Alberto Fernandez-Soto

Morphological number counts and redshift distributions to I <26 from the Hubble deep field: Implications for the evolution of ellipticals, spirals, and irregulars

We combine the photometric redshift data of Fern?ndez-Soto et al. with the morphological data of Odewahn et al. for all galaxies with I 24. The breadth of the redshift distribution at faint magnitudes implies strong clustering or an extended epoch of formation commencing at z>3. 2.?Spiral galaxies are present in numbers consistent with zero-evolution predictions to I=22. Beyond this magnitude some net positive evolution is required. Although the number counts are consistent with the passive-evolution predictions to I=26.0, the redshift distributions favor number and luminosity evolution, although few obvious mergers are seen (possibly classified as irregulars). We note that beyond z~2 very few ordered spirals are seen suggesting a formation epoch of spiral galaxies at z~1.5-2. 3.?There is no obvious explanation for the late-type/irregular class, and this category requires further subdivision. While a small fraction of the population lies at low redshift (i.e., true irregulars), the majority lie at redshifts 1 1.5 mergers are frequent and, taken in conjunction with the absence of normal spirals at z>2, the logical inference is that they represent the progenitors of normal spirals that form via hierarchical merging.

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.

An Empirical Limit on Extremely High Redshift Galaxies

We apply the Lyman absorption signature to search for galaxies at redshifts z ≈ 6–17 using optical and infrared images of the Hubble Deep Field. The infrared images are sensitive to a point-source 5 σ detection threshold of AB(22,000) = 23.8, which, adopting plausible assumptions to relate rest-frame ultraviolet flux densities to unobscured star formation rates, is easily sufficient to detect the star formation rates expected for massive elliptical galaxy formation to quite high redshifts. For q0 = 0.5, the infrared images are sensitive to an unobscured star formation rate of = 100 h-2 M⊙ yr-1 to redshifts as large as z = 17, and for q0 = 0, the infrared images are sensitive to an unobscured star formation rate of = 300 h-2 M⊙ yr-1 to redshifts as large as z = 14. The primary result of the analysis is that only one extremely high redshift galaxy candidate is identified at the 5 σ level of significance (and four at the 4 σ level). This implies a strict upper limit to the surface density of extremely high redshift galaxies of less than 1.5 arcmin-2 to a limiting magnitude threshold AB(22,000) = 23.8. This also implies a strict upper limit to the volume density of extremely high redshift galaxies if (and only if) such galaxies are not highly obscured by dust.

The Gaseous Extent of Galaxies and the Origin of Lyα Absorption Systems. IV. Lyα Absorbers Arising in a Galaxy Group

We present new Goddard High Resolution Spectrograph observations of Lyα absorption lines associated with a group of galaxies toward the QSO 1545+2101. We have identified eight distinct Lyα absorption features in the spectrum of QSO 1545+2101 at a mean redshift of z = 0.2648 with a velocity dispersion of 163 km s-1. A group of galaxies is detected in the vicinity of this QSO at a mean redshift of z = 0.2645 and velocity dispersion of 239 km s-1. The identification of discrete absorption systems indicates that they arise in clouds of neutral hydrogen rather than in a diffuse intragroup medium. Our analysis suggests that the Lyα absorption lines are associated with individual galaxies in the group, although a one-to-one relationship between absorbers and galaxies is difficult to establish in such a dense environment.

QSO Lyman-Alpha Absorbers and Galaxy Halos

We review the current status of our survey of galaxies in fields of HST target QSOs, which has allowed us to identify the galaxies responsible for a number of Lyman-α absorption systems. We emphasize the use of QSO absorption lines to study the structure and kinematics of the large gaseous halos that virtually all galaxies appear to possess.

High-Redshift Galaxies: The HDF and More

We review our present knowledge of high-redshift galaxies, emphasizing particularly their physical properties and the ways in which they relate to present-day galaxies. We also present a catalogue of photometric redshifts of galaxies in the Hubble Deep Field and discuss the possibilities that this kind of study offers to complete the standard spectroscopically based surveys.