arXiv subjects

D. Stern

Publications and source records attributed to D. Stern.

At least 343 records · Page 19Linked to original sources

arXiv2004

The Great Observatories Origins Deep Survey - VLT/FORS2 Spectroscopy in the GOODS-South Field

We present the first results of the ESO/GOODS program of spectroscopy of faint galaxies in the Chandra Deep Field South (CDF-S). 399 spectra of 303 unique targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 234 redshift determinations (the median of the redshift distribution is at 1.04). The typical redshift uncertainty is estimated to be sig(z) ~ 0.001. Galaxies have been color selected in a way that the resulting redshift distribution typically spans from z=0.5 to 2. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/ Large scale structure is clearly detected at z ~ 0.67, 0.73, 1.10 and 1.61. Three Lyman-break galaxies have also been included as targets and are confirmed to have redshifts z=4.800, 4.882 and 5.828. In a few cases, we observe clear [OII]3727 rotation curves, even at the relatively low resolution (R = 860) of the present observations. Assuming that the observed velocity structure is due to dynamically-relaxed rotation, this is an indication of large galactic masses (few times 10^(11) solar masses) at z ~ 1.

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arXiv2004

The Hubble Higher-Z Supernova Search: Supernovae to z=1.6 and Constraints on Type Ia Progenitor Models

We present results from the Hubble Higher-z Supernova Search, the first space-based open field survey for supernovae (SNe). In cooperation with the Great Observatories Origins Deep Survey, we have used the Hubble Space Telescope with the Advanced Camera for Surveys to cover 300 square arcmin in the area of the Chandra Deep Field South and the Hubble Deep Field North on five separate search epochs (separated by 45 day intervals) to a limiting magnitude of z'=26. These deep observations have allowed us to discover 42 SNe in the redshift range 0.2 < z < 1.6. As these data span a large range in redshift, they are ideal for testing the validity of Type Ia supernova progenitor models with the distribution of expected ``delay times,'' from progenitor star formation to SN Ia explosion, and the SN rates these models predict. Through a Bayesian maximum likelihood test, we determine which delay-time models best reproduce the redshift distribution of SNe Ia discovered in this survey. We find that models that require a large fraction of ``prompt'' (less than 2 Gyr) SNe Ia poorly reproduce the observed redshift distribution and are rejected at 95% confidence. We find that Gaussian models best fit the observed data for mean delay times in the range of 3 to 4 Gyr.

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arXiv2004

The IRAC Shallow Survey

The IRAC shallow survey covers 8.5 square degrees in the NOAO Deep Wide-Field Survey in Bootes with 3 or more 30 second exposures per position. An overview of the survey design, reduction, calibration, star-galaxy separation, and initial results is provided. The survey includes approximately 370,000, 280,000, 38,000, and 34,000 sources brighter than the 5 sigma limits of 6.4, 8.8, 51, and 50 microJy at 3.6, 4.5, 5.8, and 8 microns respectively, including some with unusual spectral energy distributions.

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arXiv2004

Deep mid-infrared observations of Lyman-break galaxies

As part of the In-Orbit Checkout activities for the Spitzer Space Telescope, the IRAC team carried out a deep observation (average integration time ~8 hours) of a field surrounding the bright QSO HS 1700+6416. This field contains several hundred z~3 Lyman-break galaxy candidates, and we report here on their mid-infrared properties, including the IRAC detection rate, flux densities and colors, and the results of fitting population synthesis models to the optical, near-infrared, and IRAC magnitudes. The results of the model-fitting show that previous optical/near-infrared studies of LBGs were not missing large, hidden old stellar populations. The LBG candidates' properties are consistent with those of massive, star-forming galaxies at z~3. Other IRAC sources in the same field have similar properties, so IRAC selection may prove a promising method of finding additional high-redshift galaxies.

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arXiv2004

Number Counts At 3 < lambda < 10 um From the Spitzer Space Telescope

Infrared source counts at wavelengths 3 < lambda < 10 um cover more than 10 magnitudes in source brightness, four orders of magnitude in surface density, and reach an integrated surface density of 10^5 sources/deg^2. At m<14 mag, most of the sources are Galactic stars, in agreement with models. After removal of Galactic stars, galaxy counts are consistent with what few measurements exist at nearby wavelengths. At 3.6 and 4.5 um, the galaxy counts follow the expectations of a Euclidean world model down to ~16 mag and drop below the Euclidean curve for fainter magnitudes. Counts at these wavelengths begin to show decreasing completeness around magnitude 19.5. At 5.8 and 8 um, the counts relative to a Euclidean world model show a large excess at bright magnitudes. This is probably because local galaxies emit strongly in the aromatic dust (``PAH'') features. The counts at 3.6 um resolve <50% of the Cosmic Infrared Background at that wavelength.

