Sebastian Michael Pascarelle

Unusual spectral energy distribution of a galaxy previously reported to be at redshift 6.68

Observations of distant galaxies are important both for understanding how galaxies form and for probing the physical conditions of the Universe at early times. It is, however, very difficult to identify galaxies at redshifts z > 5, because they are so faint and have few spectral characteristics. We previously reported the probable identification of a galaxy at z = 6.68, based on one line and an apparent break in the spectrum just shortwards of that, which we interpreted as Lyman α emission and the Lyman α break, where photons with shorter wavelengths are absorbed by the intervening neutral hydrogen gas. Here we present optical photometry that shows moderate detections of light in the B- and V-band images, which are inconsistent with the expected absence of flux shortwards of the Lyman α break for a galaxy at z > 5, and inconsistent with the previous flux measurement. Moreover, the spectral energy distribution for this object cannot readily be fitted by any known galaxy spectral template at any redshift, so the redshift is undetermined.

The Star Formation Rate Intensity Distribution Function: Implications for the Cosmic Star Formation Rate History of the Universe

We address the effects of cosmological surface brightness dimming on observations of faint galaxies by examining the distribution of unobscured star formation rate intensities versus redshift. We use the star formation rate intensity distribution function to assess the ultraviolet luminosity density versus redshift, based on our photometry and photometric redshift measurements of faint galaxies in the Hubble Deep Field (HDF) and the Hubble Deep Field-South (HDF-S) Wide Field Planetary Camera 2 and Near-Infrared Camera and Multi-Object Spectrometer fields. We find that (1) previous measurements have missed a dominant fraction of the ultraviolet luminosity density of the universe at high redshifts by neglecting cosmological surface brightness dimming effects, which are important at redshifts larger than z ≈ 2; (2) the incidence of the highest intensity star-forming regions increases monotonically with redshift; and (3) the ultraviolet luminosity density plausibly increases monotonically with redshift through the highest redshifts observed. By measuring the spectrum of the luminosity density versus redshift, we also find that (4) previous measurements of the ultraviolet luminosity density at redshifts z < 2 must be reduced by a factor of ≈2 to allow for the spectrum of the luminosity density between rest-frame wavelengths 1500 and 2800 A. And, by comparing with observations of high-redshift damped Lyα absorption systems detected toward background quasi-stellar objects, we further find that (5) the distribution of star formation rate intensities matches the distribution of neutral hydrogen column densities at redshifts z ≈ 2-5, which establishes a quantitative connection between high-redshift galaxies and high column density gas and suggests that high-redshift damped Lyα absorption systems trace lower star formation rate intensity regions of the same galaxies detected in starlight in the HDF and HDF-S. Because our measurements neglect the effects of obscuration by dust, they represent lower limits to the total star formation rate density.

A z = 5.34 galaxy pair in the Hubble Deep Field

?????The system as a whole is slightly brighter than L relative to the z ~ 3 Lyman break population, and the total star formation rate inferred from the UV continuum is ?22 h M? yr-1 (q0 = 0.5) assuming the absence of dust extinction. The two individual galaxies are quite small (size scales 1 h kpc). Thus these galaxies superficially resemble the building blocks of Pascarelle and coworkers; if they comprise a gravitationally bound system, the pair will likely merge in a timescale ~100 Myr.

The Ultraviolet Luminosity Density of the Universe from Photometric Redshifts of Galaxies in the Hubble Deep Field

Studies of the Hubble Deep Field (HDF) and other deep surveys have revealed an apparent peak in the ultraviolet (UV) luminosity density, and therefore the star formation rate density, of the universe at redshifts 1 2.

On the Compared Accuracy and Reliability of Spectroscopic and Photometric Redshift Measurements

We present a comparison between the catalog of spectroscopic redshifts in the Hubble Deep Field (HDF) recently published by Cohen and collaborators and the redshifts that our group has measured for the same objects using photometric techniques. This comparison is performed in order to fully characterize the errors associated with the photometric redshift technique. The compilation of spectroscopic redshifts incorporates previously published results, corrections to previously published wrong values, and new data, and it includes over 140 objects in the HDF proper. It represents the deepest, cleanest, most complete spectroscopic catalog ever compiled. We particularly study each and every object for which our redshift and the one measured by Cohen and collaborators seem to disagree. In most of those cases, the photometric evidence we put forth is strong enough to call for a careful review of the spectroscopic values, since the spectroscopic values seem to be in error. We show that it is possible to characterize the systematic errors associated with our technique, which when combined with the well-measured photometric errors allow us to obtain complete information on the redshift of each galaxy and its associated confidence interval, regardless of its apparent magnitude. One of the main conclusions of this study is that, to date, all the redshifts from our published catalog that have been checked have been shown to be correct (within the stated confidence limits). This implies that our set of spectrophotometric galaxy templates is a fair representation of the galaxy population at all redshifts (0 < z < 6) and magnitudes (R < 24) explored thus far. On the other hand, spectroscopy of faint sources is subject to unknown and uncharacterized systematic errors. These errors will in turn be transmitted to any photometric redshift technique that uses spectroscopic samples in its calibration. Our analysis proves that photometric redshift techniques can and must be used to extend the range of applicability (in redshift, signal-to-noise, and apparent magnitude) of the spectroscopic redshift measurements.

