The redshift and nature of AzTEC/COSMOS 1: A starburst galaxy at z=4.6
  Based on broad/narrow-band photometry and Keck DEIMOS spectroscopy we report
a redshift of z=4.64-0.08+0.06 for AzTEC/COSMOS 1, the brightest sub-mm galaxy
in the AzTEC/COSMOS field. In addition to the COSMOS-survey X-ray to radio
data, we report observations of the source with Herschel/PACS (100, 160
micron), CSO/SHARC II (350 micron), CARMA and PdBI (3 mm). We do not detect
CO(5-4) line emission in the covered redshift ranges, 4.56-4.76 (PdBI/CARMA)
and 4.94-5.02 (CARMA). If the line is within this bandwidth, this sets 3sigma
upper limits on the gas mass to <~8x10^9 M_Sol and <~5x10^10 M_Sol,
respectively (assuming similar conditions as observed in z~2 SMGs). This could
be explained by a low CO-excitation in the source. Our analysis of the UV-IR
spectral energy distribution of AzTEC 1 shows that it is an extremely young
(<~50 Myr), massive (M*~10^11 M_Sol), but compact (<~2 kpc) galaxy forming
stars at a rate of ~1300 M_Sol/yr. Our results imply that AzTEC 1 is forming
stars in a 'gravitationally bound' regime in which gravity prohibits the
formation of a superwind, leading to matter accumulation within the galaxy and
further generations of star formation.
