Sky Survey for a flux-limited sample of 118,000 galaxies and a volume-limited subset of 22,000 galaxies with absolute magnitude Mr < -21. Both correlation functions show subtle but systematic departures from the best-fit power law, in particular a change in slope at rp ~ 1-2 h-1 Mpc. These departures are stronger for the volume-limited sample, which is restricted to relatively luminous galaxies. We show that the inflection point in wp img1.gif can be naturally explained by contemporary models of galaxy clustering, according to which it marks the transition from a large-scale regime dominated by galaxy pairs in separate dark matter halos to a small-scale regime dominated by galaxy pairs in the same dark matter halo. For example, given the dark halo population predicted by an inflationary cold dark matter scenario, the projected correlation function of the volume-limited sample can be well reproduced by a model in which the mean number of Mr < -21 galaxies in a halo of mass M > M1 = 4.74 x 1013 h-1 M is img2.gifM = img3.gif0.89, with 75% of the galaxies residing in less massive, single-galaxy halos and simple auxiliary assumptions about the spatial distribution of galaxies within halos and the fluctuations about the mean occupation. This physically motivated model has the same number of free parameters as a power law, and it fits the wp img1.gif data better, with a kh2/dof = 0.93, compared to 6.12 (for 10 degrees of freedom, incorporating the covariance of the correlation function errors). Departures from a power-law correlation function encode information about the relation between galaxies and dark matter halos. Higher precision measurements of these departures for multiple classes of galaxies will constrain galaxy bias and provide new tests of the theory of galaxy formation.

We present estimates of cosmological parameters from the application of the Karhunen-Loeve transform to the analysis of the three-dimensional power spectrum of density fluctuations using Sloan Digital Sky Survey galaxy redshifts. We use Omh and fb = Ob/Om to describe the shape of the power spectrum, simg1.gif for the (linearly extrapolated) normalization, and b to parameterize linear theory redshift-space distortions. On scales k [?] 0.16 h Mpc-1, our maximum likelihood values are Omh = 0.264 +- 0.043, fb = 0.286 +- 0.065, simg1.gif = 0.966 +- 0.048, and b = 0.45 +- 0.12. When we take a prior on Ob from the Wilkinson Microwave Anisotropy Probe (WMAP), we find Omh = 0.207 +- 0.030, which is in excellent agreement with WMAP and the Two-Degree Field. This indicates that we have reasonably measured the gross shape of the power spectrum, but we have difficulty breaking the degeneracy between Omh and fb, because the baryon oscillations are not resolved in the current spectroscopic survey window function.

Using a catalog of 147,986 galaxy redshifts and fluxes from the Sloan Digital Sky Survey (SDSS), we measure the galaxy luminosity density at z = 0.1 in five optical bandpasses corresponding to the SDSS bandpasses shifted to match their rest-frame shape at z = 0.1. We denote the bands 0.1u, 0.1g, 0.1r, 0.1i, 0.1z with leff = (3216, 4240, 5595, 6792, 8111 A), respectively. To estimate the luminosity function, we use a maximum likelihood method that allows for a general form for the shape of the luminosity function, fits for simple luminosity and number evolution, incorporates the flux uncertainties, and accounts for the flux limits of the survey. We find luminosity densities at z = 0.1 expressed in absolute AB magnitudes in a Mpc3 to be (-14.10 +- 0.15, -15.18 +- 0.03, -15.90 +- 0.03, -16.24 +- 0.03, -16.56 +- 0.02) in (0.1u, 0.1g, 0.1r, 0.1i, 0.1z), respectively, for a cosmological model with O0 = 0.3, OL = 0.7, and h = 1 and using SDSS Petrosian magnitudes. Similar results are obtained using Sersic model magnitudes, suggesting that flux from outside the Petrosian apertures is not a major correction. In the 0.1r band, the best-fit Schechter function to our results has ph* = (1.49 +- 0.04) x 10-2 h3 Mpc-3, M* - 5 log10 h = -20.44 +- 0.01, and a = -1.05 +- 0.01. In solar luminosities, the luminosity density in 0.1r is (1.84 +- 0.04) x 108 h L0.1r, Mpc-3. Our results in the 0.1g band are consistent with other estimates of the luminosity density, from the Two-Degree Field Galaxy Redshift Survey and the Millennium Galaxy Catalog. They represent a substantial change (~0.5 mag) from earlier SDSS luminosity density results based on commissioning data, almost entirely because of the inclusion of evolution in the luminosity function model.

