Morphological number-count 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 Fernandez-Soto et al. (1997) with the morphological data of Odewahn et al. (1996) for all galaxies with I < 26.0 detected in the Hubble Deep Field. From this combined catalog we generate the morphological galaxy number-counts and corresponding redshift distributions and compare these to the predictions of high normalization zero- and passive- evolution models. From this comparison we conclude the following: (1) E/S0s are seen in numbers and over a redshift range consistent with zero- or minimal passive- evolution to I = 24. Beyond this limit fewer E/S0s are observed than predicted implying a net negative evolutionary process --- luminosity dimming, disassembly or masking by dust --- at I > 24. (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. (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 < z < 3. At z > 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 forming via hierarchical merging.Comment: Accepted for publication in ApJ Letters, colour plates available from http://www.phys.unsw.edu.au/~spd/bib.htm

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 HDF-S NICMOS field. The analysis is based on (1) infrared images obtained with the Hubble Space Telescope (HST) using the Near Infrared Camera and Multi-Object Spectrograph (NICMOS) with the F110W, F160W, and F222M filters, (2) an optical image obtained with HST using the Space Telescope Imaging Spectrograph (STIS) with no filter, and (3) optical images obtained with the European Southern Observatory (ESO) Very Large Telescope (VLT) with U, B, V, R, and I filters. 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 significantly with position, reaching a limiting depth of AB(16,000) ~ 28.7 and covering 1.01 arcmin^2 to AB(16,000) = 27 and 1.05 arcmin^2 to AB(16,000) = 26.5. The catalog of photometric redshifts identifies 21 galaxies (or 6% of the total) of redshift z > 5, 8 galaxies (or 2% of the total) of redshift z > 10, and 11 galaxies (or 3% of the total) of best-fit spectral type E/S0, of which 5 galaxies (or 1% of the total) are of redshift z > 1.Comment: 33 pages, 10 figures, accepted for publication in the Astrophysical Journal, August 1, 2000 issu

Direct Distance Measurements to Superluminal Radio Sources

We present a new technique for directly measuring the distances to superluminal radio sources. By comparing the observed proper motions of components in a parsec scale radio jet to their measured Doppler factors, we can deduce the distance to the radio source independent of the standard rungs in the cosmological distance ladder. This technique requires that the jet angle to the line of sight and the ratio of pattern to flow velocities are sufficiently constrained. We evaluate a number of possibilities for constraining these parameters and demonstrate the technique on a well defined component in the parsec scale jet of the quasar 3C279 (z = 0.536). We find an angular size distance to 3C279 of greater than 1.8 (+0.5,-0.3) n^{1/8} Gpc, where n is the ratio of the energy density in the magnetic field to the energy density in the radiating particles in that jet component. For an Einstein-de Sitter Universe, this measurement would constrain the Hubble constant to be H < 65 n^{-1/8} km/s/Mpc at the two sigma level. Similar measurements on higher redshift sources may help discriminate between cosmological models.Comment: 18 pages, 8 figures, to be published in The Astrophysical Journa