arXiv subjects

M. Davis

Publications and source records attributed to M. Davis.

53 records · Page 3Linked to original sources

arXiv2001

Extreme BAL Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a population of broad absorption line quasars with various extreme properties. Many show absorption from metastable states of FeII with varying excitations; several objects are almost completely absorbed bluewards of MgII; at least one shows stronger absorption from FeIII than FeII, indicating temperatures T>35000 K in the absorbing region; and one object even seems to have broad H-beta absorption. Many of these extreme BALs are also heavily reddened, though `normal' BALs (particularly LoBALs) from SDSS also show evidence for internal reddening.

astro-ph↗

arXiv2000

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data III: A Color Selected Sample at i^*<20 in the Fall Equatorial Stripe

This is the third paper in a series aimed at finding high-redshift quasars from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by the SDSS during its commissioning phase. In this paper, we first present the observations of 14 bright high-redshift quasars (3.66<z<4.77, i^*<20) discovered in the SDSS Fall Equatorial Stripe, and the SDSS photometry of two previously known high-redshift quasars in the same region of the sky. Combined with the quasars presented in previous papers, we define a color-selected flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i^*<20, covering a total effective area of 182 deg^2. From this sample, we estimate the average spectral power law slope in the rest-frame ultraviolet for quasars at z~4 to be -0.79 with a standard deviation of 0.34, and the average rest-frame equivalent width of the Ly alpha+N V emission line to be 69 A with a standard deviation of 18 A. The selection completeness of this multicolor sample is determined from the model colors of high-redshift quasars, taking into account the distributions of emission line strengths, intrinsic continuum slope, the line and continuum absorption from intervening material, and the effects of photometric errors. The average completeness of this sample is about 75%. The selection function calculated in this paper will be used to correct the incompleteness of this color-selected sample and to derive the high-redshift quasar luminosity function in a subsequent paper. In the Appendix, we present the observations of an additional 18 faint quasars (3.57<z<4.80, 20.1<i^*<20.8) discovered in the region on the sky that has been imaged twice. Several quasars presented in this paper exhibit interesting properties, including a radio-loud quasar at z=4.77, and a narrow-line quasar (FWHM = 1500 km s^-1) at z=3.57.

astro-ph↗

arXiv2000

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data IV: Luminosity Function from the Fall Equatorial Stripe Sampl

This is the fourth paper in a series aimed at finding high-redshift quasars from five-color imaging data taken along the Celestial Equator by the SDSS. during its commissioning phase. In this paper, we use the color-selected sample of 39 luminous high-redshift quasars presented in Paper III to derive the evolution of the quasar luminosity function over the range of 3.6<z<5.0, and -27.5<M_1450<-25.5 (Omega=1, H_0=50 km s^-1 Mpc^-1). We use the selection function derived in Paper III to correct for sample incompleteness. The luminosity function is estimated using three different methods: (1) the 1/V_a estimator; (2) a maximum likelihood solution, assuming that the density of quasars depends exponentially on redshift and as a power law in luminosity and (3) Lynden-Bell's non-parametric C^- estimator. All three methods give consistent results. The luminous quasar density decreases by a factor of ~ 6 from z=3.5 to z=5.0, consistent with the decline seen from several previous optical surveys at z<4.5. The luminosity function follows psi(L) ~ L^{-2.5} for z~4 at the bright end, significantly flatter than the bright end luminosity function psi(L) \propto L^{-3.5} found in previous studies for z<3, suggesting that the shape of the quasar luminosity function evolves with redshift as well, and that the quasar evolution from z=2 to 5 cannot be described as pure luminosity evolution. Possible selection biases and the effect of dust extinction on the redshift evolution of the quasar density are also discussed.

astro-ph↗

arXiv2000

Optical Spectroscopy of Supernova 1993J During Its First 2500 Days

We present 42 low-resolution spectra of Supernova (SN) 1993J, our complete collection from the Lick and Keck Observatories, from day 3 after explosion to day 2454, as well as one Keck high-dispersion spectrum from day 383. SN 1993J began as an apparent SN II, albeit an unusual one. After a few weeks, a dramatic transition took place, as prominent helium lines emerged in the spectrum. SN 1993J had metamorphosed from a SN II to a SN IIb. Nebular spectra of SN 1993J closely resemble those of SNe Ib and Ic, but with a persistent H_alpha line. At very late times, the H_alpha emission line dominated the spectrum, but with an unusual, box-like profile. This is interpreted as an indication of circumstellar interaction.

