Context: We have observed again two stars inside the ZZ Ceti instabilitystrip that were previously classified as not-observed-to-vary (NOV) byMukadam et al. (2004) and found them to be low-amplitude variables. Someevidence points to a pure ZZ Ceti instability strip; other evidencecontests it. Aims: The two stars previously classified as NOVhave Sloan Digital Sky Survey (SDSS) spectroscopic effectivetemperatures that place them inside the ZZ Ceti instability strip, andthey were "contaminating" the strip as constant stars, which couldindicate that the instability strip was no longer a simple evolutionarystage. A pure instability strip indicates that pulsation is a normalphase which all DAs must go through. Methods: We used effectivetemperatures derived from SDSS optical spectra by comparing them withmodel atmospheres to look for pulsators through time-resolved photometryand stars previously classified as NOV. Results: Our new resultsindicate, but do not prove, a pure instability strip, because there arestill other NOV stars that need to be observed again. Additionally, wehave discovered five other ZZ Ceti stars based on their effectivetemperatures.Partially based on observations at Observatório do Pico dosDias/LNA; the Southern Astrophysical Research telescope, a collaborationbetween CNPq-Brazil, NOAO, UNC, and MSU; and McDonald Observatory of TheUniversity of Texas at Austin.

We present new measurements of the luminosity function (LF) of luminousred galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dFSDSS LRG and Quasar (2SLAQ) survey. We have carefully quantified, andcorrected for, uncertainties in the K and evolutionary corrections,differences in the colour selection methods, and the effects ofphotometric errors, thus ensuring we are studying the same galaxypopulation in both surveys. Using a limited subset of 6326 SDSS LRGs(with 0.17 < z < 0.24) and 1725 2SLAQ LRGs (with 0.5 < z <0.6), for which the matching colour selection is most reliable, we findno evidence for any additional evolution in the LRG LF, over thisredshift range, beyond that expected from a simple passive evolutionmodel. This lack of additional evolution is quantified using thecomoving luminosity density of SDSS and 2SLAQ LRGs, brighter thanM0.2r - 5 log h0.7 = -22.5, which are2.51 +/- 0.03 × 10-7LsolarMpc-3and 2.44 +/- 0.15 ×10-7LsolarMpc-3, respectively (<10per cent uncertainty). We compare our LFs to the COMBO-17 data and findexcellent agreement over the same redshift range. Together, thesesurveys show no evidence for additional evolution (beyond passive) inthe LF of LRGs brighter than M0.2r - 5 logh0.7 = -21 (or brighter than ~L*). We test our SDSS and 2SLAQLFs against a simple `dry merger' model for the evolution of massive redgalaxies and find that at least half of the LRGs at z ~= 0.2 mustalready have been well assembled (with more than half their stellarmass) by z ~= 0.6. This limit is barely consistent with recent resultsfrom semi-analytical models of galaxy evolution.

We present a spectroscopic survey of almost 15000 candidateintermediate-redshift luminous red galaxies (LRGs) brighter than i =19.8, observed with 2dF on the Anglo-Australian Telescope. The targetswere selected photometrically from the Sloan Digital Sky Survey (SDSS)and lie along two narrow equatorial strips covering 180 deg2.Reliable redshifts were obtained for 92 per cent of the targets and theselection is very efficient: over 90 per cent have 0.45 < z < 0.8.More than 80 per cent of the ~11000 red galaxies have pureabsorption-line spectra consistent with a passively evolving old stellarpopulation. The redshift, photometric and spatial distributions of theLRGs are described. The 2SLAQ data will be released publicly frommid-2006, providing a powerful resource for observational cosmology andthe study of galaxy evolution.

We present a detailed investigation into the recent star formationhistories of 5697 luminous red galaxies (LRGs) based on the Hδ(4101 Å), and [OII] (3727 Å) lines and the D4000 index. LRGsare luminous (L > 3L*) galaxies which have been selected to havephotometric properties consistent with an old, passively evolvingstellar population. For this study, we utilize LRGs from the recentlycompleted 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of theHδ and [OII] lines are measured and used to define three spectraltypes, those with only strong Hδ absorption (k+a), those withstrong [OII] in emission (em) and those with both (em+a). All other LRGsare considered to have passive star formation histories. The vastmajority of LRGs are found to be passive (~80 per cent); however,significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and emLRGs (8.6 per cent) are identified. An investigation into the redshiftdependence of the fractions is also performed. A sample of SDSS MAINgalaxies with colours and luminosities consistent with the 2SLAQ LRGs isselected to provide a low-redshift comparison. While the em and em+afractions are consistent with the low-redshift SDSS sample, the fractionof k+a LRGs is found to increase significantly with redshift. Thisresult is interpreted as an indication of an increasing amount of recentstar formation activity in LRGs with redshift. By considering theexpected lifetime of the k+a phase, the number of LRGs which willundergo a k+a phase can be estimated. A crude comparison of thisestimate with the predictions from semi-analytic models of galaxyformation shows that the predicted level of k+a and em+a activities isnot sufficient to reconcile the predicted mass growth for massive earlytypes in a hierarchical merging scenario.

