arXiv subjects

Zeljko Ivezic

Publications and source records attributed to Zeljko Ivezic.

At least 127 records · Page 7Linked to original sources

arXiv2003

Hdelta-Selected Galaxies in the Sloan Digital Sky Survey I: The Catalog

[Abridged] We present here a new and homogeneous sample of 3340 galaxies selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed strength of their Hdelta absorption line. These galaxies are commonly known as ``post-starburst'' or ``E+A'' galaxies, and the study of these galaxies has been severely hampered by the lack of a large, statistical sample of such galaxies. In this paper, we rectify this problem by selecting a sample of galaxies which possess an absorption Hdelta equivalent width of EW(Hdelta_max) - Delta EW(Hdelta_max) > 4A from 106682 galaxies in the SDSS. We have performed extensive tests on our catalog including comparing different methodologies of measuring the Hdelta absorption and studying the effects of stellar absorption, dust extinction, emission-filling and measurement error. The measured abundance of our Hdelta-selected (HDS) galaxies is 2.6 +/- 0.1% of all galaxies within a volume-limited sample of 0.05<z<0.1 and M(r*)<-20.5, which is consistent with previous studies of such galaxies in the literature. We find that only 25 of our HDS galaxies in this volume-limited sample (3.5+/-0.7%) show no evidence for OII and Halpha emission, thus indicating that true E+A (or k+a) galaxies are extremely rare objects at low redshift, i.e., only 0.09+/-0.02% of all galaxies in this volume-limited sample are true E+A galaxies. In contrast, 89+/-5% of our HDS galaxies in the volume-limited sample have significant detections of the OII and Halpha emission lines. We find 27 galaxies in our volume-limited HDS sample that possess no detectable OII emission, but do however possess detectable Halpha emission. These galaxies may be dusty star-forming galaxies. We provide the community with this new catalog of Hdelta-selected galaxies to aid in the understanding of these galaxies.

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arXiv2003

A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6

We present the discovery of three new quasars at z>6 in 1300 deg^2 of SDSS imaging data, J114816.64+525150.3 (z=6.43), J104845.05+463718.3 (z=6.23) and J163033.90+401209.6 (z=6.05). The first two objects have weak Ly alpha emission lines; their redshifts are determined from the positions of the Lyman break. They are only accurate to 0.05 and could be affected by the presence of broad absorption line systems. The last object has a Ly alpha strength more typical of lower redshift quasars. Based on a sample of six quasars at z>5.7 that cover 2870 deg^2 presented in this paper and in Paper I, we estimate the comoving density of luminous quasars at z 6 and M_{1450} < -26.8 to be (8 +/- 3)x10^{-10} Mpc^{-3} (for H_0 = 50 km/s/Mpc, Omega = 1). HST imaging of two z>5.7 quasars and high-resolution ground-based images (seeing 0.4'') of three additional z>5.7 quasars show that none of them is gravitationally lensed. The luminosity distribution of the high-redshfit quasar sample suggests the bright end slope of the quasar luminosity function at z 6 is shallower than Psi L^{-3.5} (2-sigma), consistent with the absence of strongly lensed objects.

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arXiv2003

The Sloan Digital Sky Survey: The Cosmic Spectrum and Star-Formation History

We present a determination of the `Cosmic Optical Spectrum' of the Universe, i.e. the ensemble emission from galaxies, as determined from the red-selected Sloan Digital Sky Survey main galaxy sample and compare with previous results of the blue-selected 2dF Galaxy Redshift Survey. Broadly we find good agreement in both the spectrum and the derived star-formation histories. If we use a power-law star-formation history model where star-formation rate $\propto (1+z)^β$ out to z=1, then we find that $β$ of 2 to 3 is still the most likely model and there is no evidence for current surveys missing large amounts of star formation at high redshift. In particular `Fossil Cosmology' of the local universe gives measures of star-formation history which are consistent with direct observations at high redshift. Using the photometry of SDSS we are able to derive the cosmic spectrum in absolute units (i.e.$ W Å$^{-1}$ Mpc$^{-3}$) at 2--5Åresolution and find good agreement with published broad-band luminosity densities. For a Salpeter IMF the best fit stellar mass/light ratio is 3.7--7.5 $\Msun/\Lsun$ in the r-band (corresponding to $\omstars h = 0.0025$--0.0055) and from both the stellar emission history and the H$α$ luminosity density independently we find a cosmological star-formation rate of 0.03--0.04 h $\Msun$ yr$^{-1}$ Mpc$^{-3}$ today.

