Molecular gas in AzTEC/C159: a star-forming disk galaxy 1.3 Gyr after the Big Bang

¹ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: ericja@astro.uni-bonn.de
² International Max Planck Research School of Astronomy and Astrophysics, Universities of Bonn and Cologne, Bonn, Germany
³ Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy Pl, Socorro, NM 87801, USA
⁴ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
⁵ Cavendish Astrophysics Group, University of Cambridge, Cambridge CB3 0HE, UK
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA
⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁰ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
¹¹ Department of Physics and Astronomy, University of Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
¹² Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
¹³ NASA Goddard Space Flight Center, Code 665, Greenbelt, MD 20771, USA
¹⁴ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
¹⁵ Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
¹⁶ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁷ Science Mission Directorate, NASA Headquarters, Washington, DC 20546-0001, USA
¹⁸ Infrared Processing and Analysis Center, California Institute of Technology, MC 100-22, 770 South Wilson Ave., Pasadena, CA 91125, USA
¹⁹ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
²⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
²² Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
²³ Greenhouse Software, 3rd Floor, 110 5th Avenue, New York, NY 10011, USA
²⁴ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Athens, Greece
²⁵ Max Planck Institute for Radioastronomy, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 27 October 2017
Accepted: 9 February 2018

Abstract

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at z = 4.567, in order to better constrain the nature of the high-redshift end of the submillimeter-selected galaxy (SMG) population. We secured ¹²CO molecular line detections for the J = 2 →1 and J = 5 →4 transitions using the Karl G. Jansky Very Large Array (VLA) and the NOrthern Extended Millimeter Array (NOEMA) interferometer. The broad (FWHM ~ 750 km s⁻¹) and tentative double-peaked profiles of the two ¹²CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk, as previously revealed from [CII] 158 μm line observations. Based on the ¹²CO(2 →1) emission line, we derived L^′_CO=(3.4±0.6)×10¹⁰ K km s⁻¹ pc², which yields a molecular gas mass of M_H2(α_CO/4.3)=(1.5±0.3)×10¹¹ M_⊙ and unveils a gas-rich system with μ_gas(α_CO/4.3)≡M_H2/M_⋆=3.3±0.7. The extreme star formation efficiency of AzTEC/C159, parametrized by the ratio L_IR/L^′_CO=(216±80) L_⊙ (K km s⁻¹ pc²)⁻¹, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies and SMGs. Likewise, the ¹²CO(5 →4)/CO(2 →1) line brightness temperature ratio of r₅₂ = 0.55 ± 0.15 is consistent with high-excitation conditions as observed in SMGs. Based on mass budget considerations, we constrained the value for the L^′_CO – H₂ mass conversion factor in AzTEC/C159, that is, α_CO=3.9_−1.3^+2.7 M_⊙ K⁻¹ km⁻¹ s pc⁻², which is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps and forms stars as efficiently as in merger-driven systems and generates high gas excitation. These results support the evolutionary connection between AzTEC/C159-like systems and massive quiescent disk galaxies at z ~ 2.