2017A&A...602A..11P

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±0.47x10¹¹M_☉, with a star formation rate (SFR) of 214±44M_☉/yr putting it on the star-forming "main-sequence", but with an H-band effective radius of 2.5kpc, making it much smaller than the bulk of "main-sequence" star-forming galaxies. The intensity ratio of the line detections yield an ISM density (∼6x10⁴cm^–3) and a UV-radiation field (∼2x10⁴G₀), 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≤140Myr) 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±41km/s. 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.