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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.

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arXiv2002

Astrometric Calibration of the Sloan Digital Sky Survey

The astrometric calibration of the Sloan Digital Sky Survey is described. For point sources brighter than r ~ 20 the astrometric accuracy is 45 milliarcseconds (mas) rms per coordinate when reduced against the USNO CCD Astrograph Catalog, and 75 mas rms when reduced against Tycho-2, with an additional 20 - 30 mas systematic error in both cases. The rms errors are dominated by anomalous refraction and random errors in the primary reference catalogs. The relative astrometric accuracy between the r filter and each of the other filters (u g i z) is 25 - 35 mas rms. At the survey limit (r ~ 22), the astrometric accuracy is limited by photon statistics to approximately 100 mas rms for typical seeing. Anomalous refraction is shown to contain components correlated over two or more degrees on the sky.

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arXiv2002

Stellar Masses and Star Formation Histories for 10^5 Galaxies from the Sloan Digital Sky Survey

We develop a new method to constrain the star formation histories, dust attenuation and stellar masses of galaxies. It is based on two stellar absorption line indices, the 4000 Angstrom break strength and the Balmer absorption line index Hdelta_A. Together, these indices allow us to constrain the mean stellar ages of galaxies and the fractional stellar mass formed in bursts over the past few Gyr. A comparison with broad band photometry then yields estimates of dust attenuation and of stellar mass. We generate a large library of Monte Carlo realizations of different star formation histories, including starbursts of varying strength and a range of metallicities. We use this library to generate median likelihood estimates of burst mass fractions, dust attenuation strengths, stellar masses and stellar mass-to-light ratios for a sample of 122,208 galaxies drawn from the Sloan Digital Sky Survey. The typical 95% confidence range in our estimated stellar masses is +-40%. We study how the stellar mass-to-light ratios of galaxies vary as a function of absolute magnitude, concentration index and photometric pass-band and how dust attenuation varies as a function of absolute magnitude and 4000 Angstrom break strength. We also calculate how the total stellar mass of the present Universe is distributed over galaxies as a function of their mass, size, concentration, colour, burst mass fraction and surface mass density. We find that most of the stellar mass in the local Universe resides in galaxies that have stellar masses \~5\times 10^10 M_sol, half light radii ~3 kpc, and half- light surface mass densities ~10^9 M_sol/kpc^2. The distribution of D(4000) is strongly bimodal, showing a clear division between galaxies dominated by old stellar populations and galaxies with more recent star formation.

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arXiv2002

SDSS Survey for Resolved Milky Way Satellite Galaxies II: High-Velocity Clouds in the Early Data Release

In this paper, we limit the stellar content of 13 high-velocity clouds (HVCs; including 1 compact HVC) using the detection limits of a new survey for resolved Milky Way satellite galaxies in the Sloan Digital Sky Survey Early Data Release (EDR). Our analysis is sensitive to stellar associations within the virial radius of the Milky Way that are up to 50 times fainter than the faintest known Milky Way satellites. Statistically, we find no stellar overdensity associated with any of the clouds. These non-detections suggest lower limits of M_{HI}/L_V that range between ~ 0.2 and 250, assuming cloud distances of 50 and 300 kpc and using fiducial optical scale lengths of 3' and 7'. We explore the implication of these non-detections on the origin of the HVCs.

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arXiv2002

The Structure of Winds in AGB Stars

Most dusty winds are described by a set of similarity functions of a single independent variable that can be chosen as the overall optical depth at visual. The self-similarity implies general scaling relations among the system parameters, in agreement with observations. Dust drift through the gas has a major impact on the structure of most winds.

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arXiv2002

An IR view of mass loss in Long Period Variables

We have investigated the mass loss processes in Long Period Variables, by combining radiative transfer modeling with mid-IR imaging and spectroscopic observations. We find a correlation between variability type and temporal variations in the mass loss rate. Mira variables are more likely to maintain constant mass loss rates over long period of time. Semiregular and Irregular variables are characterized by frequent interruptions in their mass loss rate, on timescales of ~100 yr. High resolution imaging of individual Long Period Variable sources show departures from spherical symmetry. We present the case of o Cet (Mira), in which the presence of a low mass companion plays an active role in shaping the circumstellar environment.

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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.

