arXiv subjects

David J. Schlegel

Publications and source records attributed to David J. Schlegel.

At least 145 records · Page 8Linked to original sources

arXiv2004

Mass-producing spectra: The SDSS spectrographic system

The Sloan Digital Sky Survey is the largest redshift survey conducted to date, and the principal survey observations have all been conducted on the dedicated SDSS 2.5m and 0.5m telescopes at Apache Point Observatory. While the whole survey has many unique features, this article concentrates on a description of the systems surrounding the dual fibre-input spectrographs that obtain all the survey spectra and that are capable of recording 5,760 individual spectra per night on an industrial, consistent, mass-production basis. It is hoped that the successes and lessons learned will prove instructive for future large spectrographic surveys.

astro-ph↗

arXiv2004

The Lyman-alpha Forest Power Spectrum from the Sloan Digital Sky Survey

We measure the power spectrum, P_F(k,z), of the transmitted flux in the Ly-alpha forest using 3035 high redshift quasar spectra from the Sloan Digital Sky Survey. This sample is almost two orders of magnitude larger than any previously available data set, yielding statistical errors of ~0.6% and ~0.005 on, respectively, the overall amplitude and logarithmic slope of P_F(k,z). This unprecedented statistical power requires a correspondingly careful analysis of the data and of possible systematic contaminations in it. For this purpose we reanalyze the raw spectra to make use of information not preserved by the standard pipeline. We investigate the details of the noise in the data, resolution of the spectrograph, sky subtraction, quasar continuum, and metal absorption. We find that background sources such as metals contribute significantly to the total power and have to be subtracted properly. We also find clear evidence for SiIII correlations with the Ly-alpha forest and suggest a simple model to account for this contribution to the power. While it is likely that our newly developed analysis technique does not eliminate all systematic errors in the P_F(k,z) measurement below the level of the statistical errors, our tests indicate that any residual systematics in the analysis are unlikely to affect the inference of cosmological parameters from P_F(k,z). These results should provide an essential ingredient for all future attempts to constrain modeling of structure formation, cosmological parameters, and theories for the origin of primordial fluctuations.

astro-ph↗

arXiv2004

Sloan Digital Sky Survey Spectroscopic Lens Search. I. Discovery of Intermediate-Redshift Star-Forming Galaxies Behind Foreground Luminous Red Galaxies

We present a catalog of 49 spectroscopic strong gravitational lens candidates selected from a Sloan Digital Sky Survey sample of 50996 luminous red galaxies. Potentially lensed star-forming galaxies are detected through the presence of background oxygen and hydrogen nebular emission lines in the spectra of these massive foreground galaxies. This multiline selection eliminates the ambiguity of single-line identification and provides a very promising sample of candidate galaxy-galaxy lens systems at low to intermediate redshift, with foreground redshifts ranging from 0.16 to 0.49 and background redshifts from 0.25 to 0.81. Any lenses confirmed within our sample would be important new probes of early-type galaxy mass distributions, providing complementary constraints to those obtained from currently known lensed high-redshift quasars.

astro-ph↗

arXiv2003

SDSS J0903+5028: A New Gravitational Lens

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J090334.92+502819.2. This object was targeted for SDSS spectroscopy as a Luminous Red Galaxy (LRG), but manual examination of the spectrum showed the presence of a quasar at z= 3.6 in addition to a red galaxy at z=0.388, and the SDSS image showed a second possible quasar image nearby. Follow-up imaging and spectroscopy confirmed the lensing hypothesis. In images taken at the ARC 3.5-meter telescope, two quasars are separated by 2.8 arc-seconds; the lensing galaxy is clearly seen and is blended with one of the quasar images. Spectroscopy taken at the Keck II telescope shows that the quasars have identical redshifts of z=3.6 and both show the presence of the same broad absorption line-like troughs. We present simple lens models which account for the geometry and magnifications. The lens galaxy lies near two groups of galaxies and may be a part of them. The models suggest that the groups may contribute considerable shear and may have a strong effect on the lens configuration.