We study the distribution of Eddington luminosity ratios,Lbol/LEdd, of active galactic nuclei (AGNs)discovered in the AGN and Galaxy Evolution Survey (AGES). We combineHβ, Mg II, and C IV line widths with continuum luminosities toestimate black hole (BH) masses in 407 AGNs, covering the redshift rangez~0.3-4 and the bolometric luminosity rangeLbol~1045-1047 ergs s-1. Thesample consists of X-ray or mid-infrared (24 μm) point sources withoptical magnitude R<=21.5 mag and optical emission-line spectracharacteristic of AGNs. For the range of luminosity and redshift probedby AGES, the distribution of estimated Eddington ratios is welldescribed as log-normal, with a peak atLbol/LEdd~=1/4 and a dispersion of 0.3 dex. Sinceadditional sources of scatter are minimal, this dispersion must accountfor contributions from the scatter between estimated and true BH massand the scatter between estimated and true bolometric luminosity.Therefore, we conclude that (1) neither of these sources of error cancontribute more than ~0.3 dex rms, and (2) the true Eddington ratios ofoptically luminous AGNs are even more sharply peaked. Because the massestimation errors must be smaller than ~0.3 dex, we can also investigatethe distribution of Eddington ratios at fixed BH mass. We show for thefirst time that the distribution of Eddington ratios at fixed BH mass ispeaked, and that the dearth of AGNs at a factor of ~10 below Eddingtonis real and not an artifact of sample selection. These results providestrong evidence that supermassive BHs gain most of their mass whileradiating close to the Eddington limit, and they suggest that thefueling rates in luminous AGNs are ultimately determined by BHself-regulation of the accretion flow rather than galactic-scaledynamical disturbances.Observations reported here were obtained at the MMT Observatory (MMTO),a joint facility of the University of Arizona and the SmithsonianInstitution.

Using a sample of nearly 20,000 massive early-type galaxies selectedfrom the Sloan Digital Sky Survey, we study the color-magnitude relationfor the most luminous (L>~2.2L*) field galaxies in theredshift range 0.1

We present a catalog of 9316 spectroscopically confirmed white dwarfsfrom the Sloan Digital Sky Survey Data Release 4. We have selected thestars through photometric cuts and spectroscopic modeling, backed up bya set of visual inspections. About 6000 of the stars are newdiscoveries, roughly doubling the number of spectroscopically confirmedwhite dwarfs. We analyze the stars by performing temperature and surfacegravity fits to grids of pure hydrogen and helium atmospheres. Among therare outliers are a set of presumed helium-core DA white dwarfs withestimated masses below 0.3 Msolar, including two candidatesthat may be the lowest-mass yet found. We also present a list of 928 hotsubdwarfs.

We present a spectroscopic sample of 747 detached close binary systemsfrom the Sloan Digital Sky Survey (SDSS) Fourth Data Release. Themajority of these binaries consist of a white dwarf primary and alow-mass secondary (typically M dwarf) companion. We have determined thetemperature and gravity for 496 of the white dwarf primaries and thespectral type and magnetic activity properties for 661 of the low-masssecondaries. We have estimated the distances for each of the whitedwarf-main-sequence star binaries and use white dwarf evolutionary gridsto establish the age of each binary system from the white dwarf coolingtimes. With respect to a spectroscopically identified sample of ~8000isolated M dwarf stars in the SDSS, the M dwarf secondaries showenhanced activity with a higher active fraction at a given spectraltype. The white dwarf temperatures and gravities are similar to thedistribution of ~1900 DA white dwarfs from the SDSS. The ages of thebinaries in this study range from ~0.5 Myr to nearly 3 Gyr (average age~0.20 Gyr).