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arXiv2004

X-ray nondetection of the Lyman-alpha Emitters at z ~ 4.5

The Lyman-alpha emitters found at z ~ 4.5 by the Large Area Lyman Alpha (LALA) survey have high equivalent widths in the Lyman-alpha line, which can be produced by either narrow-lined active galactic nuclei (AGNs) or by stellar populations with a very high proportion of young, massive stars. To investigate the AGN scenario, we obtained two deep Chandra exposures to study the X-ray nature of the Lyman-alpha emitters. The 172 ks deep Chandra image on the LALA Bootes field was presented in a previous paper (Malhotra et al. 2003), and in this paper we present a new Chandra deep exposure (174 ks) on the LALA Cetus field, which doubled our sample of X-ray imaged Lyman-alpha sources, and imaged the brightest source among our Lyman-alpha emitters. None of the 101 Lyman-alpha sources covered by two Chandra exposures was detected individually in X-ray, with a 3 sigma limiting X-ray flux of F_{0.5-10.0 keV} < 3.3 x 10^{-16} ergs/cm^2/s for on-axis targets. The sources remain undetectable in the stacked image, implying a 3 sigma limit to the average luminosity of L_{2-8keV} < 2.8 x 10^{42} ergs/s. The resulting X-ray to Lyman-alpha ratio is > 21 times lower than the ratios for known high redshift type-II quasars. Together with optical spectra obtained at Keck, we conclude that no evidence of AGN activity was found among our Lyman-alpha emitters at z ~ 4.5.

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arXiv2004

High-redshift QSOs in the GOODS

The Great Observatories Origins Deep Survey provides significant constraints on the space density of less luminous QSOs at high redshift, which is particularly important to understand the interplay between the formation of galaxies and super-massive black holes and to measure the QSO contribution to the UV ionizing background. We present the results of a search for high-z QSOs, identified in the two GOODS fields on the basis of deep imaging in the optical (with HST) and X-ray (Chandra), and discuss the allowed space density of QSOs in the early universe.

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arXiv2003

A Deep, Wide Field, Optical, and Near Infrared Catalog of a Large Area around the Hubble Deep Field North

We have conducted a deep multi-color imaging survey of 0.2 degrees^2 centered on the Hubble Deep Field North (HDF-N). We shall refer to this region as the Hawaii-HDF-N. Deep data were collected in U, B, V, R, I, and z' bands over the central 0.2 degrees^2 and in HK' over a smaller region covering the Chandra Deep Field North (CDF-N). The data were reduced to have accurate relative photometry and astrometry across the entire field to facilitate photometric redshifts and spectroscopic followup. We have compiled a catalog of 48,858 objects in the central 0.2 degrees^2 detected at 5 sigma significance in a 3" aperture in either R or z' band. Number counts and color-magnitude diagrams are presented and shown to be consistent with previous observations. Using color selection we have measured the density of objects at 3 5.5 using the Lyman break technique suffer from more contamination by low redshift objects than suggested by previous studies.

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arXiv2003

The Rest-Frame UV Luminosity Density of Star-Forming Galaxies at Redshifts z>3.5

We have measured the rest--frame lambda~1500 Ang comoving specific luminosity density of star--forming galaxies at redshift 3.5<z<6.5 from deep images taken with the Hubble Space Telescope (HST)and the Advanced Camera for Surveys (ACS), obtained as part of the Great Observatories Origins Deep Survey (GOODS). We used color selection criteria to construct samples of star--forming galaxies at redshifts z~4, 5 and 6, identified by the signature of the 912 Ang Lyman continuum discontinuity and Lyman-alpha forest blanketing in their rest--frame UV colors (Lyman--break galaxies). The ACS samples cover ~0.09 square degree, and are also relatively deep, reaching between 0.2 and 0.5 L_3^*, depending on the redshift, where $L_3^*$ is the characteristic UV luminosity of Lyman--break galaxies at z~3. The specific luminosity density of Lyman--break galaxies appears to be nearly constant with redshift from z~3 to z~6, although the measure at z~6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the z~6 sample. If Lyman--break galaxies are fair tracers of the cosmic star formation activity, our results suggest that at z~6 the universe was already producing stars as vigorously as it did near its maximum several Gyr later, at 1<~z<~3. Thus, the onset of large--scale star formation in the universe is to be sought at around z~6 or higher, namely at less than ~7% of the current cosmic age.

