Richard Elston

The Hubble Space Telescope Northern-Hemisphere grid of stellar polarimetric standards

Results are presented from a comprehensive program of optical polarimetry of strongly polarized and null-polarization standard stars. Potential sources of systematic error either instrumental in origin or due to absolute calibration are critically discussed. Although the project emphasized the requirements of a spaceborne optical-UV observatory (HST), the resulting grid of standards represents a distinct improvement over previous such tabulations and offers an opportunity to reference future multiwavelength polarimetry to a common system

The optical polarization properties of X-ray-selected BL Lacertae objects

We discuss the optical polarization properties of X-ray-selected BL Lacertae objects (XSBLs) as determined from 3 yr of monitoring the polarization of 37 BL Lac objects and candidates. The observed objects include a complete X-ray flux limited sample drawn from the Einstein Extended Medium Sensitivity Survey (EMSS). We find that the majority of the XSBLs classified solely on the appearance of their optical spectra are true members of the class of BL Lacertae objects since they possess intrinsically polarized and variable continua. The duty cycle of polarized emission (fraction of time spent with the degree of polarization greater than 4%) from XSBLs is 44%. The maximum observed percent polarizations are of order 10%, considerably below the maximum values observed for radio-selected BL Lacertae objects and blazars (30%-40%). While XSBLs have variable polarized emission, the majority (approximately equal to 85%) have preferred polarization position angles on timescales at least as long as 3 yr. This reflects stability in the geometry of the region emitting the linearly polarized optical emission. We describe the spectral dependence of the degree of polarization and discuss some of the possible mechanisms producing the observed characteristics. While dilution of the polarized emission by the host galaxy starlight is certainly present in some objects, we demonstrate that the average polarization properties of XSBLs derived from our observations are not drastically affected by this effect. While the confirmed BL Lac objects are shown to be photometric variables, the objects in our monitored sample did not display the larger than 1 mag variations generally used to characterize the optical variability of radio-selected BL Lacertae objects or blazars in general.

Chandra detection of a Type II quasar at z = 3.288

We report on observations of a type II quasar at redshift z ¼ 3:288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-Ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXO J084837.9+445352 (hereafter CXO 52) shows an unusually hard Xray spectrum from which we infer an absorbing column density NH ¼ð 4:8 � 2:1 Þ� 10 23 cm � 2 (90% confidence) and an implied unabsorbed 2 10 keV rest-frame luminosity of L2 10 ¼ 3:3 � 10 44 ergs s � 1 , well within the quasar regime. Hubble Space Telescope imaging shows CXO 52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO 52 shows high-ionization emission lines with velocity widths � 1000 km s � 1 and flux ratios similar to a Seyfert 2 galaxy or radio galaxy. The latter are the only class of high-redshift type II luminous active galactic nuclei that have been extensively studied to date. Unlike radio galaxies, however, CXO 52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f4:8 GHz < 40 lJy. High-redshift type II quasars, expected from unification models of active galaxies and long thought necessary to explain the X-ray background, are poorly constrained observationally, with few such systems known. We discuss recent observations of similar type II quasars and detail search techniques for such systems, namely, (1) X-ray selection, (2) radio selection, (3) multicolor imaging selection, and (4) narrowband imaging selection. Such studies are likely to begin identifying luminous, high-redshift type II systems in large numbers. We discuss the prospects for these studies and their implications for our understanding of the X-ray background. Subject headings: cosmology: observations — galaxies: active — quasars: individual (CXO J084837.9+445352) — X-rays: galaxies

Photometric redshifts in the IRAC shallow survey

Accurate photometric redshifts are calculated for nearly 200,000 galaxies to a 4.5 micron flux limit of {approx} 13 {micro}Jy in the 8.5 deg{sup 2} Spitzer/IRAC Shallow survey. Using a hybrid photometric redshift algorithm incorporating both neural-net and template-fitting techniques, calibrated with over 15,000 spectroscopic redshifts, a redshift accuracy of {sigma} = 0.06 (1+z) is achieved for 95% of galaxies at 0 1) galaxy clusters. We present one such spectroscopically confirmed cluster at = 1.24, ISCS J1434.2+3426. Finally, we present a measurement of the 4.5 {micro}m-selected galaxy redshift distribution.

Deep 2 micron imaging of the sky - Evidence for a new extragalactic population

Deep near-infrared imaging was obtained to study extragalactic populations, and a color-magnitude diagram was used to identify both normal galaxies with z = 0.3-1 and what appears to be a new distinct population. The latter has properties roughly similar to what would be expected for large galaxies at z greater than six undergoing a rapid luminous phase of star formation. It is suggested that these galaxies are good candidates for primeval galaxies. The normal galaxies identified are also important for studies of galaxy evolution, since they are found in the near-infrared, which is only weakly affected by evolution of the stellar populations. 17 references.

