Resolving the Faint Galaxy Excess with HST: Results from the Medium Deep Survey

Abstract

WFPC2 on HST has provided the means to make rapid progress towards solving a long-standing problem in observational cosmology, viz. the nature of the objects contributing to the faint blue galaxy counts. The solution to this problem has been found in the increasing number of irregular and peculiar systems seen as HST probes to greater depths. Galaxies in the Medium Deep Survey (MDS) have been reliably classified to magnitudes I ₈₁₄ ≲ 22.0 in the F814W band, at a mean redshift z ∼0.5. A high proportion (∼40%) of these objects are irregular or anomalous and they have great diversity. They include compact galaxies, galaxies or protogalaxies with superluminous starforming regions, interacting pairs, and diffuse low surface brightness galaxies of various forms. These diverse objects contribute most of the excess counts in the I-band at our limiting magnitude, and when MDS images with BVI data are taken into account, they also explain most of the ‘faint blue galaxy‘ excess. Of the irregular population (40% of the total at I=22), about 30% show multiple, high surface brightness components indicative of starburst activity.

But roughly half of the faint galaxies (I < 22) appear to be similar to regular Hubble-sequence examples observed at low redshift, with relative numbers of spheroidal and disk systems roughly consistent with nearby smaples, indicating that the bulk of the local giant galaxy population was in place at half the Hubble time. Moreover, the giant population has certainly faded since z = 1, by about 1 mag. for the E/S0s and about the same for the early-type spirals. For the first time, we have observed directly the evolution of the luminosity function of E/S0s galaxies. We have found evidence for weak gravitational shear in the vicinity of E/S0 field galaxies, and have used this effect to constrain their masses and the cut-off radii of their dark matter halos.

The “major” merger rate has revolved only slowly with look-back time, as (1+ z)^1.2 out to z = 1 or 1.5, although there is already evidence from the deeper surveys that the rate may have been greater at higher redshifts. “Major” merging is probably not a majpr component of the explanation for the faint blue galaxy excess (to I = 22. “Minor merging” is a different story, and more difficult to address observationally. The deeper HST surveys, addressing the redshift range from 1 to 2, show increasing evidence of “conglomerations”, i.e. groupings of what appear to be sub-galactic ‘clumps’.

The clear picture which has thus emerged from the HST MDS is one in which the giant ellipticals and spirals have passively faded but have otherwise been relatively stable since z = 1, whereas there has been rapid evolution of the irregular/peculiar galaxy population - about 20% of them were brighter by about 2 mags. only 6 Gyr. ago. This latter population is not at all homogenous, however , and seems to comprise galaxies in formation as well as fading dwarf irregulars.