We present results from Chandra observations of SDSS J1004+4112, astrongly lensed quasar system with a maximum image separation of 15".All four bright images of the quasar, as well as resolved X-ray emissionoriginating from the lensing cluster, are clearly detected. The emissionfrom the lensing cluster extends out to approximately 1.5 arcmin. Wemeasure the bolometric X-ray luminosity and temperature of the lensingcluster to be 4.7×1044 ergs s-1 and 6.4 keV,consistent with the luminosity-temperature relation for distantclusters. The mass estimated from the X-ray observation shows excellentagreement with the mass derived from gravitational lensing. The X-rayflux ratios of the quasar images differ markedly from the optical fluxratios, and the combined X-ray spectrum of the images possesses anunusually strong Fe Kα emission line, both of which are indicativeof microlensing.

We measure the large-scale real-space power spectrum P(k) using luminousred galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) and use thismeasurement to sharpen constraints on cosmological parameters from theWilkinson Microwave Anisotropy Probe (WMAP). We employ a matrix-basedpower spectrum estimation method using Pseudo-Karhunen-Loèveeigenmodes, producing uncorrelated minimum-variance measurements in 20k-bands of both the clustering power and its anisotropy due toredshift-space distortions, with narrow and well-behaved windowfunctions in the range 0.01h/Mpcm and the baryon fraction in goodagreement with WMAP. Within the context of flat ΛCDM models, ourLRG measurements complement WMAP by sharpening the constraints on thematter density, the neutrino density and the tensor amplitude by about afactor of 2, giving Ωm=0.24±0.02 (1σ),

We report the discovery of eleven new ZZ Cetis using telescopes at OPD(Observatório do Pico dos Dias/LNA) in Brazil, the 4.1 m SOAR(Southern Astrophysical Research) telescope at Cerro Pachon, Chile, andthe 2.1 m Otto Struve telescope at McDonald observatory. The candidateswere selected from the SDSS (Sloan Digital Sky Survey) and SPY (ESO SNIa progenitor survey), based on their Teff obtained fromoptical spectra fitting. This selection criterion yields the highest

We present the discovery of three z>5 quasars in the AGN and GalaxyEvolution Survey spectroscopic observations of the NOAO Deep Wide-FieldSurvey (NDWFS) Bootes Field. These quasars were selected as part of alarger Spitzer mid-infrared quasar sample, with no selection based onoptical colors. The highest redshift object, NDWFS J142516.3+325409, atz=5.85, is the lowest luminosity z>5.8 quasar currently known. Wecompare mid-infrared techniques for identifying z>5 quasars to moretraditional optical techniques and show that mid-infrared colors allowfor the selection of high-redshift quasars even at redshifts at whichquasars lie near the optical stellar locus and at z>7, where opticalselection is impossible. Using the superb multiwavelength coverageavailable in the NDWFS Bootes field, we construct the spectral energydistributions (SEDs) of high-redshift quasars from observedBW band to 24 μm (rest-frame 600 Å-3.7 μm). Weshow that the three high-redshift quasars have quite similar SEDs, andthe rest-frame composite SED of low-redshift quasars from the literatureshows little evolution compared to our high-redshift objects. We comparethe number of z>5 quasars we have discovered to the expected numberfrom published quasar luminosity functions. While analyses of the quasarluminosity function are tenuous based on only three objects, we findthat a relatively steep luminosity function with Ψ~L-3.2provides the best agreement with the number of high-redshift quasarsdiscovered in our survey.

This paper describes the Fourth Data Release of the Sloan Digital SkySurvey (SDSS), including all survey-quality data taken through 2004June. The data release includes five-band photometric data for 180million objects selected over 6670 deg2 and 673,280 spectraof galaxies, quasars, and stars selected from 4783 deg2 ofthose imaging data using the standard SDSS target selection algorithms.

We present ugriz photometry and optical spectroscopy for 28 DB and DOwhite dwarfs with temperatures between 28,000 and 45,000 K. About 10 ofthese are particularly well observed; the remainder are candidates.These are the hottest DB stars yet found, and they populate the ``DBgap'' between the hotter DO stars and the familiar DB stars cooler than30,000 K. Nevertheless, after carefully matching the survey volumes wefind that the ratio of DA stars to DB stars is a factor of 2.5 larger at30,000 than at 20,000 K, suggesting that the ``DB gap'' is indeeddeficient and that some kind of atmospheric transformation takes place

We present a spectroscopic survey using the MMT Hectospec fiberspectrograph of 24 μm sources selected with the Spitzer SpaceTelescope in the Spitzer First Look Survey. We report 1296 new redshiftsfor 24 μm sources, including 599 with fν(24μm)>=1 mJy. Combined with 291 additional redshifts for sourcesfrom the Sloan Digital Sky Survey (SDSS), our observing program was