astro-ph↗

arXiv2000

The Discovery of a Luminous z=5.80 Quasar from the Sloan Digital Sky Survey

We present observations of SDSSp J104433.04--012502.2, a luminous quasar at z=5.80 discovered from Sloan Digital Sky Survey (SDSS) multicolor imaging data. This object was selected as an i'-band dropout object, with i*=21.8 +/- 0.2, z*=19.2 +/- 0.1. It has an absolute magnitude M1450 = -27.2 (H_0 =50 km/s/Mpc, q0 = 0.5). The spectrum shows a strong and broad Ly alpha emission line, strong Ly alpha forest absorption lines with a mean continuum decrement D_A = 0.91, and a Lyman Limit System at z=5.72. The spectrum also shows strong OI and SiIV emission lines similar to those of quasars at z<= 5, suggesting that these metals were produced at redshift beyond six. The lack of a Gunn-Peterson trough in the spectrum indicates that the universe is already highly ionized at z ~ 5.8. Using a high-resolution spectrum in the Ly alpha forest region, we place a conservative upper limit of the optical depth due to the Gunn-Peterson effect of tau < 0.5 in regions of minimum absorption. The Ly alpha forest absorption in this object is much stronger than that in quasars at z<= 5. The object is unresolved in a deep image with excellent seeing, implying that it is unlensed. The black hole mass of this quasar is ~3 x 10^9 M_solar if we assume that it is radiating at the Eddington luminosity and no lensing amplification, implying that it resides in a very massive dark matter halo. The discovery of one quasar at M_1450 < -27 in a survey area of 600 deg^2 is consistent with an extrapolation of the observed luminosity function at lower redshift. The abundance and evolution of such quasars can provide sensitive tests of models of quasar and galaxy formation.

astro-ph↗

arXiv2000

Five High-Redshift Quasars Discovered in Commissioning Imaging Data of the Sloan Digital Sky Survey

We report the discovery of five quasars with redshifts of 4.67 - 5.27 and z'-band magnitudes of 19.5-20.7 M_B ~ -27. All were originally selected as distant quasar candidates in optical/near-infrared photometry from the Sloan Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift quasars by supplementing the SDSS data with J and K measurements. The quasars possess strong, broad Lyman-alpha emission lines, with the characteristic sharp cutoff on the blue side produced by Lyman-alpha forest absorption. Three quasars contain strong, broad absorption features, and one of them exhibits very strong N V emission. The amount of absorption produced by the Lyman-alpha forest increases toward higher redshift, and that in the z=5.27 object (D_A ~ 0.7) is consistent with a smooth extrapolation of the absorption seen in lower redshift quasars. The high luminosity of these objects relative to most other known objects at z >~ 5 makes them potentially valuable as probes of early quasar properties and of the intervening intergalactic medium.

astro-ph↗

arXiv2000

Evidence for a low-density Universe from the relative velocities of galaxies

The motions of galaxies can be used to constrain the cosmological density parameter Omega and the clustering amplitude of matter on large scales. The mean relative velocity of galaxy pairs, estimated from the Mark III survey, indicates that Omega = 0.35 +0.35/-0.25. If the clustering of galaxies is unbiased on large scales, Omega = 0.35 +/- 0.15, so that an unbiased Einstein-de Sitter model (Omega = 1) is inconsistent with the data.

astro-ph↗

arXiv1999

Streaming velocities as a dynamical estimator of Omega

It is well known that estimating the pairwise velocity of galaxies, v_{12}, from the redshift space galaxy correlation function is difficult because this method is highly sensitive to the assumed model of the pairwise velocity dispersion. Here we propose an alternative method to estimate v_{12} directly from peculiar velocity samples, which contain redshift-independent distances as well as galaxy redshifts. In contrast to other dynamical measures which determine beta = sigma_8 x Omega^{0.6}, our method can provide an estimate of (sigma_8)^2 x Omega^{0.6} for a range of sigma_8 (here Omega is the cosmological mass density parameter while sigma_8 is the standard normalization parameter for the spectrum of matter density fluctuations). We demonstrate how to measure this quantity from realistic catalogues.

astro-ph↗

arXiv1999

Comparing the SBF Survey Velocity Field with the Gravity Field from Redshift Surveys