Photometry and Photometric Redshifts of Faint Galaxies in the Hubble Deep Field South NICMOS Field

We present a catalog of photometry and photometric redshifts of 335 faint objects in the Hubble Deep Field South near-infrared camera and multiobject spectrograph (NICMOS) —eld. The analysis is based on (1) infrared images obtained with the Hubble Space Telescope (HST ) using the NICMOS with the F110W, F160W, and F222M —lters; (2) an optical image obtained with HST using the Space Telescope Imaging Spectrograph with no —lter; and (3) optical images obtained with the European Southern Observatory Very Large Telescope with U, B, V , R, and I —lters. The primary utility of the catalog of photometric redshifts is as a survey of faint galaxies detected in the NICMOS F160W and F222M images. The sensitivity of the survey varies signi—cantly with position, reaching a limiting depth of AB(16000) B 28.7 and covering 1.01 arcmin2 to AB(16000) \ 27 and 1.05 arcmin2 to AB(16000) \ 26.5. The catalog of photometric redshifts identi—es 21 galaxies (or 6% of the total) of redshift z ( 5, eight galaxies (or 2% of the total) of redshift z ( 10, and 11 galaxies (or 3% of the total) of best-—t spectral type E/S0, of which —ve galaxies (or 1% of the total) are of redshift z ( 1. Subject headings: cosmology: observationsgalaxies: distances and redshifts ¨ galaxies: photometrygalaxies: statistics

Spectroscopic identification of a galaxy at a probable redshift of z = 6.68

The detection and identification of distant galaxies is an important goal of observational cosmology, as such galaxies are seen at a time when the Universe was very young. The development of new techniques and instrumentation permits the search for ever-fainter galaxies, and so aids attempts to determine when the first stars and galaxies formed. Here we report the identification of a galaxy at a probable redshift of 6.68, the most distant object yet detected. The galaxys spectrum is characterized by an abrupt discontinuity at a wavelength λ≈ 9,300 Å, which we interpret as arising from the absorption of light at shorter wavelengths by hydrogen gas along the line of sight (the Lyman-α decrement), and by an emission line at λ≈ 9,334 Å, which we interpret as the Lyman-α line at a redshift of 6.68. The galaxy is relatively bright: the ultraviolet luminosity density contributed by this one galaxy is almost ten times the value measured at z = 3.

Compact Lyα-emitting candidates at z ≃ 2.4 in deep medium-band Hubble Space Telescope WFPC2 images

Medium-band imaging with Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) in the F410M filter has previously revealed a population of compact Lyα emission objects around the radio galaxy 53W002 at z 2.4. We report detections of similar objects at z 2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyα candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z 2.4 subgalactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.

Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images

Medium-band imaging with Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) in the F410M filter has previously revealed a population of compact Lyα emission objects around the radio galaxy 53W002 at z 2.4. We report detections of similar objects at z 2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyα candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z 2.4 subgalactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.

The Theta-z relation for HST bulges and disks out to z approximately equal 0.8

We present Hubble Space Telescope (HST) scale lengths and ground-based redshifts for 63 faint field galaxies down to I less than or approximately 21.5 mag from the Medium-Deep Survey. These have measured redshifts z less than or approximately 0.8 and half-light radii 0.1 sec less than or approximately r(sub e), r(sub s) less than or approximately 5 sec. We present the Theta-z relation for r(exp 1/4)-bulges and exponential disks separately for world models with q(sub 0) = 0.0-1.0. We show that selection against low surface brightness galaxies in the HST images and ground-based spectra is comparable to that in local surveys. We compare our HST disk-dominated galaxies to a magnitude-limited subsample of the European Southern Observatory (ESO)-Uppsala local spirals. Extrapolating the best-fit local exponential disk scale length (r(sub s) = 3.5 kpc for H(sub 0) = 75) out to z = 0.8, we find a rather symmetrical distribution of HST disks around this value. This implies that galaxy disks have been stable since z is approximately equal to 0.8. We compare our HST bulge-dominated galaxies with r(exp 1/4)-profiles to a magnitude-limited subsample of local Seven Samurai ellipticals. The latter have a local best fit r(sub e) = 5.7 kpc. Our HST bulge sample shows a similar distribution for z less than or approximately 0.8. Elliptical galaxy scale lengths have thus also remained rather constant since z less than or approximately 0.8. We set limits to their possible evolution.