Internal structure of molecular gas in a main sequence galaxy with a UV clump at z = 1.45

TL;DR

This study uses high-resolution ALMA observations to analyze the molecular gas distribution and properties in a main sequence galaxy at z=1.45, revealing a concentrated gas structure associated with a UV clump and a rotating disk morphology.

Contribution

First detailed ALMA analysis of molecular gas distribution and properties in a z=1.45 main sequence galaxy with a UV clump, linking gas density to galaxy structure.

Findings

CO emission lines detected with flux ratio ~1, similar to the Milky Way.

The UV clump coincides with a peak in CO(5-4)/CO(2-1) ratio, indicating high gas density or temperature.

Molecular gas is more centrally concentrated than stars, with a decreasing gas fraction outward.

Abstract

We present results of sub-arcsec ALMA observations of CO(2-1) and CO(5-4) toward a massive main sequence galaxy at z = 1.45 in the SXDS/UDS field, aiming at examining the internal distribution and properties of molecular gas in the galaxy. Our target galaxy consists of the bulge and disk, and has a UV clump in the HST images. The CO emission lines are clearly detected and the CO(5-4)/CO(2-1) flux ratio (R_52) is ~1, similar to that of the Milky Way. Assuming a metallicity dependent CO-toH_2 conversion factor and a CO(2-1)/CO(1-0) flux ratio of 2 (the Milky Way value), the molecular gas mass and the gas mass fraction (f_gas = molecular gas mass / (molecular gas mass + stellar mass)) are estimated to be ~1.5x10^11 M_Sun and ~0.55, respectively. We find that R_52 peak coincides with the position of the UV clump and its value is approximately two times higher than the galactic average. This result implies high gas density and/or high temperature in the UV clump, which qualitatively agrees with a numerical simulation of a clumpy galaxy. The CO(2-1) distribution is well represented by a rotating disk model and its half-light radius is ~2.3 kpc. Compared to the stellar distribution, the molecular gas is more concentrated in the central region of the galaxy. We also find that f_gas decreases from ~0.6 at the galactic center to ~0.2 at 3xhalf-light radius, indicating that the molecular gas is distributed in more central region of the galaxy than stars and seems to associate with the bulge rather than the stellar disk.