astro-ph↗

arXiv2003

Double-Peaked Low-Ionization Emission Lines in Active Galactic Nuclei

We present a new sample of 116 double-peaked Balmer line Active Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey. Double-peaked emission lines are believed to originate in the accretion disks of AGN, a few hundred gravitational radii (Rg) from the supermassive black hole. We investigate the properties of the candidate disk emitters with respect to the full sample of AGN over the same redshifts, focusing on optical, radio and X-ray flux, broad line shapes and narrow line equivalent widths and line flux-ratios. We find that the disk-emitters have medium luminosities (~10^44erg/s) and FWHM on average six times broader than the AGN in the parent sample. The double-peaked AGN are 1.6 times more likely to be radio-sources and are predominantly (76%) radio quiet, with about 12% of the objects classified as LINERs. Statistical comparison of the observed double-peaked line profiles with those produced by axisymmetric and non-axisymmetric accretion disk models allows us to impose constraints on accretion disk parameters. The observed Halpha line profiles are consistent with accretion disks with inclinations smaller than 50 deg, surface emissivity slopes of 1.0-2.5, outer radii larger than ~2000 Rg, inner radii between 200-800Rg, and local turbulent broadening of 780-1800 km/s. The comparison suggests that 60% of accretion disks require some form of asymmetry (e.g., elliptical disks, warps, spiral shocks or hot spots).

astro-ph↗

arXiv2003

The dependence on environment of the color-magnitude relation of galaxies

The distribution in color and absolute magnitude is presented for 55,158 galaxies taken from the Sloan Digital Sky Survey in the redshift range $0.08<z<0.12$, as a function of galaxy number overdensity in a line-of-sight cylinder of transverse radius $1 h^{-1} \mathrm{Mpc}$. In all environments, bulge-dominated galaxies (defined to be those with radial profiles best fit with large Sérsic indices) form a narrow, well defined color--magnitude relation. Although the most luminous galaxies reside preferentially in the highest density regions, there is only a barely detectable variation in the color (zero-point) or slope of the color--magnitude relation ($<0.02 \mathrm{mag}$ in $^{0.1}[g-r]$ or $[B-V]$). These results constrain variations with environmental density in the ages or metallicities of typical bulge-dominated galaxies to be under 20 percent.

astro-ph↗

arXiv2003

Selection and photometric properties of K+A galaxies

Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on $156,395$ fiber spectra of galaxies with $r<17.77$ mag taken from the Sloan Digital Sky Survey in the redshift range $0.05<z<0.20$: a ratio $\Aamp / \Kamp$ found by fitting a linear sum of an average old stellar poplulation spectrum (\Kamp) and average A-star spectrum (\Aamp) to the galaxy spectrum, and the equivalent width (EW) of the $\Halpha$ emission line. The two measures are strongly correlated, but there is a small clearly separated population of outliers from the median correlation that display excess $\Aamp /\Kamp$ relative to \Halpha EW. These ``K+A'' (or ``E+A'') galaxies must have dramatically decreased their star-formation rates over the last $\sim 1$ Gyr. The K+A luminosity distribution is very similar to that of the total galaxy population. The K+A population appears to be bulge-dominated, but bluer and higher surface-brightness than normal bulge-dominated galaxies; it appears that K+A galaxies will fade with time into normal bulge-dominated galaxies. The inferred rate density for K+A galaxy formation is $\sim 10^{-4} h^3 Mpc^{-3} Gyr^{-1}$ at redshift $z\sim 0.1$. These events are taking place in the field; K+A galaxies don't primarily lie in the high-density environments or clusters typical of bulge-dominated populations.

astro-ph↗

arXiv2003

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 sq. degrees on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to M_B = -14; most of the satellites have M_B=-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 r.m.s. line-of-sight velocity changes from ~120 km/s at 20 kpc to ~60 km/s 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 rho_DM propto 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 MOND. We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as sigma propto L^{0.3}; this is very close to the Tully-Fisher relation for normal spiral galaxies.

astro-ph↗

arXiv2002

The overdensities of galaxy environments as a function of luminosity and color

We study the mean environments of galaxies in the Sloan Digital Sky Survey as a function of rest-frame luminosity and color. Overdensities in galaxy number are estimated in $8 h^{-1} \mathrm{Mpc}$ and $1 h^{-1} \mathrm{Mpc}$ spheres centered on $125,000$ galaxies taken from the SDSS spectroscopic sample. We find that, at constant color, overdensity is independent of luminosity for galaxies with the blue colors of spirals. This suggests that, at fixed star-formation history, spiral-galaxy mass is a very weak function of environment. Overdensity does depend on luminosity for galaxies with the red colors of early types; both low-luminosity and high-luminosity red galaxies are found to be in highly overdense regions.

