A Proto-Galaxy Candidate at z=2.7 Discovered by its Young Stellar Population

Abstract

A proto-galaxy candidate at z = 2.72 has been discovered serendipitously by the CNOC cluster redshift survey. The candidate is an extremely luminous (V = 20.5 mag, absolute mag -26) and well resolved (2"X 3") disk-like galaxy. The redshift is identified from a dozen strong UV absorption lines, including lines with P-Cygni profiles, which are indicative of the presence of young O and B stars. No emission lines are found between 1000 and 2000 A (rest), including Lyalpha. The surface brightness profile of the galaxy fits an exponential law with a scale length of ~3.5 kpc. The multi-color photometric data fit the spectral energy distributions of a stellar population from 400 million years to an arbitrary young age, dependent on the amount of dust extinction. However, the presence of a strong P-Cygni profile in CIVlambda1550 indicates that a very substantial component of the stellar population must be younger than ~10 Myr. These models predict that this galaxy will evolve into a bright galaxy of several L^*^ in brightness. We can interpret this object as an early type galaxy observed within about 100 million years of the initial burst of star formation which created most of its stellar mass, producing the extremely high luminosity. Because of the resolved, regular, and smooth nature of the object, it is unlikely that the high luminosity is due to gravitational lensing. We estimate the sky density of this type of objects observable at any one time to be 10^0+/-1^ per square degree.