We present measurements of parameters of the three-dimensional power spectrum of galaxy clustering from 222 square degrees of early imaging data in the Sloan Digital Sky Survey (SDSS). The projected galaxy distribution on the sky is expanded over a set of Karhunen-Loeve (KL) eigenfunctions, which optimize the signal-to-noise ratio in our analysis. A maximum likelihood analysis is used to estimate parameters that set the shape and amplitude of the three-dimensional power spectrum of galaxies in the SDSS magnitude-limited sample with r* < 21. Our best estimates are G = 0.188 +- 0.04 and s8L = 0.915 +- 0.06 (statistical errors only), for a flat universe with a cosmological constant. We demonstrate that our measurements contain signal from scales at or beyond the peak of the three-dimensional power spectrum. We discuss how the results scale with systematic uncertainties, like the radial selection function. We find that the central values satisfy the analytically estimated scaling relation. We have also explored the effects of evolutionary corrections, various truncations of the KL basis, seeing, sample size, and limiting magnitude. We find that the impact of most of these uncertainties stay within the 2 s uncertainties of our fiducial result.

We present the first measurements of clustering in the Sloan Digital Sky Survey (SDSS) galaxy redshift survey. Our sample consists of 29,300 galaxies with redshifts 5700 km s-1 [?] cz [?] 39,000 km s-1, distributed in several long but narrow (2.deg5-5deg) segments, covering 690 deg2. For the full, flux-limited sample, the redshift-space correlation length is approximately 8 h-1 Mpc. The two-dimensional correlation function x(rp,p) shows clear signatures of both the small-scale, "fingers-of-God" distortion caused by velocity dispersions in collapsed objects and the large-scale compression caused by coherent flows, though the latter cannot be measured with high precision in the present sample. The inferred real-space correlation function is well described by a power law, x(r) = (r/6.1 +- 0.2 h-1 Mpc)-1.75+-0.03, for 0.1 h-1 Mpc [?] r [?] 16 h-1 Mpc. The galaxy pairwise velocity dispersion is s12 [?] 600 +- 100 km s-1 for projected separations 0.15 h-1 Mpc [?] rp [?] 5 h-1 Mpc. When we divide the sample by color, the red galaxies exhibit a stronger and steeper real-space correlation function and a higher pairwise velocity dispersion than do the blue galaxies. The relative behavior of subsamples defined by high/low profile concentration or high/low surface brightness is qualitatively similar to that of the red/blue subsamples. Our most striking result is a clear measurement of scale-independent luminosity bias at r [?] 10 h-1 Mpc: subsamples with absolute magnitude ranges centered on M* - 1.5, M*, and M* + 1.5 have real-space correlation functions that are parallel power laws of slope [?]-1.8 with correlation lengths of approximately 7.4, 6.3, and 4.7 h-1 Mpc, respectively.

The emission of 21 cm radiation from a neutral intergalactic medium (IGM) at high redshift is discussed in connection with the thermal and ionization history of the universe. The physical mechanisms that make such radiation detectable against the cosmic microwave background include Lya coupling of the hydrogen spin temperature to the kinetic temperature of the gas and preheating of the IGM by the first generation of stars and quasars. Three different signatures are investigated in detail: (a) the fluctuations in the redshifted 21 cm emission induced by the gas density inhomogeneities that develop at early times in cold dark matter (CDM) dominated cosmologies, (b) the sharp absorption feature in the radio sky due to the rapid rise of the Lya continuum background that marks the birth of the first UV sources in the universe, and (c) the 21 cm emission and absorption shells that are generated on several Mpc scales around the first bright quasars. Future radio observations with projected facilities like the Giant Meterwave Radio Telescope and the Square Kilometer Array may shed light on the power spectrum of density fluctuations at z > 5 and map the end of the "dark ages," i.e., the transition from the postrecombination universe to one populated with radiation sources.