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arXiv2001

The Cut & Enhance method : selecting clusters of galaxies from the SDSS commissioning data

We describe an automated method, the Cut & Enhance method (CE) for detecting clusters of galaxies in multi-color optical imaging surveys. This method uses simple color cuts, combined with a density enhancement algorithm, to up-weight pairs of galaxies that are close in both angular separation and color. The method is semi-parametric since it uses minimal assumptions about cluster properties in order to minimize possible biases. No assumptions are made about the shape of clusters, their radial profile or their luminosity function. The method is successful in finding systems ranging from poor to rich clusters of galaxies, of both regular and irregular shape. We determine the selection function of the CE method via extensive Monte Carlo simulations which use both the real, observed background of galaxies and a randomized background of galaxies. We use position shuffled and color shuffled data to perform the false positive test. We have also visually checked all the clusters detected by the CE method. We apply the CE method to the 350 deg^2 of the SDSS (Sloan Digital Sky Survey) commissioning data and construct a SDSS CE galaxy cluster catalog with an estimated redshift and richness for each cluster. The CE method is compared with other cluster selection methods used on SDSS data such as the Matched Filter (Postman et al. 1996, Kim et al. 2001), maxBCG technique (Annis et al. 2001) and Voronoi Tessellation (Kim et al. 2001). The CE method can be adopted for cluster selection in any multi-color imaging surveys.

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arXiv2001

LOTIS, Super-LOTIS, SDSS and Tautenburg Observations of GRB 010921

We present multi-instrument optical observations of the High Energy Transient Explorer (HETE-2)/Interplanetary Network (IPN) error box of GRB 010921. This event was the first gamma ray burst (GRB) localized by HETE-2 which has resulted in the detection of an optical afterglow. In this paper we report the earliest known observations of the GRB010921 field, taken with the 0.11-m Livermore Optical Transient Imaging System (LOTIS) telescope, and the earliest known detection of the GRB010921 optical afterglow, using the 0.5-m Sloan Digital Sky Survey Photometric Telescope (SDSS PT). Observations with the LOTIS telescope began during a routine sky patrol 52 minutes after the burst. Observations were made with the SDSS PT, the 0.6-m Super-LOTIS telescope, and the 1.34-m Tautenburg Schmidt telescope at 21.3, 21.8, and 37.5 hours after the GRB, respectively. In addition, the host galaxy was observed with the USNOFS 1.0-m telescope 56 days after the burst. We find that at later times (t > 1 day after the burst), the optical afterglow exhibited a power-law decline with a slope of $α= 1.75 \pm 0.28$. However, our earliest observations show that this power-law decline can not have extended to early times (t < 0.035 day).

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arXiv2001

The Ghost of Sagittarius and Lumps in the Halo of the Milky Way

We identify new structures in the halo of the Milky Way Galaxy from positions, colors and magnitudes of five million stars detected in the Sloan Digital Sky Survey. Most of these stars are within 1.26 degrees of the celestial equator. We present color-magnitude diagrams (CMDs) for stars in two previously discovered, tidally disrupted structures. The CMDs and turnoff colors are consistent with those of the Sagittarius dwarf galaxy, as had been predicted. In one direction, we are even able to detect a clump of red stars, similar to that of the Sagittarius dwarf, from stars spread across 110 square degrees of sky. Focusing on stars with the colors of F turnoff objects, we identify at least five additional overdensities of stars. Four of these may be pieces of the same halo structure, which would cover a region of the sky at least 40 degrees in diameter, at a distance of 11 kpc from the Sun (18 kpc from the center of the Galaxy). The turnoff is significantly bluer than that of thick disk stars, and closer to the Galactic plane than a power-law spheroid. We suggest two models to explain this new structure. One possibility is that this new structure could be a new dwarf satellite of the Milky Way, hidden in the Galactic plane, and in the process of being tidally disrupted. The other possibility is that it could be part of a disk-like distribution of stars which is metal-poor, with a scale height of approximately 2 kpc and a scale length of approximately 10 kpc. The fifth overdensity, which is 20 kpc away, is some distance from the Sagittarius dwarf streamer orbit and is not associated with any known structure in the Galactic plane. It is likely that there are many smaller streams of stars in the Galactic halo.

