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 \pm 0.47 \times 10 ^ { 11 } M _ { \odot } , with a star formation rate of 214 \pm 44 M _ { \odot } 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 ( \sim 6 \times 10 ^ { 4 } cm ^ { -3 } ) and a UV-radiation field ( \sim 2 \times 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 _ { H 2 ,dep } \leq 140 Myr ) we find in 3D-HST GS30274 using three different proxies for the H _ { 2 } \xspace 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 \sim 750 \pm 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 .