Infrared Observations of Galaxy Clusters

However evidence is growing for the presence of well evolved rich clusters and cluster galaxies at high redshift (above z=0.5-1; Dickinson 1997, Mushotzky and Loewenstein 1997), which sets a key constraint on galaxy evolution models and on structure formation models as well. After IRAS, which proved that starbursts emitted mostly in the infrared (Sanders & Mirabel 1996), ISOCAM is finding a strong excess of faint mid-IR (12-18 μm) emitting galaxies (Cesarsky 1997), demonstrating that dust may affect our view of the star formation history of the universe. This is confirmed at large redshifts by the effect of absorption in U and B drop-out selected galaxies (Pettini et al. 1997). The fact that galaxy interaction plays a dominant role both in the hierarchical clustering scenario and in IRAS ultra-luminous galaxies, strengthens the interest of studying the effect of the environment on the evolution of cluster galaxies, whereas the presence of dust requires the use of infrared telescopes like ISO to complete the analysis. Observations of galaxy clusters can also set strong constraints on the evolution of galaxies through the analysis of intra-cluster iron (Elbaz et al. 1995), but also on diffuse dust emission outside galaxies, which could give real-time information on the current status of the cluster galaxies which would necessarily be replanishing dust (see below).