Volume 602, June 2017
|Number of page(s)||10|
|Published online||24 May 2017|
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
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
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 (tH2,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.
Key words: galaxies: high-redshift / galaxies: evolution / galaxies: ISM / ISM: atoms / ISM: molecules / ISM: lines and bands
© ESO, 2017
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