Do submillimeter galaxies really trace the most massive dark-matter halos? Discovery of a high-z cluster in a highly active phase of evolution

Abstract

We present detailed observations of a z ~ 1.99 cluster of submillimeter galaxies (SMGs), discovered as the strongest redshift spike in our entire survey of ~100 SMGs across 800 arcmin^2. It is the largest blank-field SMG concentration currently known and has <0.01% chance of being drawn from the underlying selection function for SMGs. We have compared UV observations of galaxies at this redshift, where we find a much less dramatic overdensity, having an 11% chance of being drawn from its selection function. We use this z ~ 1.99 overdensity to compare the biasing of UV- and submillimeter-selected galaxies, and test whether SMGs could reside in less overdense environments, with their apparent clustering signal being dominated by highly active merger periods in modest mass structures. We discuss the probable mechanisms for the apparently different bias we see at the two wavelengths. This impressively active formation phase in a low-mass cluster is not something seen in simulations, although we propose a toy model using merger bias, which could account for the bias seen in the SMGs. While enhanced buildup of stellar mass appears characteristic of other high-z galaxy clusters, neither the UV galaxies nor SMGs in this structure exhibit larger stellar masses than their field galaxy counterparts (although the excess of SMGs in the structure represents a larger volume-averaged stellar mass than the field). Our findings have strong implications for future surveys of high-z galaxies at long wavelengths such as SCUBA2 and Herschel. We suggest that since these surveys will select galaxies during their episodes of peak starbursts, they could probe a much wider range of environments than just the progenitors of rich clusters, revealing more completely the key events and stages in galaxy formation and assembly.