Molecular gas properties of a lensed star-forming galaxy at z ~ 3.6: a case study^⋆

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
e-mail: miroslava.dessauges@unige.ch
² Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, 10330 Bangkok, Thailand
³ Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo Institutes for Advanced Study, Kashiwa, 277-8583 Chiba, Japan
⁴ Université Lyon, Université Lyon 1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR 5574, 69230 Saint-Genis-Laval, France
⁵ CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France
⁶ Observatoire de Paris, LERMA, 61 Avenue de l’Observatoire, 75014 Paris, France
⁷ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁸ ESA/Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁰ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Versoix, Switzerland
¹¹ Aix Marseille Université, CNRS, LAM, UMR 7326, 13388 Marseille, France
¹² National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
¹³ University of California, Center for Astrophysics and Space Sciences, 9500 Gilman Drive, San Diego, CA 92093, USA

Received: 14 March 2016
Accepted: 12 June 2017

Abstract

We report on the galaxy MACSJ0032-arc at z_CO = 3.6314 discovered during the Herschel Lensing snapshot Survey of massive galaxy clusters, and strongly lensed by the cluster MACS J0032.1+1808. The successful detections of its rest-frame ultraviolet (UV), optical, far-infrared (FIR), millimeter, and radio continua, and of its CO emission enable us to characterize, for the first time at such a high redshift, the stellar, dust, and molecular gas properties of a compact star-forming galaxy with a size smaller than 2.5 kpc, a fairly low stellar mass of 4.8^{+ 0.5}_-1.0 × 10⁹M_⊙, and a moderate IR luminosity of 4.8^{+ 1.2}_-0.6 × 10¹¹L_⊙. By combining the stretching effect of the lens with the high angular resolution imaging of the CO(1–0) line emission and the radio continuum at 5 GHz, we find that the bulk of the molecular gas mass and star formation seems to be spatially decoupled from the rest-frame UV emission. About 90% of the total star formation rate is undetected at rest-frame UV wavelengths because of severe obscuration by dust, but is seen through the thermal FIR dust emission and the radio synchrotron radiation. The observed CO(4–3) and CO(6–5) lines demonstrate that high-J transitions, at least up to J = 6, remain excited in this galaxy, whose CO spectral line energy distribution resembles that of high-redshift submm galaxies, even though the IR luminosity of MACSJ0032-arc is ten times lower. This high CO excitation is possibly due to the compactness of the galaxy. We find evidence that this high CO excitation has to be considered in the balance when estimating the CO-to-H₂ conversion factor. Indeed, the respective CO-to-H₂ conversion factors as derived from the correlation with metallicity and the FIR dust continuum can only be reconciled if excitation is accounted for. The inferred depletion time of the molecular gas in MACSJ0032-arc supports the decrease in the gas depletion timescale of galaxies with redshift, although to a lesser degree than predicted by galaxy evolution models. Instead, the measured molecular gas fraction as high as 60–79% in MACSJ0032-arc favors the continued increase in the gas fraction of galaxies with redshift as expected, despite the plateau observed between z ~ 1.5 and z ~ 2.5.