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

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 x 10(11) M-circle dot, with a star formation rate (SFR) of 214 +/- 44 M-circle dot 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 (similar to 6 x 10(4) cm(-3)) and a UV-radiation field (similar to 2 x 10(4) G(0)), 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 H-2 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 similar to 750 +/- 41 km s(-1). The CO and [CI] FWHM are consistent with a previously measured H alpha 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.