Kiloparsec-scale imaging of the CO(1-0)-traced cold molecular gas reservoir in a z~3.4 submillimeter galaxy

Abstract

We present a high-resolution study of the cold molecular gas as traced by CO(1-0) in the unlensed z$\sim$3.4 submillimeter galaxy SMM J13120+4242, using multi-configuration observations with the Karl G. Jansky Very Large Array (JVLA). The gas reservoir, imaged on 0.39" ($\sim$3 kpc) scales, is resolved into two components separated by $\sim$11 kpc with a total extent of 16 $\sim$3 kpc. Despite the large spatial extent of the reservoir, the observations show a CO(1-0) FWHM linewidth of only 267 $\pm$ 64 km s${}^{-1}$. We derive a revised line luminosity of L'${}_{\mathrm{CO}(1-0)}$ = (10 $\pm$ 3) $\times$ 10${}^{10}$ K km s${}^{-1}$ pc${}^2$ and a molecular gas mass of M${}_{\mathrm{gas}}$ = (13 $\pm$ 3) $\times$ 10${}^{10}$ ($\hat{I}{\pm }_{\mathrm{CO}}$/1) M${}_{\odot }$. Despite the presence of a velocity gradient (consistent with previous resolved CO(6-5) imaging), the CO(1-0) imaging shows evidence for significant turbulent motions which are preventing the gas from fully settling into a disk. The system likely represents a merger in an advanced stage. Although the dynamical mass is highly uncertain, we use it to place an upper limit on the CO-to-H${}_2$ mass conversion factor $\hat{I}{\pm }_{\mathrm{CO}}$ of 1.4. We revisit the SED fitting, finding that this galaxy lies on the very massive end of the main sequence at z = 3.4. Based on the low gas fraction, short gas depletion time and evidence for a central AGN, we propose that SMM J13120 is in a rapid transitional phase between a merger-driven starburst and an unobscured quasar. The case of SMM J13120 highlights the how mergers may drive important physical changes in galaxies without pushing them off the main sequence.

19 pages, 12 figures, accepted for publication in ApJ