First results from a deep spectroscopic survey of faint red galaxies : clues on the nature of low redshift dwarf galaxies.
Deep multi-object spectroscopy of a sample of 44 faint galaxies selected on deep I band images is presented. The combination of long integration times with careful data reduction has led to a completeness in redshift measurement of 93% to a limiting magnitude I = 22.1. Although we have selected our galaxies from their old stellar content we confirm the existence of an over-abundant population of dwarf galaxies at low redshifts, which are probably responsible for the excess in the blue number counts. Combined with a slight deficiency of high-z galaxies (no galaxies beyond z = 1),our data provide red counts apparently consistent with an absence of luminosity and number evolution. Most of the low redshift dwarf galaxies in our sample are star-forming galaxies which is an indication of a high level of activity at relatively low redshifts. We present the discovery of a dwarf galaxy at z = 0.21, seemingly forming its first generation of population II stars, making it one of the best primordial galaxy candidate. Evidence for galaxy-galaxy interactions in several other dwarfs is presented, especially in the most active ones. We finally stress that differences between red and blue counts result from a complex mixture of effects which will have to be studied from a much larger spectroscopic sample.
Astronomy and Astrophysics
- Pub Date:
- September 1993