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Z. Ivezic

Publications and source records attributed to Z. Ivezic.

At least 55 records · Page 3Linked to original sources
arXiv2005

A sample of radio-loud AGN in the Sloan Digital Sky Survey

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. The comparison is carried out in a multi-stage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multi-component radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95% completeness and 98.9% reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud AGN and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000Ang break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star forming galaxies brighter than 5mJy at 1.4GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-IR correlation, the radio luminosity function of star forming galaxies is also compared to the luminosity function derived in the far-infrared. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to far-IR correlation breaks down below about 10^22 W/Hz at 1.4GHz.

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arXiv2005

Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies

We present the large-scale correlation function measured from a spectroscopic sample of 46,748 luminous red galaxies from the Sloan Digital Sky Survey. The survey region covers 0.72 h^{-3} Gpc^3 over 3816 square degrees and 0.16<z<0.47, making it the best sample yet for the study of large-scale structure. We find a well-detected peak in the correlation function at 100h^{-1} Mpc separation that is an excellent match to the predicted shape and location of the imprint of the recombination-epoch acoustic oscillations on the low-redshift clustering of matter. This detection demonstrates the linear growth of structure by gravitational instability between z=1000 and the present and confirms a firm prediction of the standard cosmological theory. The acoustic peak provides a standard ruler by which we can measure the ratio of the distances to z=0.35 and z=1089 to 4% fractional accuracy and the absolute distance to z=0.35 to 5% accuracy. From the overall shape of the correlation function, we measure the matter density Omega_mh^2 to 8% and find agreement with the value from cosmic microwave background (CMB) anisotropies. Independent of the constraints provided by the CMB acoustic scale, we find Omega_m = 0.273 +- 0.025 + 0.123 (1+w_0) + 0.137 Omega_K. Including the CMB acoustic scale, we find that the spatial curvature is Omega_K=-0.010+-0.009 if the dark energy is a cosmological constant. More generally, our results provide a measurement of cosmological distance, and hence an argument for dark energy, based on a geometric method with the same simple physics as the microwave background anisotropies. The standard cosmological model convincingly passes these new and robust tests of its fundamental properties.

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arXiv2004

SDSS Data Management and Photometric Quality Assessment

We summarize the Sloan Digital Sky Survey data acquisition and processing steps, and describe runQA, a pipeline designed for automated data quality assessment. In particular, we show how the position of the stellar locus in color-color diagrams can be used to estimate the accuracy of photometric zeropoint calibration to better than 0.01 mag in 0.03 deg2 patches. Using this method, we estimate that typical photometric zeropoint calibration errors for SDSS imaging data are not larger than ~0.01 mag in the g, r, and i bands, 0.02 mag in the z band, and 0.03 mag in the u band (root-mean-scatter for zeropoint offsets).

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arXiv2004

Cosmological parameter analysis including SDSS Ly-alpha forest and galaxy bias: constraints on the primordial spectrum of fluctuations, neutrino mass, and dark energy

We combine the constraints from the recent Ly-alpha forest and bias analysis of the SDSS with previous constraints from SDSS galaxy clustering, the latest supernovae, and WMAP . Combining WMAP and the lya we find for the primordial slope n_s=0.98\pm 0.02. We see no evidence of running, dn/d\ln k=-0.003\pm 0.010, a factor of 3 improvement over previous constraints. We also find no evidence of tensors, r<0.36 (95% c.l.). A positive correlation between tensors and primordial slope disfavors chaotic inflation type models with steep slopes: V \propto ϕ^4 is outside the 3-sigma contour. For the amplitude we find sigma_8=0.90\pm 0.03 from the lyaf and WMAP alone. We find no evidence of neutrino mass: for the case of 3 massive neutrino families with an inflationary prior, \sum m_ν<0.42eV and the mass of lightest neutrino is m_1<0.13eV at 95% c.l. For the 3 massless + 1 massive neutrino case we find m_ν<0.79eV for the massive neutrino, excluding at 95% c.l. all neutrino mass solutions compatible with the LSND results. We explore dark energy constraints in models with a fairly general time dependence of dark energy equation of state, finding Omega_lambda=0.72\pm 0.02, w(z=0.3)=-0.98^{+0.10}_{-0.12}, the latter changing to w(z=0.3)=-0.92^{+0.09}_{-0.10} if tensors are allowed. We find no evidence for variation of the equation of state with redshift, w(z=1)=-1.03^{+0.21}_{-0.28}. These results rely on the current understanding of the lyaf and other probes, which need to be explored further both observationally and theoretically, but extensive tests reveal no evidence of inconsistency among different data sets used here.

