Molecular gas in two companion cluster galaxies at z = 1.2

¹ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 75014 Paris, France
e-mail: gianluca.castignani@obspm.fr
² Collège de France, 11 Place Marcelin Berthelot, 75231 Paris, France
³ INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁴ Department of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
⁵ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
⁶ Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
⁷ International Centre for Radio Astronomy Research, University of Western Australia M468, 35 Stirling Highway, Crawley WA 6009, Australia
⁸ National Radio Astronomy Observatory, PO Box O, Socorro NM 87801, USA
⁹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

Received: 23 February 2018
Accepted: 5 June 2018

Abstract

Context. Probing both star formation history and evolution of distant cluster galaxies is essential to evaluate the effect of dense environment on shaping the galaxy properties we observe today.

Aims. We investigate the effect of cluster environment on the processing of the molecular gas in distant cluster galaxies. We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the Irac Shallow Cluster Survey, at a redshift z = 1.2, that is, ~ 2 Gyr after the cosmic star formation density peak. This work describes the first CO detection from 1 < z < 1.4 star-forming cluster galaxies with no clear reported evidence of active galactic nuclei.

Methods. We exploit observations taken with the NOEMA interferometer at ~3 mm to detect CO(2−1) line emission from the two selected galaxies, unresolved by our observations.

Results. Based on the CO(2−1) spectrum, we estimate a total molecular gas mass M(H₂) = (2.2^+0.5_0.4) × 10¹⁰ M_⊙, where fully excited gas is assumed, and a dust mass M_dust < 4.2 × 10⁸ M_⊙ for the two blended sources. The two galaxies have similar stellar masses and Hα-based star formation rates (SFRs) found in previous work, as well as a large relative velocity of ~400 km s⁻¹ estimated from the CO(2−1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2−1). Using the archival Spitzer MIPS flux at 24 μm we estimate an SFR (24μm) = (28⁺¹²₋₈) M_⊙/yr for each of the two galaxies. Assuming that the two sources contribute equally to the observed CO(2−1), our analysis yields a depletion timescale of τ_dep = (3.9^+1.4_−1.8) × 10⁸ yr, and a molecular gas to stellar mass ratio of 0.17 ± 0.13 for each of two sources, separately. We also provide a new, more precise measurement of an unknown weighted mean of the redshifts of the two galaxies, z = 1.163 ± 0.001.

Conclusions. Our results are in overall agreement with those of other distant cluster galaxies and with model predictions for main sequence (MS) field galaxies at similar redshifts. The two target galaxies have molecular gas mass and depletion times that are marginally compatible with, but smaller than those of MS field galaxies, suggesting that the molecular gas has not been sufficiently refueled. We speculate that the cluster environment might have played a role in preventing the refueling via environmental mechanisms such as galaxy harassment, strangulation, ram-pressure, or tidal stripping. Higher-resolution and higher-frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.