astro-ph↗

arXiv2002

Average spectra of massive galaxies in the SDSS

We combine Sloan Digital Sky Survey spectra of 22,000 luminous, red, bulge-dominated galaxies to get high S/N average spectra in the rest-frame optical and ultraviolet (2600A to 7000A). The average spectra of these massive, quiescent galaxies are early-type with weak emission lines and with absorption lines indicating an apparent excess of alpha elements over solar abundance ratios. We make average spectra of subsamples selected by luminosity, environment and redshift. The average spectra are remarkable in their similarity. What variations do exist in the average spectra as a function of luminosity and environment are found to form a nearly one-parameter family in spectrum space. We present a high signal-to-noise ratio spectrum of the variation. We measure the properties of the variation with a modified version of the Lick index system and compare to model spectra from stellar population syntheses. The variation may be a combination of age and chemical abundance differences, but the conservative conclusion is that the quality of the data considerably exceeds the current state of the models.

astro-ph↗

arXiv2002

Estimating Fixed Frame Galaxy Magnitudes in the SDSS

Broad-band measurements of flux for galaxies at different redshifts measure different regions of the rest-frame galaxy spectrum. Certain astronomical questions, such as the evolution of the luminosity function of galaxies, require transforming these magnitudes into redshift-independent quantities. To prepare to address these astronomical questions, investigated in detail in subsequent papers, we fit spectral energy distributions (SEDs) to broad band photometric observations, in the context of the optical observations of the Sloan Digital Sky Survey (SDSS). Linear combinations of four spectral templates can reproduce the five SDSS magnitudes of all galaxies to the precision of the photometry. Expressed in the appropriate coordinate system, the locus of the coefficients multiplying the templates is planar, and in fact nearly linear. The resulting reconstructed SEDs can be used to recover fixed frame magnitudes over a range of redshifts. This process yields consistent results, in the sense that within each sample the intrinsic colors of similar type galaxies are nearly constant with redshift. We compare our results to simpler interpolation methods and galaxy spectrophotometry from the SDSS. The software that generates these results is publicly available and easily adapted to handle a wide range of galaxy observations.

astro-ph↗

arXiv2002

The Redshift of a Lensing Galaxy in PMN J0134-0931

The Sloan Digital Sky Survey (SDSS) automatically targeted as a quasar candidate the recently discovered, gravitationally lensed, extremely reddened z=2.2 quasar PMN J0134-0931. The SDSS spectrum exhibits Ca II absorption at z=0.76451, which we identify as the redshift of a lensing galaxy. Hubble Space Telescope imaging shows that components CDE of the system are significantly redder than components A or B and detects faint galaxy emission between D and A+B. The redshift of the dust responsible for the reddening remains unconstrained with current data. However, we outline a model wherein lensing and differential reddening by a z=0.76451 galaxy pair can entirely explain this system. Detecting mm-wave molecular line absorption from the lensing galaxy or galaxies may be possible in PMN J0134-0931, just as in the lenses PKS1830-211 and B0218+357. Well-constructed optical quasar surveys like the SDSS can contribute to the detection and study of reddened quasars. (Expanded)

astro-ph↗

arXiv2002

Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample

We describe the algorithm for selecting quasar candidates for optical spectroscopy in the Sloan Digital Sky Survey. Quasar candidates are selected via their non-stellar colors in "ugriz" broad-band photometry, and by matching unresolved sources to the FIRST radio catalogs. The automated algorithm is sensitive to quasars at all redshifts lower than z=5.8. Extended sources are also targeted as low-redshift quasar candidates in order to investigate the evolution of Active Galactic Nuclei (AGN) at the faint end of the luminosity function. Nearly 95% of previously known quasars are recovered (based on 1540 quasars in 446 square degrees). The overall completeness, estimated from simulated quasars, is expected to be over 90%, whereas the overall efficiency (quasars:quasar candidates) is better than 65%. The selection algorithm targets ultraviolet excess quasars to i^*=19.1 and higher-redshift (z>3) quasars to i^*=20.2, yielding approximately 18 candidates per square degree. In addition to selecting ``normal'' quasars, the design of the algorithm makes it sensitive to atypical AGN such as Broad Absorption Line quasars and heavily reddened quasars.

astro-ph↗

arXiv2001

Tentative Detection of Electric Dipole Emission from Rapidly Rotating Dust Grains

