Discovery of a [CI]-faint, CO-bright Galaxy: ALMA Observations of the Merging Galaxy NGC 6052

Abstract

We report sensitive [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ and ${}^{12}$CO~$J$=4--3 observations of the nearby merging galaxy NGC 6052 using the Morita (Atacama Compact) Array of ALMA. We detect ${}^{12}$CO~$J$=4--3 toward the northern part of NGC 6052, but [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ is not detected with a [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ to ${}^{12}$CO~$J$=4--3 line luminosity ratio of$\lesssim 0.07$. According to models of photodissociation regions, the unusual weakness of [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ relative to ${}^{12}$CO~$J$=4--3 can be explained if the interstellar medium has a hydrogen density larger than $10^5{\mathrm{cm}}^{-3}$, conditions that might arise naturally in the ongoing merging process in NGC 6052. Its [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ emission is also weaker than expected given the molecular gas mass inferred from previous measurements of ${}^{12}$CO~$J$=1--0 and ${}^{12}$CO~$J$=2--1. This suggests that [\ion{C}{1}]~${}^3{P}_1$--${}^3{P}_0$ may not be a reliable tracer of molecular gas mass in this galaxy. NGC 6052 is a unique laboratory to investigate how the merger process impacts the molecular gas distribution.

7 pages, 3 figures, accepted for publication in The Astrophysical Journal Letters