We present measurements of the Hubble diagram for 103 Type Ia supernovae (SNe) with redshifts 0.04 < z < 0.42, discovered during the first season (Fall 2005) of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey. These data fill in the redshift "desert" between low-and high-redshift SN Ia surveys. Within the framework of the mlcs2k2 light-curve fitting method, we use the SDSS-II SN sample to infer the mean reddening parameter for host galaxies, R V = 2.18 ± 0.14 stat ± 0.48 syst , and find that the intrinsic distribution of host-galaxy extinction is well fitted by an exponential function, P (A V) = exp(−A V /τ V), with τ V = 0.334 ± 0.088 mag. We combine the SDSS-II measurements with new distance estimates for published SN data from the ESSENCE survey, the Supernova Legacy Survey (SNLS), the Hubble Space Telescope (HST), and a compilation of Nearby SN Ia measurements. A new feature in our analysis is the use of detailed Monte Carlo simulations of all surveys to account for selection biases, including those from spectroscopic targeting. Combining the SN Hubble diagram with measurements of baryon acoustic oscillations from the SDSS Luminous Red Galaxy sample and with cosmic microwave background temperature anisotropy measurements from the Wilkinson Microwave Anisotropy Probe, we estimate the cosmological parameters w and Ω M , assuming a spatially flat cosmological model (FwCDM) with constant dark energy equation of state parameter, w. We also consider constraints upon Ω M and Ω Λ for a cosmological constant model (ΛCDM) with w = −1 and non-zero spatial curvature. For the FwCDM model and the combined sample of 288 SNe Ia,

We present constraints on the dark energy equation-of-state parameter, w ¼ P/( c 2 ), using 60 SNe Ia from the ESSENCE supernova survey. We derive a set of constraints on the nature of the dark energy assuming a flat universe. By including constraints on ( M , w) from baryon acoustic oscillations, we obtain a value for a static equation-of-state parameter w ¼ À1:05 þ0:13 À0:12 (stat 1 ) AE 0:13 (sys) and M ¼ 0:274 þ0:033 À0:020 (stat 1 ) with a bestfit 2 /dof of 0.96. These results are consistent with those reported by the Supernova Legacy Survey from the first year of a similar program measuring supernova distances and redshifts. We evaluate sources of systematic error that afflict supernova observations and present Monte Carlo simulations that explore these effects. Currently, the largest systematic with the potential to affect our measurements is the treatment of extinction due to dust in the supernova host galaxies. Combining our set of ESSENCE SNe Ia with the first-results Supernova Legacy Survey SNe Ia, we obtain a joint constraint of w ¼ À1:07 þ0:09 À0:09 (stat 1 ) AE 0:13 (sys), M ¼ 0:267 þ0:028 À0:018 (stat 1 ) with a best-fit 2 /dof of 0.91. The current global SN Ia data alone rule out empty ( M ¼ 0), matter-only M ¼ 0:3, and M ¼ 1 universes at >4.5 . The current SN Ia data are fully consistent with a cosmological constant.

In addition to optical photometry of unprecedented quality, the Sloan Digital Sky Survey (SDSS) is producing a massive spectroscopic database which already contains over 280,000 stellar spectra. Using effective temperature and metallicity derived from SDSS spectra for ∼60,000 F and G type main sequence stars (0.2 < g − r < 0.6), we develop polynomial models, reminiscent of traditional methods based on the U BV photometry, for estimating these parameters from the SDSS u−g and g−r colors. These estimators reproduce SDSS spectroscopic parameters with a root-mean-square scatter of 100 K for effective temperature, and 0.2 dex for metallicity (limited by photometric errors), which are similar to random and systematic uncertainties in spectroscopic determinations. We apply this method to a photometric catalog of coadded SDSS observations and study the photometric metallicity distribution of ∼200,000 F and G type stars observed in 300 deg 2 of high Galactic latitude sky. These deeper (g < 20.5) and photometrically precise (∼0.01 mag) coadded data enable an accurate measurement of the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, that correspond to disk and halo. The best-fit number ratio of the two components is consistent with that implied by the decomposition of stellar counts profiles into exponential disk and power-law halo components by Jurić et al. (2008). The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component can be modeled as a spatially invariant Gaussian distribution with a mean of [F e/H] = −1.46 and a standard deviation of ∼0.3 dex. The disk metallicity distribution is non-Gaussian, with a remarkably small scatter (rms∼0.16 dex) and the median smoothly decreasing with distance from the plane from −0.6 at 500 pc to −0.8 beyond several kpc. Similarly, we find using proper motion measurements that a non-Gaussian rotational velocity distribution of disk stars shifts by ∼50 km/s as the distance from the plane increases from 500 pc to several kpc. Despite this similarity, the metallicity and rotational velocity distributions of disk stars are not correlated (Kendall's τ = 0.017 ± 0.018). This absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks, which predicts a strong correlation (τ = −0.30 ± 0.04) at ∼1 kpc from the mid-plane. Instead, the variation of the metallicity and rotational velocity distributions can be modeled using non-Gaussian functions that retain their shapes and only shift as the distance from the mid-plane increases. We also study the metallicity distribution using a shallower (g < 19.5) but much larger sample of close to three million stars in 8500 sq. deg. of sky included in SDSS Data Release 6. The large sky coverage enables the detection of...

