= = the butcher-oemler effect (butcher and oemler 1978, 1984; bo-effect in what follows) is the evidence for the existence of a different population of galaxies in distant clusters that is not found in nearby ones. this population consists of a mix of starburst and post-starburst galaxies, both blue spirals and red early-type galaxies. the physical process(es) responsible for this effect, i.e. a rapid evolution of cluster galaxies, has (have) not been found yet, although many have been proposed (e.g. oemler 1992). the difficulty lies in the fact that these galaxies are quite distant (redshift of about 0.3). recently, however, caldwell et al. (1993) have identified a low-redshift analogue of the bo-effect in a sample of 17 early-type galaxies in the coma cluster. these galaxies have an enhanced balmer absorption, which is abnormal for their red colours, plus (in some cases) emission-lines. their spectrum is indicative of a post-starburst phase that would have occurred 1 gyr ago in an early-type galaxy, or of a truncated stellar formation in a late-type one. most important, their spectra are very similar to those seen in a distant cluster of galaxies which shows evidence of the bo-effect, so that studying the properties of these easy-access relatively nearby galaxies will finally explain what is the cause of the bo-effect. ir observations with iso will be extremely useful for characterizing the post-starburst properties of these galaxies, thanks to the high sensitivity achievable and the large wavelength range that can be covered.
Instrument
CAM01
Temporal Coverage
1997-07-24T21:52:15Z/1997-07-25T00:25:45Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the world's first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
