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 lies in the increasing number of irregular and peculiar systems seen as a function of increasing magnitude. 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 z̄ ∼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 probably explain most of the 'faint blue galaxy' excess. Of the irregular population (40% of the total), about 30% show multiple, high surface brightness nuclear components. At least half of the faint galaxies, however, appear to be similar to regular Hubble-sequence examples observed at low redshift. Furthermore, the relative proportion of spheroidal and disk systems of normal appearance is as expected from nearby samples, indicating that the bulk of the local giant galaxy population was in place at half the Hubble time. Little evolution in the properties of these giant galaxies has been observed. The clear picture which thus emerges from the MDS is one in which the giant ellipticals and spirals have been relatively stable since z = 0.7, whereas there is rapid evolution of the irregular galaxy population. This latter population is not at all homogeneous, however, and seems to comprise galaxies in formation as well as fading dwarf irregulars. We have found evidence for weak gravitational shear in the vicinity of field galaxies, and gravitational lenses in MDS and archival data, including the first two examples of HST-discovered "Einstein crosses" centered on elliptical galaxies.
|Original language||English (US)|
|Number of pages||7|
|Journal||Astrophysical Letters and Communications|
|State||Published - Dec 1 1997|
ASJC Scopus subject areas
- Astronomy and Astrophysics