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arXiv2003

Photometric Redshifts for Galaxies in the GOODS Southern Field

We use extensive multi-wavelength photometric data from the Great Observatories Origins Deep Survey (GOODS) to estimate photometric redshifts for a sample of 434 galaxies with spectroscopic redshifts in the Chandra Deep Field South. Using the Bayesian method, which incorporates redshift/magnitude priors, we estimate photometric redshifts for galaxies in the range 18 < R (AB) < 25.5, giving an rms scatter of 0.11. The outlier fraction is < 10%, with the outlier-clipped rms being 0.047. We examine the accuracy of photometric redshifts for several, special sub--classes of objects. The results for extremely red objects are more accurate than those for the sample as a whole, with rms of 0.051 and very few outliers (3%). Photometric redshifts for active galaxies, identified from their X-ray emission, have a dispersion of 0.104, with 10% outlier fraction, similar to that for normal galaxies. Employing a redshift/magnitude prior in this process seems to be crucial in improving the agreement between photometric and spectroscopic redshifts.

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arXiv2003

The Space Density of High-Redshift QSOs in the GOODS Survey

We present a sample of 17 high-redshift (3.5<z<5.2) QSO candidates in the 320 sq.arcmin area of the Great Observatories Origins Deep Survey, selected in the magnitude range 22.45<z_{850}<25.25 using deep imaging with the Advanced Camera for Surveys onboard the Hubble Space Telescope and the Advanced CCD Imaging Spectrometer onboard the Chandra X-ray Observatory. On the basis of seven spectroscopic and ten photometric redshifts we estimate that the final sample will contain between two and four QSOs with 4<z<5.2. A dearth of high-redshift, moderate-luminosity (M_{145}=~-23) QSOs is observed with respect to predictions based on a) the extrapolation of the z~2.7 luminosity function (LF), according to a pure luminosity evolution calibrated by the results of the Sloan Digital Sky Survey; and b) a constant universal efficiency in the formation of super-massive black holes (SMBHs) in dark-matter halos. Evidence is gathered in favor of a density evolution of the LF at high redshift and of a suppression of the formation or feeding of SMBHs in low-mass halos.

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arXiv2003

Color-selected galaxies at z~6 in the Great Observatories Origins Deep Survey

We report early results on galaxies at z~6, selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and VLT observatories a shows a strong continuum break and asymmetric line emission, identified as Lya at z=5.83. We detect only five spatially extended, z~6 candidates with signal-to-noise ratios > 10, two of which have spectroscopic confirmation. This is many fewer than would be expected if galaxies at z=6 had the same luminosity function as those at z=3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z=6 galaxy population with fainter luminosities, higher space density, and similar co-moving ultraviolet emissivity to that at z=3, but this depends critically on counts at fluxes fainter than those reliably probed by the current data.

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arXiv2003

Massive star formation in a gravitationally-lensed HII-galaxy at z=3.357

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R-K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an \hii galaxy magnified by a factor of ~10 by a complex cluster environment. The high intrinsic luminosity, the emission line spectrum, the absorption components seen in Lyαand C IV, and the restframe ultraviolet continuum are all consistent with a simple HII region model containing ~ 10^6 hot O stars. The best fit parameters for this model imply a very hot ionizing continuum (T_BB ~ 80,000 K), high ionization parameter (log U ~ -1), and low nebular metallicity (Z / Z_\odot ~ 0.05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair instability supernovæ, requiring stars more massive than ~140 M_\odot.

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arXiv2003

Discovery of a Clustered Quasar Pair at z ~ 5: Biased Peaks in Early Structure Formation

We report a discovery of a quasar at z = 4.96 +- 0.03 within a few Mpc of the quasar SDSS 0338+0021 at z = 5.02 +- 0.02. The newly found quasar has the SDSS i and z magnitudes of ~ 21.2, and an estimated absolute magnitude M_B ~ -25.2. The projected separation on the sky is 196 arcsec, and the redshift difference Delta z = 0.063 +- 0.008. The probability of finding this quasar pair by chance in the absence of clustering in this particular volume is ~ 10^-4 to 10^-3. We conclude that the two objects probably mark a large-scale structure, possibly a protocluster, at z ~ 5. This is the most distant such structure currently known. Our search in the field of 13 other QSOs at z >~ 4.8 so far has not resulted in any detections of comparable luminous QSO pairs, and it is thus not yet clear how representative is this structure at z ~ 5. However, along with the other evidence for clustering of quasars and young galaxies at somewhat lower redshifts, the observations are at least qualitatively consistent with a strong biasing of the first luminous and massive objects, in agreement with general predictions of theoretical models. More extensive searches for clustered quasars and luminous galaxies at these redshifts will provide valuable empirical constraints for our understanding of early galaxy and structure formation.

