The implications of the surprising existence of a large, massive CO disk in a distant protocluster

It is not yet known if the properties of molecular gas in distant protocluster galaxies are significantly a ff ected by their environment as galaxies are in local clusters. Through a deep, 64 h of e ff ective on-source integration with the Australian Telescope Compact Array (ATCA), we discovered a massive, M-mol = 2 : 0 +/- 0 : 2 x 10(11) M-circle dot, extended, similar to 40 kpc, CO(1-0)-emitting disk in the protocluster surrounding the radio galaxy, MRC1138 262. The galaxy, at zCO = 2 : 1478, is a clumpy, massive disk galaxy, M-* similar to 5 x 10(11) M-circle dot, which lies 250 kpc in projection from MRC1138-262 and is a known H ff emitter, named HAE229. This source has a molecular gas fraction of similar to 30%. The CO emission has a kinematic gradient along its major axis, centered on the highest surface brightness rest-frame optical emission, consistent with HAE229 being a rotating disk. Surprisingly, a significant fraction of the CO emission lies outside of the UV /optical emission. In spite of this, HAE229 follows the same relation between star-formation rate and molecular gas mass as normal field galaxies. HAE229 is the first CO(1-0) detection of an ordinary, star-forming galaxy in a protocluster. We compare a sample of cluster members at z > 0 : 4 that are detected in low-order CO transitions, with a similar sample of sources drawn from the field. We confirm findings that the CO-luminosity and full-width at half maximum are correlated in starbursts and show that this relation is valid for normal high-z galaxies as well as for those in overdensities. We do not find a clear dichotomy in the integrated Schmidt-Kennicutt relation for protocluster and field galaxies. Our results suggest that environment does not have an impact on the star-formation e ffi ciency or the molecular gas content of high-redshift galaxies. Not finding any environmental dependence in these characteristics, especially for such an extended CO disk, suggests that environmentally-specific processes such as ram pressure stripping do not operate e ffi ciently in (proto) clusters.