The K-Band Luminosity Function in Galaxy Clusters to z

We present K-band luminosity functions for galaxies in a heterogeneous sample of 38 clusters at 0.1 0.4 and is consistent with the behavior of a simple, passive luminosity evolution model in whichgalaxies form all their stars in a single burst at zf = 2(3) in an H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7(0) universe. This differs from the flat or negative infrared luminosity evolution, which has been reported for high-redshift field galaxy samples. We find that the observed evolution appears to be insensitive to cluster X-ray luminosity or optical richness, implying little variation in the evolutionary history of galaxies over the range of environmental densities spanned by our cluster sample. These results support and extend previous analyses based on the color evolution of high-redshift cluster E/S0 galaxies, indicating not only that their stellar populations formed at high-redshift, but that the assembly of the galaxies themselves was largely complete by z ≈ 1 and that subsequent evolution down to the present epoch was primarily passive.

The FLAMINGOS Extragalactic Survey

Using the Florida Multi-object Imaging Near-IR Grism Observational Spectrometer (FLAMINGOS), we have conducted the FLAMINGOS Extragalactic Survey (FLAMEX), a deep imaging survey covering 7.1 deg2 within the 18.6 deg2 NOAO Deep Wide-Field Survey (NDWFS) regions. FLAMEX is the first deep, wide-area, near-infrared survey to image in both the J and Ks filters, and is larger than any previous NIR survey of comparable depth. The intent of FLAMEX is to facilitate the study of galaxy and galaxy cluster evolution at 1 1 galaxy clusters detected using the joint FLAMEX, NDWFS, and Spitzer IRAC Shallow Survey data sets.

The [ital K]-Band Luminosity Function in Galaxy Clusters to [bold [ital z]] [approximately]1

We present K-band luminosity functions for galaxies in a heterogeneous sample of 38 clusters at 0.1 0.4 and is consistent with the behavior of a simple, passive luminosity evolution model in whichgalaxies form all their stars in a single burst at zf = 2(3) in an H0 = 65 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7(0) universe. This differs from the flat or negative infrared luminosity evolution, which has been reported for high-redshift field galaxy samples. We find that the observed evolution appears to be insensitive to cluster X-ray luminosity or optical richness, implying little variation in the evolutionary history of galaxies over the range of environmental densities spanned by our cluster sample. These results support and extend previous analyses based on the color evolution of high-redshift cluster E/S0 galaxies, indicating not only that their stellar populations formed at high-redshift, but that the assembly of the galaxies themselves was largely complete by z ≈ 1 and that subsequent evolution down to the present epoch was primarily passive.

The Unusual Infrared Object HDF-N J123656.3+621322* **

We describe an object in the Hubble Deep Field North with very unusual near-infrared properties. It is readily visible in Hubble Space Telescope NICMOS images at 1.6 ?m and from the ground at 2.2 ?m, but it is undetected (with S/N 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1 ?m. The f? flux density drops by a factor 8.3 (97.7% confidence) from 1.6 to 1.1 ?m. The object is compact but may be slightly resolved in the NICMOS 1.6 ?m image. In a low-resolution, near-infrared spectrogram, we find a possible emission line at 1.643 ?m, but a reobservation at higher spectral resolution failed to confirm the line, leaving its reality in doubt. We consider various hypotheses for the nature of this object. Its colors are unlike those of known Galactic stars, except perhaps the most extreme carbon stars or Mira variables with thick circumstellar dust shells. It does not appear to be possible to explain its spectral energy distribution as that of a normal galaxy at any redshift without additional opacity from either dust or intergalactic neutral hydrogen. The colors can be matched by those of a dusty galaxy at z 2, by a maximally old elliptical galaxy at z 3 (perhaps with some additional reddening), or by an object at z 10 whose optical and 1.1 ?m light have been suppressed by the intergalactic medium. Under the latter hypothesis, if the luminosity results from stars and not an AGN, the object would resemble a classical, unobscured protogalaxy, with a star formation rate 100 M? yr-1. Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a space density several hundred times lower than that of present-day L* galaxies.

Identification of a galaxy responsible for a high-redshift Lyman-α absorption system

DAMPEDLyman-α systems are high-column-density intergalactic clouds of hydrogen, the existence of which is inferred from absorption lines appearing in the emission spectra of distant quasars. The galaxies believed to be responsible for these absorption systems have been suggested as possible progenitors of the normal disk galaxies observed in the local Universe1. Indeed, Lyman-α systems appear to contain a substantial fraction of the baryons known to exist in galaxies today2,3. Here we report the optical detection of a galaxy (designated DLA2233 + 131) associated with a known4 damped Lyman-α absorption system at a redshift of z = 3.150. The properties of this galaxy correspond closely to those expected of a young disk galaxy in the early stages of formation, and show no evidence for an active nucleus. This finding gives strong support to the idea that damped Lyman-α systems represent a population of young galaxies at high red-shifts.