We compare the predicted local peculiar velocity field from the IRAS 1.2 Jy flux-limited redshift survey and the Optical Redshift Survey (ORS) to the measured peculiar velocities from the recently completed SBF Survey of Galaxy Distances. The analysis produces a value of β= Ω^{0.6}/b for the redshift surveys, where b is the linear biasing factor, and a tie to the Hubble flow, i.e., a value of H_0, for the SBF Survey. There is covariance between these parameters, but we find good fits with H_0 \approx 74 \kmsM for the SBF distances, β_I \approx 0.44 for the IRAS survey predictions, and β_O \approx 0.3 for the ORS. The small-scale velocity error \sigv \sim 200 \kms is similar to, though slightly larger than, the value obtained in our parametric flow modeling with SBF.

astro-ph↗

arXiv1999

A First Comparison of the SBF Survey Distances with the Galaxy Density Field: Implications for H_0 and Omega

We compare the peculiar velocities measured in the SBF Survey of Galaxy Distances with the predictions from the density fields of the IRAS 1.2 Jy flux-limited redshift survey and the Optical Redshift Survey (ORS) to derive simultaneous constraints on the Hubble constant $H_0$ and the density parameter $β= Ω^{0.6}/b$, where $b$ is the linear bias. We find $β_I=0.42^{+0.10}_{-0.06}$ and $β_O=0.26\pm0.08$ for the IRAS and ORS comparisons, respectively, and $H_0=74\pm4$ \kmsMpc (with an additional 9% uncertainty due to the Cepheids themselves). The match between predicted and observed peculiar velocities is good for these values of $H_0$ and $β$, and although there is covariance between the two parameters, our results clearly point toward low-density cosmologies. Thus, the unresolved discrepancy between the ``velocity-velocity'' and ``density-density'' measurements of $β$ continues.

astro-ph↗

arXiv1999

Measuring Omega with Galaxy Streaming Velocities

The mean pairwise velocity of galaxies has traditionally been estimated from the redshift space galaxy correlation function. This method is notorious for being highly sensitive to the assumed model of the pairwise velocity dispersion. Here we propose an alternative method to estimate the streaming velocity directly from peculiar velocity samples, which contain redshift-independent distances as well as galaxy redshifts. This method can provide an estimate of $Ω^{0.6}σ_8^2$ for a range of $σ_8$ where $Ω$ is the cosmological density parameter, while $σ_8$ is the standard normalization for the power spectrum of density fluctuations. We demonstrate how to measure this quantity from realistic catalogues and identify the main sources of bias and errors

astro-ph↗

arXiv1998

The Supergalactic Plane revisited with the Optical Redshift Survey

We re-examine the existence and extent of the planar structure in the local galaxy density field, the so-called Supergalactic Plane (SGP). This structure is studied here in three dimensions using both the new Optical Redshift Survey (ORS) and the IRAS 1.2 Jy redshift survey. The density contrast in a slab of thickness of 20 Mpc/h and diameter of 80 Mpc/h aligned with the standard de Vaucouleurs' Supergalactic coordinates, is delta_sgp =0.5 for both ORS and IRAS. The structure of the SGP is not well described by a homogeneous ellipsoid, although it does appear to be a flattened structure, which we quantify by calculating the moment of inertia tensor of the density field. The directions of the principal axes vary with radius, but the minor axis remains within 30 deg of the standard SGP Z-axis, out to a radius of 80 Mpc/h, for both ORS and \iras. However, the structure changes shape with radius, varying between a flattened pancake and a dumbbell, the latter at a radius of ~50 Mpc/h, where the Great Attractor and Perseus-Pisces superclusters dominate the distribution. This calls to question the connectivity of the `plane' beyond ~40 Mpc/h. The configuration found here can be viewed as part of a web of filaments and sheets, rather than as an isolated pancake-like structure. An optimal minimum variance reconstruction of the density field using Wiener filtering which corrects for both redshift distortion and shot noise, yields a similar misalignment angle and behaviour of axes. The background-independent statistic of axes proposed here can be best used for testing cosmological models by comparison with N-body simulations.