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arXiv2004

Multiwavelength View of SDSS Galaxies

We summarize the detection rates at wavelengths other than optical for \~99,000 galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 1 ``main'' spectroscopic sample. The analysis is based on positional cross-correlation with source catalogs from ROSAT, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS surveys. We find that the rest-frame UV-IR broad-band galaxy SEDs form a remarkably uniform, nearly one parameter, family. As an example, the SDSS u and r band data, supplemented with redshift, can be used to predict K band magnitudes measured by 2MASS with an rms scatter of only 0.2 mag; when measurement uncertainties are taken into account, the astrophysical scatter appears not larger than ~0.1 mag.

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arXiv2004

Variable Faint Optical Sources Discovered by Comparing POSS and SDSS Catalogs

We present a study of variable faint optical sources discovered by comparing the Sloan Digital Sky Survey (SDSS) and the Palomar Observatory Sky Survey (POSS) catalogs. We use SDSS measurements to photometrically recalibrate several publicly available POSS catalogs; a piecewise recalibration in 100 arcmin2 patches generally results in an improvement of photometric accuracy (rms) by nearly a factor of two, compared to the original data. The POSS I magnitudes can be improved to ~0.15 mag accuracy, and POSS II magnitudes to \~0.10 mag accuracy. We use the recalibrated catalogs for the ~2,000 deg2 of sky in the SDSS Data Release 1 to construct a catalog of ~60,000 sources variable on time scales 10-50 years. A series of statistical tests based on the morphology of SDSS color-color diagrams, as well as visual comparison of images and comparison with repeated SDSS observations, demonstrate the robustness of the selection methods. We quantify the distribution of variable sources in the SDSS color-color diagrams, and the variability characteristics of quasars. We detect a turn-over in quasar structure function which suggests that the characteristic time scale for quasar variability is of the order one year. The long-term (>1 year) quasar variability decreases with luminosity and rest-frame wavelength similarly to the short-term (<1 year) behavior. We also demonstrate that candidate RR Lyrae stars trace the same halo structures, such as the Sgr dwarf tidal stream, that were discovered using repeated SDSS observations. We utilize the POSS-SDSS selected candidates to constrain the halo structure in the parts of sky for which repeated SDSS observations do not exist. (abridged)

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arXiv2004

The Distribution of Quasars and Galaxies in Radio Color-Color and Morphology Diagrams

We positionally match the 6 cm GB6, 20 cm FIRST and NVSS, and 92 cm WENSS radio catalogs and find 16,500 matches in ~3,000 deg2 of sky. Using this unified radio database, we construct radio "color-magnitude-morphology" diagrams and find that they display a clear structure, rather than a random scatter. We propose a simple, yet powerful, method for morphological classification of radio sources based on FIRST and NVSS measurements. For a subset of matched sources, we find optical identifications using the SDSS Data Release 1 catalogs, and separate them into quasars and galaxies. Compact radio sources with flat radio spectra are dominated by quasars, while compact sources with steep spectra, and resolved radio sources, contain substantial numbers of both quasars and galaxies.

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arXiv2004

Color-Induced Displacement double stars in SDSS

We report the first successful application of the astrometric color-induced displacement technique (CID, the displacement of the photocenter between different bandpasses due to a varying contribution of differently colored components to the total light), originally proposed by Wielen (1996) for discovering unresolved binary stars. Using the Sloan Digital Sky Survey (SDSS) Data Release 1 with 2.5 million stars brighter than 21m in the u and g bands, we select 419 candidate binary stars with CID greater than 0.5 arcsec. The SDSS colors of the majority of these candidates are consistent with binary systems including a white dwarf and any main sequence star with spectral type later than ~K7. The astrometric CID method discussed here is complementary to the photometric selection of binary stars in SDSS discussed by Smolcic et al. (2004), but there is considerable overlap (15%) between the two samples of selected candidates. This overlap testifies both to the physical soundness of both methods, as well as to the astrometric and photometric quality of SDSS data.

