ALMA reveals starburst-like interstellar medium conditions in a compact star-forming galaxy at z ~ 2 using [CI] and CO

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: gpopping@eso.org
² Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr., 85748 Garching, Germany
⁴ Aix Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁵ Astronomy Department, Yale University, New Haven, CT 06511, USA
⁶ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁷ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands

Received: 4 January 2017
Accepted: 9 March 2017

Abstract

We present ALMA detections of the [CI] 1–0, CO J = 3–2, and CO J = 4–3 emission lines, as well as the ALMA band 4 continuum for a compact star-forming galaxy (cSFG) at z = 2.225, 3D-HST GS30274. As is typical for cSFGs, this galaxy has a stellar mass of 1.89 ± 0.47 × 10¹¹M_⊙, with a star formation rate (SFR) of 214 ± 44 M_⊙ yr^-1 putting it on the star-forming “main-sequence”, but with an H-band effective radius of 2.5 kpc, making it much smaller than the bulk of “main-sequence” star-forming galaxies. The intensity ratio of the line detections yield an ISM density (~ 6 × 10⁴ cm^-3) and a UV-radiation field (~ 2 × 10⁴G₀), similar to the values in local starburst and ultra-luminous infrared galaxy environments. A starburst phase is consistent with the short depletion times (t_H2,dep ≤ 140 Myr) we find in 3D-HST GS30274 using three different proxies for the H₂ mass ([CI], CO, dust mass). This depletion time is significantly shorter than in more extended SFGs with similar stellar masses and SFRs. Moreover, the gas fraction of 3D-HST GS30274 is smaller than typically found in extended galaxies. We measure the CO and [CI] kinematics and find a FWHM line width of ~ 750 ± 41 km s^-1. The CO and [CI] FWHM are consistent with a previously measured Hα FWHM for this source. The line widths are consistent with gravitational motions, suggesting we are seeing a compact molecular gas reservoir. A previous merger event, as suggested by the asymmetric light profile, may be responsible for the compact distribution of gas and has triggered a central starburst event. This event gives rise to the starburst-like ISM properties and short depletion times in 3D-HST GS30274. The centrally located and efficient star formation is quickly building up a dense core of stars, responsible for the compact distribution of stellar light in 3D-HST GS30274.