astro-ph↗

arXiv1998

Application of SFD Dust Maps to Galaxy Counts and CMB Experiments

We have constructed a full-sky map of the far-infrared suitable for measuring Galactic reddening and extinction (Schlegel, Finkbeiner & Davis 1998: SFD). The SFD map is based upon extensive re-analysis of data from the COBE/DIRBE and IRAS satellite missions. We demonstrate that the maps can correct for extinction problems in the APM galaxy survey. We also determine the most dust-free regions for conducting cosmic microwave background or soft X-ray experiments of extragalactic objects.

astro-ph↗

arXiv1997

Resolving the Stellar Populations in a z=4.04 Lensed Galaxy

We have recently obtained deep near-IR Keck imaging of a newly-discovered z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground cluster Abell 2390 (z=0.23) into highly-magnified arcs 3-5" in length. Our H- and K'-band Keck/NIRC imaging allows us to map the Balmer+4000Ang break amplitude. In combination with high-quality archival HST/WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which may correspond to sites of active star formation. Indeed, in some spatial regions the Keck/LRIS discovery spectra are consistent with OB-star spectral energy distributions in the rest-ultraviolet. However, there are considerable portions of the arcs which appear redder with no Ly-alpha emission, consistent with being post-starburst regions.

astro-ph↗

arXiv1997

Faint Infrared-Excess Field Galaxies: FROGs

Deep near-infrared and optical imaging surveys in the field reveal a curious population of galaxies that are infrared-bright (I-K>4), yet with relatively blue optical colors (V-I<2). Their surface density, several per square arcminute at K>20, is high enough that if placed at z>1 as our models suggest, their space densities are about one-tenth of phi-*. The colors of these ``faint red outlier galaxies'' (fROGs) may derive from exceedingly old underlying stellar populations, a dust-embedded starburst or AGN, or a combination thereof. Determining the nature of these fROGs, and their relation with the I-K>6 ``extremely red objects,'' has implications for our understanding of the processes that give rise to infrared-excess galaxies in general. We report on an ongoing study of several targets with HST & Keck imaging and Keck/LRIS multislit spectroscopy.

astro-ph↗

arXiv1994

The Two-Point Correlation Function of Rich Clusters of Galaxies: Results from an Extended APM Cluster Redshift Survey

We present new estimates of the spatial two-point correlation function of rich clusters of galaxies selected from the APM Galaxy Survey. We have measured redshifts for a sample of $364$ clusters out to a depth of $\sim 450\hmpc$. The clusters have a mean space density of $\bar{n} = 3.4\times 10^{-5}\hmpccc$. The two-point correlation function, $ξ_{cc}$, for this sample is equal to unity at a pair-separation of $r_0 = 14.3\pm1.75\hmpc$ (2$σ$ errors), consistent with our earlier results from a smaller sample. The new observations provide an accurate determination of the shape of $ξ_{cc}$ to pair-separations of about $50\hmpc$. Our results show that $ξ_{cc}$ has a higher amplitude than expected according to the standard $Ω=1$ cold dark matter (CDM) model on spatial scales $2\simlt s \simlt 50\hmpc$, but are in good agreement with scale-invariant fluctuations in either a low density CDM model or a critical density universe made up of a mixture of hot and cold dark matter. Our results provide strong constraints on so called `co-operative' models of galaxy formation in which the galaxy formation process introduces large-scale structure in the galaxy distribution.

astro-ph↗

arXiv1994

The Optical Redshift Survey: Sample Selection and the Galaxy Distribution

This is the first in a series of papers describing the {\it Optical Redshift Survey} (ORS), a redshift survey of optically selected galaxies covering 98\% of the sky above $|b| = 20^\circ$ (8.09 ster). The survey is drawn from the UGC, ESO, and ESGC galaxy catalogues, and contains two sub-samples, one complete to a $B$ magnitude of 14.5, the other complete to a $B$ major axis diameter of $1.9^\prime$. The entire sample consists of 8457 objects, of which redshifts are now available for 8286; 171 objects remain without measured redshifts. The ORS provides the most detailed and homogeneous sampling of the large-scale galaxy distribution to date in these areas. The density field of bright optical galaxies is well-defined to $8000 \kms$, and is dominated by the Virgo, Telescopium-Pavo-Indus, Hydra-Centaurus, Pisces-Perseus, and Coma-A1367 Superclusters. The dense sampling provided by ORS allows a detailed analysis of the galaxy density field, and will be used to test its dependence on morphology and other galaxy parameters.

astro-ph↗