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arXiv2004

A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars

We report the discovery of a locus of stars in the SDSS g-r vs. u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1:2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1. The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude--color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al. (1995).

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arXiv2003

A Gravitationally Lensed Quasar with Quadruple Images Separated by 14.62 Arcseconds

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004+4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

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arXiv2003

Counts of Low-redshift SDSS Quasar Candidates

We analyze the counts of low-redshift quasar candidates selected using nine-epoch SDSS imaging data. The co-added catalogs are more than 1 mag deeper than single-epoch SDSS data, and allow the selection of low-redshift quasar candidates using UV-excess and also variability techniques. The counts of selected candidates are robustly determined down to g=21.5. This is about 2 magnitudes deeper than the position of a change in the slope of the counts reported by Boyle et al. (1990, 2000) for a sample selected by UV-excess, and questioned by Hawkins & Veron (1995), who utilized a variability-selected sample. Using SDSS data, we confirm a change in the slope of the counts for both UV-excess and variability selected samples, providing strong support for the Boyle et al. results.

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arXiv2003

Quasar Radio Dichotomy: Two Peaks, or not Two Peaks, that is the Question

Recent claims by Ivezic et al. (2002) that the distribution of the radio-to-optical flux ratio, R, for quasars is bimodal (the so-called quasar radio dichotomy) were questioned on statistical grounds by Cirasuolo et al. (2003). We apply the approach suggested by Cirasuolo et al. to a sample of \~10,000 objects detected by SDSS and FIRST, and find support for the quasar radio dichotomy. The discrepancy between the claims by Cirasuolo et al. and the results presented here is most likely because 1) the ~100 times larger sample based on two homogeneous surveys that is used here allows a direct determination of the R distribution, rather than relying on indirect inferences based on Monte Carlo simulations of several heterogeneous surveys 2) the accurate SDSS colors and redshift information allow robust determination of the K-correction for R, which, if unaccounted for, introduces significant scatter that masks the intrinsic properties of the quasar R distribution.

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arXiv2003

The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey

Using a sample of over 25000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest frame optical/UV regime depends upon rest frame time lag, luminosity, rest wavelength, redshift, the presence of radio and X-ray emission, and the presence of broad absorption line systems. The time dependence of variability (the structure function) is well-fit by a single power law on timescales from days to years. There is an anti-correlation of variability amplitude with rest wavelength, and quasars are systematically bluer when brighter at all redshifts. There is a strong anti-correlation of variability with quasar luminosity. There is also a significant positive correlation of variability amplitude with redshift, indicating evolution of the quasar population or the variability mechanism. We parameterize all of these relationships. Quasars with RASS X-ray detections are significantly more variable (at optical/UV wavelengths) than those without, and radio loud quasars are marginally more variable than their radio weak counterparts. We find no significant difference in the variability of quasars with and without broad absorption line troughs. Models involving multiple discrete events or gravitational microlensing are unlikely by themselves to account for the data. So-called accretion disk instability models are promising, but more quantitative predictions are needed.

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arXiv2003

Quantifying the bimodal color-magnitude distribution of galaxies

We analyse the bivariate distribution, in color versus absolute magnitude (u-r vs. M_r), of a low redshift sample of galaxies from the Sloan Digital Sky Survey (SDSS; 2400 deg^2, 0.004<z<0.08, -23.5<M_r<-15.5). We trace the bimodality of the distribution from luminous to faint galaxies by fitting double-Gaussians to the color functions separated in absolute magnitude bins. Color-magnitude (CM) relations are obtained for red and blue distributions (early- and late-type, predominantly field, galaxies) without using any cut in morphology. Instead, the analysis is based on the assumption of normal Gaussian distributions in color. We find that the CM relations are well fit by a straight line plus a tanh function. Both relations can be described by a shallow CM trend (slopes of about -0.04, -0.05) plus a steeper transition in the average galaxy properties over about two magnitudes. The midpoints of the transitions (M_r=-19.8 and -20.8 for the red and blue distributions, respectively) occur around 2x10^10 M_solar after converting luminosities to stellar mass. Separate luminosity functions are obtained for the two distributions. The red distribution has a more luminous characteristic magnitude and a shallower faint-end slope (M^*=-21.5, alpha=-0.8) compared to the blue distribution (alpha=-1.3 depending on the parameterization). These are approximately converted to galaxy stellar mass functions. The red distribution galaxies have a higher number density per magnitude for masses greater than about 3x10^10 M_solar. Using a simple merger model, we show that the differences between the two functions are consistent with the red distribution being formed from major galaxy mergers.

