F. Prada

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.

Halo concentrations in the standard Λ cold dark matter cosmology

We study the concentration of dark matter halos and its evolution in N-body simulations of the standard LCDM cosmology. The results presented in this paper are based on 4 large N-body simulations with about 10 billion particles each: the Millennium-I and II, Bolshoi, and MultiDark simulations. The MultiDark (or BigBolshoi) simulation is introduced in this paper. This suite of simulations with high mass resolution over a large volume allows us to compute with unprecedented accuracy the concentration over a large range of scales (about six orders of magnitude in mass), which constitutes the state-of-the-art of our current knowledge on this basic property of dark matter halos in the LCDM cosmology. We find that there is consistency among the different simulation data sets. We confirm a novel feature for halo concentrations at high redshifts: a flattening and upturn with increasing mass. The concentration c(M,z) as a function of mass and the redshift and for different cosmological parameters shows a remarkably complex pattern. However, when expressed in terms of the linear rms fluctuation of the density field sigma(M,z), the halo concentration c(sigma) shows a nearly-universal simple U-shaped behaviour with a minimum at a well defined scale at sigma=0.71. Yet, some small dependences with redshift and cosmology still remain. At the high-mass end (sigma < 1) the median halo kinematic profiles show large signatures of infall and highly radial orbits. This c-sigma(M,z) relation can be accurately parametrized and provides an analytical model for the dependence of concentration on halo mass. When applied to galaxy clusters, our estimates of concentrations are substantially larger -- by a factor up to 1.5 -- than previous results from smaller simulations, and are in much better agreement with results of observations. (abridged)

THE CLUSTERING OF MASSIVE GALAXIES AT z ∼ 0.5 FROM THE FIRST SEMESTER OF BOSS DATA

We calculate the real- and redshift-space clustering of massive galaxies at z ∼ 0.5 using the first semester of data by the Baryon Oscillation Spectroscopic Survey (BOSS). We study the correlation functions of a sample of 44,000 massive galaxies in the redshift range 0.4 <z< 0.7. We present a halo-occupation distribution modeling of the clustering results and discuss the implications for the manner in which massive galaxies at z ∼ 0.5 occupy dark matter halos. The majority of our galaxies are central galaxies living in halos of mass 10 13 h −1 M� ,b ut 10% are satellites living in halos 10 times more massive. These results are broadly in agreement with earlier investigations of massive galaxies at z ∼ 0.5. The inferred large-scale bias (b � 2) and relatively high number density ( ¯ n = 3 × 10 −4 h 3 Mpc −3 ) imply that BOSS galaxies are excellent tracers of large-scale structure, suggesting BOSS will enable a wide range of investigations on the distance scale, the growth of large-scale structure, massive galaxy evolution, and other topics.

Observing the Dark Matter Density Profile of Isolated Galaxies

Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass distribution by studying the velocities of satellites orbiting isolated galaxies. In a subsample that covers 2500 deg2 on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to MB = -14; most of the satellites have MB = -16 to -18, comparable to the magnitudes of M32 and the Magellanic Clouds. After a careful, model-independent removal of interlopers, we find that the line-of-sight velocity dispersion of satellites declines with distance to the primary. For an L* galaxy the rms line-of-sight velocity changes from ≈120 km s-1 at 20 kpc to ≈60 km s-1 at 350 kpc. This decline agrees remarkably well with theoretical expectations, as all modern cosmological models predict that the density of dark matter in the peripheral parts of galaxies declines as ρDM r-3. Thus, for the first time we find direct observational evidence of the density decline predicted by cosmological models; we also note that this result contradicts alternative theories of gravity such as modified Newtonian dynamics (MOND). We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as σ L0.3; this is very close to the Tully-Fisher relation for normal spiral galaxies.

