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

¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
e-mail: helmut@iac.es
² Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Universität Wien, Institut für Astrophysik, Türkenschanzstraße 17, 1180 Vienna, Austria
⁴ Sorbonne Universités, UPMC Univ. Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
⁵ Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷ Herschel Science Centre, European Space Astronomy Centre, ESA, 28691 Villanueva de la Cañada, Spain
⁸ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁹ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
¹⁰ Optical and Infrared Astronomy Division, National Astronomical Observatory of Japan, Mitaka, 181-8588 Tokyo, Japan
¹¹ Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, 252-5210 Kanagawa, Japan
¹² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
¹³ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁴ University of Florida, Department of Astronomy, 211 Bryant Space Sciences Center, Gainesville, FL 32611-2055, USA
¹⁵ CSIRO Astronomy & Space Science, PO Box 76, Epping, NSW 1710, Australia
¹⁶ Western Sydney University, Locked Bag 1797, Penrith South, NSW 1797, Australia
¹⁷ Observatório Nacional, Rua José Cristino 77, 20921-400 São Cristóvão, Rio de Janeiro, Brazil
¹⁸ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁹ International Center for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²⁰ Johns Hopkins University Bloomberg Center, Department of Physics & Astronomy, 3400 N. Charles Street, Baltimore, MD 21218, USA

Received: 17 January 2017
Accepted: 3 July 2017

Abstract

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 h of effective on-source integration with the Australian Telescope Compact Array (ATCA), we discovered a massive, M_mol = 2.0 ± 0.2× 10¹¹ M_⊙, extended, ~40 kpc, CO(1–0)-emitting disk in the protocluster surrounding the radio galaxy, MRC 1138−262. The galaxy, at z_CO = 2.1478, is a clumpy, massive disk galaxy, M_∗ ~ 5 × 10¹¹ M_⊙, which lies 250 kpc in projection from MRC 1138−262 and is a known Hα emitter, named HAE229. This source 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 thatare 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 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 do not operate efficiently in (proto)clusters.