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

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(2017)cite arxiv:1703.05764Comment: Accepted for publication in A&A.
DOI: 10.1051/0004-6361/201730391

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 $1.89 0.47\,10^11\,M_ødot$, with a star formation rate of $214\pm44\,M_ødot\,yr^-1$ 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 $\times 10^4\,cm^-3$) and a UV-radiation field ( ~2 $10^4\,G_0$), 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 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 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. 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.

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