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arXiv2002

Old elliptical galaxies at z=1.5 and the Kormendy relation

Deep spectroscopy of the two mJy radio galaxies LBDS 53W069 and 53W091 has previously shown them to have old (>3 Gyr) stellar populations at z=1.5. Here we present the results of HST observations of these galaxies. 53W069 has an r^1/4 profile in both the F814W & F110W data with an effective radius of 0.30 arcsec (2.7 kpc). The restframe U-B colour gradient requires a stellar population of super-solar (3Z_sun) metallicity that formed on a very short timescale at high redshift (z>5). 53W091 has a regular r^1/4 profile in F110W with an effective radius of 0.32 arcsec (2.9 kpc). The F814W profile is more extended and is consistent with a blue exponential disk that contributes 20% of the flux within r_e. The restframe U-B colour gradient is significantly larger than that observed in field ellipticals at z<1, implying a stellar population of mixed metallicity (1-3Z_sun) that formed in a high-redshift rapid burst. We have compared these two LBDS radio galaxies with the Kormendy relation of ten 3CR radio galaxies at z=0.8. The LBDS galaxies follow the same relation as the more radio-luminous 3CR galaxies, assuming passive evolution of their stellar populations, although they are smaller than the 3CR galaxies whose mean effective radius is 12 kpc. Their sizes and radio luminosities are consistent with scaling relations applied to the 3CR galaxies, in which both radio power and effective radius scale with galaxy mass. Compared with a sample of z=0.4 cluster ellipticals, 53W069 & 53W091 lie well within the scatter of the Kormendy relation. We conclude that the hosts of these mJy radio sources at z=1.5 are passively-evolving elliptical galaxies that will evolve into ordinary L* ellipticals by the present day. (Abridged)

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arXiv2001

An X-ray Selected Galaxy Cluster at z=1.11 in the Rosat Deep Cluster Survey

We report the discovery of an X-ray luminous galaxy cluster at z =1.11. RDCS J0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent in a Rosat PSPC image. Deep optical and near-IR imaging reveal a red galaxy overdensity around the peak of the X-ray emission, with a significant excess of objects with J-K and I-K colors typical of elliptical galaxies at z ~ 1.0. Spectroscopic observations at the Keck II telescope secured 9 galaxy redshifts in the range 1.095 =1.106. Eight of these galaxies lie within a 30 arcsec radius around the peak X-ray emission. A deep Chandra ACIS exposure on this field shows extended X-ray morphology and allows the X-ray spectrum of the intracluster medium to be measured. The cluster has a bolometric luminosity L_x = 2.48^{+0.33}_{-0.26} x 10^44 ergs/s, a temperature of kT = 7.2^{+2.2}_{-1.4} keV, and a mass within r = 1 Mpc of 7.0 x 10^14 M_sun (H_0=65 km/s/Mpc, Omega_m = 0.3, and Lambda = 0.7). The spatial distribution of the cluster members is elongated, which is not due to an observational selection effect, and followed by the X-ray morphology. The X-ray surface brightness profile and the spectrophotometric properties of the cluster members suggest that this is an example of a massive cluster in an advanced stage of formation with a hot ICM and an old galaxy population already in place at z > 1.

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arXiv2001

On the Threshold of the Reionization Epoch

Discovery of the cosmic reionization epoch would represent a significant milestone in cosmology. We present Keck spectroscopy of the quasar SDSS 1044-0125, at z = 5.73. The spectrum shows a dramatic increase in the optical depth at observed wavelengths lambda >~7550 A, corresponding to z_abs >~ 5.2. Only a few small, narrow transmission regions are present in the spectrum beyond that point, and out to the redshifts where the quasar signal begins. We interpret this result as a signature of the trailing edge of the cosmic reionization epoch, which we estimate to occur around ~ 6 (as indeed confirmed by subsequent observations by Becker et al.), and extending down to z \~ 5.2. This behavior is expected in the modern theoretical models of the reionization era, which predict a patchy and gradual onset of reionization. The remaining transmission windows we see may correspond to the individual reionization bubbles (Stromgren spheres) embedded in a still largely neutral intergalactic medium, intersected by the line of sight to the quasar. Future spectroscopic observations of quasars at comparable or larger redshifts will provide a more detailed insight into the structure and extent of the reionization era.

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arXiv2000

The Fraction of High Redshift Galaxies in Deep Infrared Selected Field Galaxy Samples

Recent results on the incidence of red galaxies in a > 100 square arcminute field galaxy survey to K=20 and a K=22 survey of the Hubble Deep Field are presented. We argue that a simple photometric redshift indicator, based on J-K color and supported by spectroscopic results obtained with Keck, gives a reliable lower limit of ~25% for the fraction of z>1 galaxies in the 100 square arcminute survey. This fraction is substantially higher than found in previous smaller samples, and is at least as consistent with predictions for pure luminosity evolution as with those for hierarchical models. The same technique yields a very low fraction for the HDF, which appears to be unusually underabundant in red galaxies.

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