THE TULLY-FISHER RELATION AND ITS RESIDUALS FOR A BROADLY SELECTED SAMPLE OF GALAXIES

We measure the relation between galaxy luminosity and disk circular velocity (the Tully-Fisher [TF] relation), in the g, r, i, and z bands, for a broadly selected sample of galaxies from the Sloan Digital Sky Survey, with the goal of providing well-defined observational constraints for theoretical models of galaxy formation. The input sample of 234 galaxies has a roughly flat distribution of absolute magnitudes in the range -18.5 > Mr > -22, and our only morphological selection is an isophotal axis ratio cut b/a < 0.6 to allow accurate inclination corrections. Long-slit spectroscopy from the Calar Alto and MDM observatories yields usable Hα rotation curves for 162 galaxies (69%), with a representative color and morphology distribution. We define circular velocities V80 by evaluating the rotation curve at the radius containing 80% of the i-band light. Observational errors, including estimated distance errors due to peculiar velocities, are small compared to the intrinsic scatter of the TF relation. The slope of the forward TF relation steepens from -5.5 ± 0.2 mag (log10 km s-1)-1 in the g band to -6.6 ± 0.2 mag (log10 km s-1)-1 in the z band. The intrinsic scatter is σ ≈ 0.4 mag in all bands, and residuals from either the forward or inverse relations have an approximately Gaussian distribution. We discuss how Malmquist-type biases may affect the observed slope, intercept, and scatter. The scatter is not dominated by rare outliers or by any particular class of galaxies, although it drops slightly, to σ ≈ 0.36 mag, if we restrict the sample to nearly bulgeless systems. Correlations of TF residuals with other galaxy properties are weak: bluer galaxies are significantly brighter than average in the g-band TF relation but only marginally brighter in the i band; more concentrated (earlier type) galaxies are slightly fainter than average, and the TF residual is virtually independent of half-light radius, contrary to the trend expected for gravitationally dominant disks. The observed residual correlations do not account for most of the intrinsic scatter, implying that this scatter is instead driven largely by variations in the ratio of dark to luminous matter within the disk galaxy population.

The SDSS DR6 luminosity functions of galaxies

We present number counts, luminosity functions (LFs) and luminosity densities of galaxies obtained using the Sloan Digital Sky Survey Sixth Data Release in all SDSS photometric bands. Thanks to the SDSS DR6, galaxy statistics have increased by a factor of ∼ 9 in the u-band and by a factor of ∼ 4 − 5 in the rest of the SDSS bands with respect to the previous work of Blanton et al. (2003b). In addition, we have achieved a high redshift completeness in our galaxy samples. Firstly, by making use of the survey masks, provided by the NYU-VAGC DR6, we have been able to define an area on the sky of high angular redshift completeness. Secondly, we guarantee that brightness-dependent redshift incompleteness is small within the magnitude ranges that define our galaxy samples. With these advances, we have estimated very accurate SDSS DR6 LFs in both the bright and the faint end. In the 0.1 r-band, our SDSS DR6 luminosity function is well fitted by a Schechter LF with parameters �� = 0.90 ±0.07, M�−5log10h = −20.73±0.04and � = −1.23±0.02. As compared with previous results, we find some notable differences. In the bright end of the 0.1 u-band luminosity function we find a remarkable excess, of ∼ 1.7 dex at M0.1u ≃ −20.5, with respect to the bestfit Schechter LF. This excess weakens in the 0.1 g-band, fading away towards the very red 0.1 z-band. A preliminary analysis on the nature of this bright-end bump reveals that it is mostly comprised of active galaxies and QSOs. It seems, therefore, that an important fraction of this exceeding luminosity may come from nuclear activity. In the faint end of the SDSS DR6 luminosity functions, where we can reach 1 − 1.5 magnitudes deeper than the previous SDSS LF estimation, we obtain a steeper slope, that increases from the 0.1 u-band, with � = −1.01 ± 0.03, to the very red 0.1 z-band, with � = −1.26±0.03. These state-of-the-art results may be used to constrain a variety of aspects of star formation histories and/or feed-back processes in galaxy formation models.