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arXiv2003

Candidate Type II Quasars from the Sloan Digital Sky Survey: I. Selection and Optical Properties of a Sample at 0.3<Z<0.83

Type II quasars are the long-sought luminous analogs of type II (narrow emission line) Seyfert galaxies, suggested by unification models of active galactic nuclei (AGN) and postulated to account for an appreciable fraction of the cosmic hard X-ray background. We present a sample of 291 type II AGN at redshifts 0.3<Z<0.83 from the spectroscopic data of the Sloan Digital Sky Survey. These objects have narrow (FWHM<2000 km/s), high equivalent width emission lines with high-ionization line ratios. We describe the selection procedure and discuss the optical properties of the sample. About 50% of the objects have [OIII] λ_air 5007 line luminosities in the range 3\times 10^8-10^10 L_Sun, comparable to those of luminous (-27<M_B<-23) quasars; this, along with other evidence, suggests that the objects in the luminous subsample are type II quasars.

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arXiv2003

Star formation rate indicators in the Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) first data release provides a database of 106000 unique galaxies in the main galaxy sample with measured spectra. A sample of star-forming (SF) galaxies are identified from among the 3079 of these having 1.4 GHz luminosities from FIRST, by using optical spectral diagnostics. Using 1.4 GHz luminosities as a reference star formation rate (SFR) estimator insensitive to obscuration effects, the SFRs derived from the measured SDSS Halpha, [OII] and u-band luminosities, as well as far-infrared luminosities from IRAS, are compared. It is established that straightforward corrections for obscuration and aperture effects reliably bring the SDSS emission line and photometric SFR estimates into agreement with those at 1.4 GHz, although considerable scatter (~60%) remains in the relations. It thus appears feasible to perform detailed investigations of star formation for large and varied samples of SF galaxies through the available spectroscopic and photometric measurements from the SDSS. We provide herein exact prescriptions for determining the SFR for SDSS galaxies. The expected strong correlation between [OII] and Halpha line fluxes for SF galaxies is seen, but with a median line flux ratio F_[OII]/F_Halpha=0.23, about a factor of two smaller than that found in the sample of Kennicutt (1992). This correlation, used in deriving the [OII] SFRs, is consistent with the luminosity-dependent relation found by Jansen et al. (2001). The median obscuration for the SDSS SF systems is found to be A_Halpha=1.2 mag, while for the radio detected sample the median obscuration is notably higher, 1.6 mag, and with a broader distribution.

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arXiv2003

Color Variability of Asteroids in SDSS Moving Object Catalog

We report a detection of statistically significant color variations for a sample of 7,531 multiply observed asteroids that are listed in the Sloan Digital Sky Survey Moving Object Catalog. Using 5-band photometric observations accurate to ~0.02 mag, we detect color variations in the range 0.06-0.11 mag (rms). These variations appear uncorrelated with asteroids physical characteristics such as diameter (in the probed 1-10 km range), taxonomic class, and family membership. Despite such a lack of correlation, which implies a random nature for the cause of color variability, a suite of tests suggest that the detected variations are not instrumental effects. In particular, the observed color variations are incompatible with photometric errors, and, for objects observed at least four times, the color change in the first pair of observations is correlated with the color change in the second pair. These facts strongly suggest that the observed effect is real, and also indicate that for some asteroids color variations are larger than for others. The detected color variations can be explained as due to inhomogeneous albedo distribution over an asteroid surface. Although relatively small, these variations suggest that fairly large patches with different color than their surroundings exist on a significant fraction of asteroids. This conclusion is in agreement with spatially resolved color images of several large asteroids obtained by NEAR spacecraft and HST.

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