The first cosmological results from the ESSENCE supernova survey (Wood-Vasey et al. 2007) are extended to a wider range of cosmological models including dynamical dark energy and non-standard cosmological models. We fold in a greater number of external data sets such as the recent Higher-z release of high-redshift supernovae ) as well as several complementary cosmological probes. Model comparison statistics such as the Bayesian and Akaike information criteria are applied to gauge the worth of models. These statistics favor models that give a good fit with fewer parameters.Based on this analysis, the preferred cosmological model is the flat cosmological constant model, where the expansion history of the universe can be adequately described with only one free parameter describing the energy content of the universe. Among the more exotic models that provide good fits to the data, we note a preference for models whose best-fit parameters reduce them to the cosmological constant model.

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u, g, r, i, z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ~ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800--9200 A, with a resolution of 1800--2100. Further characteristics of the data are described, as are the data products themselves.Comment: Submitted to The Astronomical Journal. 16 pages. For associated documentation, see http://www.sdss.org/dr

We describe a standard star catalog constructed using multiple SDSS photometric observations (at least four per band, with a median of 10) in the ugriz system. The catalog includes 1.01 million nonvariable unresolved objects from the equatorial stripe 82 (j J2000:0 j< 1:266 ) in the right ascension range 20 h 34 m Y 4 h 00 m and with the corresponding r-band (approximately Johnson V-band) magnitudes in the range 14Y22. The distributions of measurements for individual sources demonstrate that the photometric pipeline correctly estimates random photometric errors, which are below 0.01 mag for stars brighter than 19.5, 20.5, 20.5, 20, and 18.5 in ugriz, respectively (about twice as good as for individual SDSS runs). Several independent tests of the internal consistency suggest that the spatial variation of photometric zero points is not larger than $0.01 mag (rms). In addition to being the largest available data set with optical photometry internally consistent at the $1% level, this catalog provides a practical definition of the SDSS photometric system. Using this catalog, we show that photometric zero points for SDSS observing runs can be calibrated within a nominal uncertainty of 2% even for data obtained through 1 mag thick clouds, and we demonstrate the existence of He and H white dwarf sequences using photometric data alone. Based on the properties of this catalog, we conclude that upcoming large-scale optical surveys such as the Large Synoptic Survey Telescope will be capable of delivering robust 1% photometry for billions of sources.

We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, and contains 58 million photometric observations in the SDSS ugriz system for 1.4 million unresolved sources. In each photometric bandpass we compute various low-order lightcurve statistics and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g=20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ~2 million low-redshift quasars, and will discover at least 50 million variable stars.Comment: 41 pages, 14 figures, submitted to Astronomical Journa

We present the results from a survey of i-dropout objects selected from D1550 deg2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at Objects with z Z 5.8. i*[z* [ 2.2 and z* \ 20.2 are selected, and follow-up J-band photometry is used to separate L-and T-type cool dwarfs from high-redshift quasars. We describe the discovery of three new quasars, SDSSp J083643.85]005453.3 (z \ 5.82), J130608.26]035626.3 (z \ 5.99), and J103027.10]052455.0 (z \ 6.28). The quasar SDSSp J083643.85]005453.3 is a radio source with Ñux of 1.1 mJy at 20 cm. The spectra of all three quasars show strong and broad Lya ] N V emission lines and very strong Lya forest absorption, with a mean continuum decrementThe ARC 3.5 m spectrum of SDSSp D A[ 0.90. J103027.10]052455.0 shows that over a range of D300 immediately blueward of the Lya emission, Ó the average transmitted Ñux is only 0.003^0.020 times that of the continuum level, consistent with zero Ñux over a D300 range of the Lya forest region and suggesting a tentative detection of the complete Ó Gunn-Peterson trough. The existence of strong metal lines in the quasar spectra suggests early metal enrichment in the quasar environment. The three new objects, together with the previously published z \ 5.8 quasar SDSSp J104433.04[012502.2, form a complete color-selected Ñux-limited sample at z Z 5.8. We estimate the selection function of this sample, taking into account the estimated variations in the quasar spectral energy distribution, as well as observational photometric errors. We Ðnd that at z \ 6, 2 This paper is dedicated to the memory of Arthur Davidsen, who was a pioneer of the study of the intergalactic medium and who showed great leadership as chairman of the Advisory Council of the Sloan Digital Sky Survey.3 Institute for Advanced Study, Olden Lane, Princeton, NJ 08540. 4 Princeton University Observatory, Princeton, NJ 08544. . This is a factor of D2 lower than that at z D 5 and is consistent with an extrapolation of the observed quasar evolution at z \ 5. Using the current sample, we discuss the constraint on the shape of the quasar luminosity function and the implications for the contribution of quasars to the ionizing background at z D 6. The luminous quasars discussed in the paper have central black hole masses of several times 109 by the Eddington argument, with likely dark halo masses on the order M _ of 1013Their observed space density provides a sensitive test of models of quasar and galaxy for-M _ . mation at high redshift.