We present the first tentative detection of spinning dust emission from specific astronomical sources. All other detections in the current literature are statistical. The Green Bank 140 foot telescope was used to observe 10 dust clouds at 5, 8, and 10 GHz. In some cases, the observed emission was consistent with the negative spectral slope expected for free-free emission (thermal bremsstrahlung), but in two cases it was not. One HII region yields a rising spectrum, inconsistent with free-free or synchrotron emission at the 10 sigma level. One dark cloud (L 1622) has a similar spectrum with lower significance. Both spectra are consistent with electric dipole emission from rapidly rotating dust grains ("spinning dust"), as predicted by Draine & Lazarian.

astro-ph↗

arXiv2001

A photometricity and extinction monitor at the Apache Point Observatory

An unsupervised software ``robot'' that automatically and robustly reduces and analyzes CCD observations of photometric standard stars is described. The robot measures extinction coefficients and other photometric parameters in real time and, more carefully, on the next day. It also reduces and analyzes data from an all-sky $10 μm$ camera to detect clouds; photometric data taken during cloudy periods are automatically rejected. The robot reports its findings back to observers and data analysts via the World-Wide Web. It can be used to assess photometricity, and to build data on site conditions. The robot's automated and uniform site monitoring represents a minimum standard for any observing site with queue scheduling, a public data archive, or likely participation in any future National Virtual Observatory.

astro-ph↗

arXiv2000

Detection of a Far IR Excess with DIRBE at 60 and 100 Microns

From analysis of the DIRBE weekly averaged sky maps, we have detected substantial flux in the 60 and 100 micron channels in excess of expected zodiacal and Galactic emission. Two methods are used to separate zodiacal light from more distant emission. Method I makes use of the time-dependence of the North-South annual variation observed at the ecliptic poles. This method is robust against errors in the inter-planetary dust (IPD) model, but does not demonstrate isotropy of the background. Method II measures the ecliptic latitude dependence of the dust over a range of ecliptic latitudes at solar elongation e=90 degrees. This allows the excess to be determined in each week of the DIRBE mission for high redundancy, but the results depend weakly on the IPD model. Both methods give consistent results at 60 and 100 microns. The observed signal is consistent with an isotropic background at the level νI_ν= 28.1 +- 1.8 +- 7(syst) nW/m^2/sr at 60 microns and 24.6 +- 2.5 +- 8 nW/m^2/sr at 100 microns. The IR excess detected at 140 and 240 microns by these methods agrees with previous measurements, which are thought to be the cosmic infra-red background (CIB). The detections at 60 and 100 microns are new. While this new excess is not necessarily the CIB, we have ruled out all known sources of emission in the solar system and Galaxy. We therefore tentatively interpret this signal as the CIB and consider the implications of such energy production from the viewpoint of star formation efficiency and black hole accretion efficiency. However, the IR excess exceeds limits on the CIB derived from the inferred opacity of the IGM to observed TeV photons, thus casting doubt on this interpretation. There is currently no satisfactory explanation for the 60-100 micron excess.

astro-ph↗

arXiv1999

Bulk-flow and beta_I from the SMAC project

The SMAC project is a Fundamental Plane peculiar velocity survey of 56 clusters of galaxies to a depth of cz ~ 12000 km/s. We present here some results from the analysis of the SMAC velocity field, focussing on three specific features: the best-fitting bulk-flow model for the SMAC data; the agreement between the observed velocity field and predictions from the IRAS-PSCz redshift survey; the role of the Great Attractor and Shapley Concentration in generating the local flows. We argue that the local mass distribution, as probed by the PSCz, can fully account for the observed cluster velocities.

astro-ph↗

arXiv1999

Are Recent Peculiar Velocity Surveys Consistent?

We compare the bulk flow of the SMAC sample to the predictions of popular cosmological models and to other recent large-scale peculiar velocity surveys. Both analyses account for aliasing of small-scale power due to the sparse and non-uniform sampling of the surveys. We conclude that the SMAC bulk flow is in marginal conflict with flat COBE-normalized Lambda-CDM models which fit the cluster abundance constraint. However, power spectra which are steeper shortward of the peak are consistent with all of the above constraints. When recent large-scale peculiar velocity surveys are compared, we conclude that all measured bulk flows (with the possible exception of that of Lauer & Postman) are consistent with each other given the errors, provided the latter allow for `cosmic covariance'. A rough estimate of the mean bulk flow of all surveys (except Lauer & Postman) is ~400 km/s towards l=270, b=0.

astro-ph↗