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arXiv2001

The Angular Clustering of Galaxy Pairs

We identify close pairs of galaxies from 278 deg^2 of Sloan Digital Sky Survey commissioning imaging data. The pairs are drawn from a sample of 330,041 galaxies with 18 < r^* < 20. We determine the angular correlation function of galaxy pairs, and find it to be stronger than the correlation function of single galaxies by a factor of 2.9 +/- 0.4. The two correlation functions have the same logarithmic slope of 0.77. We invert Limber's equation to estimate the three-dimensional correlation functions; we find clustering lengths of r_0= 4.2 +/- 0.4 h^{-1} Mpc for galaxies and 7.8 +/- 0.7 h^{-1} Mpc for galaxy pairs. These results agree well with the global richness dependence of the correlation functions of galaxy systems.

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arXiv2001

An SDSS Survey For Resolved Milky Way Satellite Galaxies I: Detection Limits

We present the detection limits of a new survey for resolved low surface brightness satellite galaxies to the Milky Way, based on the Sloan Digital Sky Survey (SDSS). Our survey exploits SDSS's major strengths (multi-color photometry, depth, large-scale, and uniformity) by combining filter smoothing with limits in both magnitude and color space to search for low surface brightness galaxies and stellar counterparts to the Compact High Velocity Clouds out to the Milky Way's virial radius (350 kpc). Our calculated detection limits for a purely old stellar population vary with galaxy size and distance between mu_{V,0} = 26.7 and 30.1 mag/sq ''. These limits will allow us to detect systems whose surface brightnesses are .5-3.9 mag/sq '' fainter than Sextans, the lowest surface brightness Local Group member known. Our survey not only is sensitive to lower surface brightness stellar populations than possible with previous Local Group surveys, but will also allow us to make an unbiased and well defined assessment of the completeness of the observed Local Group galaxy luminosity function, so that we may compare the results with the predictions of various structure formation scenarios.

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arXiv2001

L Dwarfs Found in Sloan Digital Sky Survey Commissioning Data II. Hobby-Eberly Telescope Observations

Low dispersion optical spectra have been obtained with the Hobby-Eberly Telescope of 22 very red objects found in early imaging data from the Sloan Digital Sky Survey. The objects are assigned spectral types on the 2MASS system (Kirkpatrick et al. 1999) and are found to range from late M to late L. The red- and near-infrared colors from SDSS and 2MASS correlate closely with each other, and most of the colors are closely related to spectral type in this range; the exception is the (i^* - z^*) color, which appears to be independent of spectral type between about M7 and L4. The spectra suggest that this independence is due to the disappearance of the TiO and VO absorption in the i-band for later spectral types; to the presence of strong Na I and K I absorption in the i-band; and to the gradual disappearance of the 8400 Angstrom absorption of TiO and FeH in the z-band.

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arXiv2001

Detecting Clusters of Galaxies in the Sloan Digital Sky Survey I : Monte Carlo Comparison of Cluster Detection Algorithms

We present a comparison of three cluster finding algorithms from imaging data using Monte Carlo simulations of clusters embedded in a 25 deg^2 region of Sloan Digital Sky Survey (SDSS) imaging data: the Matched Filter (MF; Postman et al. 1996), the Adaptive Matched Filter (AMF; Kepner et al. 1999) and a color-magnitude filtered Voronoi Tessellation Technique (VTT). Among the two matched filters, we find that the MF is more efficient in detecting faint clusters, whereas the AMF evaluates the redshifts and richnesses more accurately, therefore suggesting a hybrid method (HMF) that combines the two. The HMF outperforms the VTT when using a background that is uniform, but it is more sensitive to the presence of a non-uniform galaxy background than is the VTT; this is due to the assumption of a uniform background in the HMF model. We thus find that for the detection thresholds we determine to be appropriate for the SDSS data, the performance of both algorithms are similar; we present the selection function for each method evaluated with these thresholds as a function of redshift and richness. For simulated clusters generated with a Schechter luminosity function (M_r^* = -21.5 and alpha = -1.1) both algorithms are complete for Abell richness >= 1 clusters up to z ~ 0.4 for a sample magnitude limited to r = 21. While the cluster parameter evaluation shows a mild correlation with the local background density, the detection efficiency is not significantly affected by the background fluctuations, unlike previous shallower surveys.

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arXiv2001

Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar

We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly Alpha absorption in the spectra of these quasars evolves strongly with redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is 0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A (5.95 150, and is consistent with zero flux in the Ly Alpha forest region immediately blueward of the Ly Alpha emission line, compared with a drop by a factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha absorption at z=6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z~5 to 6, and the universe is approaching the reionization epoch at z~6.

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