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.