A PROTO-GALAXY CANDIDATE AT Z=2.7 DISCOVERED BY ITS YOUNG STELLAR POPULATION

A proto-galaxy candidate at z=2.72 has been discovered serendipitously by the CNOC cluster redshift survey, The candidate is an extremely lu minous (V=20.5 mag, absolute mag -26) and well resolved (2 '' x3 '') d isk-like galaxy, The redshift is identified from a dozen strong UV abs orption lines, including lines with P-Cygni profiles, which are indica tive of the presence of young O and B stars. No emission lines are fou nd between 1000 and 2000 Angstrom (rest), including Ly alpha. The surf ace brightness profile of the galaxy fits an exponential law with a sc ale length of similar to 3.5 kpc. The multi-color photometric data fit the spectral energy distributions of a stellar population from 400 mi llion years to an arbitrary young age, dependent on the amount of dust extinction, However, the presence of a strong P-Cygni profile in CIV lambda 1550 indicates that a very substantial component of the stellar population must be younger than similar to 10 Myr. These models predi ct that this galaxy will evolve into a bright galaxy of several L in brightness. We can interpret this object as an early type galaxy obser ved within about 100 million years of the initial burst of star format ion which created most of its stellar mass, producing the extremely hi gh luminosity, Because of the resolved, regular, and smooth nature of the object, it is unlikely that the high luminosity is due to gravitat ional lensing. We estimate the sky density of this type of objects obs ervable at any one time to be 10(0+/-1) per square degree. (C) 1996 Am erican Astronomical Society.