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

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posted on 2023-06-09, 08:54 authored by H Dannerbauer, M D Lehnert, B Emonts, B Ziegler, C De Breuck, N Hatch, T Kodama, Y Koyama, J Kurk, T Matiz, G Miley, D Narayanan, R Norris, R Overzier, H J A Röttgering, Mark Sargent, N Seymour, M Tanaka, I Valtchanov, D Wylezalek
It is not yet known if the properties of molecular gas in distant protocluster galaxies are significantly affected by their environment as galaxies are in local clusters. Through a deep, 64 hours of effective on-source integration with the Australian Telescope Compact Array (ATCA), we discovered a massive, Mmol = 2.0 ± 0.2 × 10^11 M?, extended, ~40 kpc, CO(1-0)-emitting disk in the protocluster surrounding the radio galaxy, MRC 1138-262. The galaxy, at zCO = 2.1478, is a clumpy, massive disk galaxy, M* ~ 5 × 10^11 M?, which lies 250 kpc in projection from MRC 1138-262 and is a known Ha emitter, HAE229. HAE229 has a molecular gas fraction of ~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 FWHM are correlated in starbursts and show that this relation is valid for normal high-z galaxies as well as those in overdensities. We do not find a clear dichotomy in the integrated Schmidt-Kennicutt relation for protocluster and field galaxies. Our results uggest that environment does not impact the “star-formation efficiency” 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 are not operating efficiently in (proto)clusters. We discuss why this might be so.


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