Accurate photometric redshifts are calculated for nearly 200,000galaxies to a 4.5 μm flux limit of ~13 μJy in the 8.5deg2 Spitzer IRAC Shallow Survey. Using a hybrid photometricredshift algorithm incorporating both neural net and template-fittingtechniques, calibrated with over 15,000 spectroscopic redshifts, a

Using high-resolution cosmological simulations, we study hydrogen andhelium gravitational cooling radiation from gas accretion by younggalaxies. We focus on the He II cooling lines, which arise from gas witha different temperature history (Tmax~105 K) thanH I line-emitting gas. We examine whether three major atomic coolinglines, H I λ1216, He II λ1640, and He II λ304, areobservable, finding that Lyα and He II λ1640 coolingemission at z=2-3 are potentially detectable with deep narrowband(R>100) imaging and/or spectroscopy from the ground. While theexpected strength of H I λ1216 cooling emission depends stronglyon the treatment of the self-shielded phase of the IGM in thesimulations, our predictions for the He II λ1640 line are morerobust, because the He II emissivity is negligible belowT~104.5 K and less sensitive to the UV background. AlthoughHe II λ1640 cooling emission is fainter than Lyα by atleast a factor of 10 and, unlike Lyα, might not be resolvedspatially with current observational facilities, it is more suitable tostudy gas accretion in the galaxy formation process because it isoptically thin and less contaminated by the recombination lines fromstar-forming galaxies. The He II λ1640 line can be used todistinguish among mechanisms for powering the so-called Lyαblobs-including gravitational cooling radiation, photoionization bystellar populations, and starburst-driven superwinds-because (1) He IIλ1640 emission is limited to very low metallicity[log(Z/Zsolar)<~-5.3] and Population III stars and (2) theblob's kinematics are probed unambiguously through the He II line width,which for cooling radiation is narrower (σ<400 kms-1) than typical wind speeds.

We present the first data release of the Radial Velocity Experiment(RAVE), an ambitious spectroscopic survey to measure radial velocitiesand stellar atmosphere parameters (temperature, metallicity, and surfacegravity) of up to one million stars using the Six Degree Fieldmultiobject spectrograph on the 1.2 m UK Schmidt Telescope of theAnglo-Australian Observatory. The RAVE program started in 2003,obtaining medium-resolution spectra (median R=7500) in the Ca-tripletregion (8410-8795 Å) for southern hemisphere stars drawn from theTycho-2 and SuperCOSMOS catalogs, in the magnitude range 92. The average signal-to-noise ratio of

We investigate the relationship between the colors, luminosities, andenvironments of galaxies in the Sloan Digital Sky Survey spectroscopicsample, using environmental measurements on scales ranging from 0.2 to 6h-1 Mpc. We find that (1) the relationship between color andenvironment persists even to the lowest luminosities we probe(Mr-5log10h~-14) (2) at luminosities and colors for which thegalaxy correlation function has a large amplitude, it also has a steepslope; and (3) in regions of a given overdensity on small scales (1h-1 Mpc), the overdensity on large scales (6 h-1Mpc) does not appear to relate to the recent star formation history ofthe galaxies. Of these results, the last has the most immediateapplication to galaxy formation theory. In particular, it lends supportto the notion that a galaxy's properties are related only to the mass ofits host dark matter halo, and not to the larger scale environment.Based on observations obtained with the Sloan Digital Sky Survey.

We report follow-up observations of two gravitational lens candidatesidentified in the Sloan Digital Sky Survey (SDSS) data set. We haveconfirmed that SDSS J102111.02+491330.4 is a previously unknowngravitationally lensed quasar. This lens system exhibits two images of az=1.72 quasar, with an image separation of 1.14"+/-0.04". Optical andnear-IR imaging of the system reveals the presence of the lensing galaxybetween the two quasar images. Observations of SDSS J112012.12+671116.0indicate that it is more likely a binary quasar than a gravitationallens. This system has two quasars at a redshift of z=1.49, with anangular separation of 1.49"+/-0.02". However, the two quasars havemarkedly different spectral energy distributions, and no lens galaxy isapparent in optical and near-IR images of this system. We also present alist of 31 SDSS lens candidates that follow-up observations haveconfirmed are not gravitational lenses.Observations reported here were obtained at the MMT Observatory, a jointfacility of the University of Arizona and the Smithsonian Institution.