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

H. Dannerbauer, M. D. Lehnert, B. H. C. Emonts, B. Ziegler, B. Altieri, C. De Breuck, N. Hatch, T. Kodama, Y. Koyama, J. D. Kurk, T. Matiz, G. Miley, D. Narayanan, R. Norris, R. Overzier, H. J. A. Roettgering, M. Sargent, N. Seymour, M. Tanaka, I. Valtchanov, D. Wylezalek. 2017. The implications of the surprising existence of a large, massive CO disk in a distant protocluster. Astronomy and Astrophysics 608, DOI: 10.1051/0004-6361/201730449

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.

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