Galaxies at High Redshifts - Observing Galaxies in the Cradle

Abstract

Due to the invention of new powerful instruments in the recent past (e.g. 10m class telescopes) high redshift galaxies are no longer a curiosity. High redshift young star forming galaxies can be effectively discriminated from the much more abundant foreground galaxies by their special spectral properties: a prominent break at the Lyman limit (i.e. a complete absence of flux at wavelength below the Lyman limit), an intrinsically flat spectrum at wavelength long-ward of the Lyman limit and in the very early phase of evolution a strong Ly-alpha emission line with high equivalent width. In the last couple of years several hundred star forming galaxies with 2.5 < z < 3.5 could be identified using deep broad band images which identify the Lyman break (Steidel et al. 1996, 1998). Spectroscopic followup observations confirmed their high redshifts. I summarize the main properties of these Lyman break galaxies (LBGs). Furthermore, the very recent discoveries of very high-z strong Ly-alpha emitting galaxies at z>5 and new large systematic surveys (the Calar Alto Deep Imaging Survey and the survey of Hu et al. 1998) for such objects, suggest that the cosmic star formation rate in strong Ly-alpha emitters does not decrease at redshifts z > 3.5 as suggested for the Lyman break galaxy sample. I discuss the galaxies with the highest redshifts we know today and give an overview over the survey of Hu et al. and the Calar Alto Deep Imaging Survey.