The kinematic connection between galaxies and dark matter haloes

Using estimates of dark halo masses from satellite kinematics, weak gravitational lensing and halo abundance matching, combined with the Tully-Fisher (TF) and Faber-Jackson relations, we derive the mean relation between the optical, V opt, and virial, V 200, circular velocities of early- and late-type galaxies at redshift z � 0. For late-type galaxies, VoptV200 over the velocity range V opt = 90-260 km s −1 , and is consistent with V opt = V max,h (the maximum circular velocity of NFW dark matter haloes in the concordancecold dark matter (� CDM) cosmology). However, for early-type galaxies VoptV200, with the exception of early-type galaxies with Vopt � 350 km s −1 . This is inconsistent with early-type galaxies being, in general, globally isothermal. For low-mass (Vopt 250 km s −1 ) early-types V opt > V max,h, indicating that baryons have modified the potential well, while high-mass (Vopt 400 km s −1 ) early-types have V opt < V max,h. Folding in measurements of the black hole mass-velocity dispersion relation, our results imply that the supermassive black hole-halo mass relation has a logarithmic slope which varies from � 1. 4a t halo masses of� 10 12 h −1 Mto � 0.65 at halo masses of 10 13.5 h −1 M� . The values of V opt/V 200 we infer for the Milky Way (MW) and M31 are lower than the values currently favoured by direct observations and dynamical models. This offset is due to the fact that the MW and M31 have higher V opt and lower V 200 compared to typical late-type galaxies of the same stellar masses. We show that current high-resolution cosmological hydrodynamical simulations are unable to form galaxies which simultaneously reproduce both the V opt/V 200 ratio and the V opt-Mstar (Tully-Fisher/Faber-Jackson) relation.

Ameliorating systematic uncertainties in the angular clustering of galaxies: a study using the SDSS-III

We investigate the effects of potential sources of systematic error on the angular and photometric redshift, zphot, distributions of a sample of redshift 0.4 0.5, the magnitude of the corrections we apply is greater than the statistical uncertainty in w(θ). The photometric redshift catalogue we produce will be made publicly available at http://portal.nersc.gov/project/boss/galaxy/photoz/.

How far do they go? The outer structure of galactic dark matter halos

We study the density profiles of collapsed galaxy-size dark matter halos with masses 1011 to 5 ? 1012 M? focusing mostly on the halo outer regions from the formal virial radius Rvir up to 5Rvir-7Rvir. We find that isolated halos in this mass range extend well beyond Rvir exhibiting all properties of virialized objects up to 2Rvir-3Rvir: relatively smooth density profiles and no systematic infall velocities. The dark matter halos in this mass range do not grow as one naively may expect through a steady accretion of satellites; i.e., on average there is no mass infall. This is strikingly different from more massive halos, which have large infall velocities outside the virial radius. We provide an accurate fit for the density profile of these isolated galaxy-size halos. For a wide range 0.01Rvir-2Rvir of radii the halo density profiles are fitted with the approximation ? = ?s exp + ?m, where x ? r/rs, ?m is the mean matter density of the universe, and the index n is in the range n = 6-7.5. These profiles do not show a sudden change of behavior beyond the virial radius. For larger radii we combine the statistics of the initial fluctuations with the spherical collapse model to obtain predictions for the mean and most probable density profiles for halos of several masses. The model gives excellent results beyond 2-3 formal virial radii for the most probable profile and qualitatively correct predictions for the mean profile.

The MultiDark Database: Release of the Bolshoi and MultiDark cosmological simulations

We present the online MultiDark Database -- a Virtual Observatory-oriented, relational database for hosting various cosmological simulations. The data is accessible via an SQL (Structured Query Language) query interface, which also allows users to directly pose scientific questions, as shown in a number of examples in this paper. Further examples for the usage of the database are given in its extensive online documentation (www.multidark.org). The database is based on the same technology as the Millennium Database, a fact that will greatly facilitate the usage of both suites of cosmological simulations. The first release of the MultiDark Database hosts two 8.6 billion particle cosmological N-body simulations: the Bolshoi (250/h Mpc simulation box, 1/h kpc resolution) and MultiDark Run1 simulation (MDR1, or BigBolshoi, 1000/h Mpc simulation box, 7/h kpc resolution). The extraction methods for halos/subhalos from the raw simulation data, and how this data is structured in the database are explained in this paper. With the first data release, users get full access to halo/subhalo catalogs, various profiles of the halos at redshifts z=0-15, and raw dark matter data for one time-step of the Bolshoi and four time-steps of the MultiDark simulation. Later releases will also include galaxy mock catalogs and additional merging trees for both simulations as well as new large volume simulations with high resolution. This project is further proof of the viability to store and present complex data using relational database technology. We encourage other simulators to publish their results in a similar manner.