Discovery of a [CI]-faint, CO-bright Galaxy: ALMA Observations of the
  Merging Galaxy NGC 6052
  We report sensitive [\ion{C}{1}]~$^3P_1$--$^3P_0$ 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}]~$^3P_1$--$^3P_0$ is not detected with a
[\ion{C}{1}]~$^3P_1$--$^3P_0$ to $^{12}$CO~$J$=4--3 line luminosity ratio
of$~\lesssim0.07$. According to models of photodissociation regions, the
unusual weakness of [\ion{C}{1}]~$^3P_1$--$^3P_0$ relative to
$^{12}$CO~$J$=4--3 can be explained if the interstellar medium has a hydrogen
density larger than $10^5\,{\rm cm}^{-3}$, conditions that might arise
naturally in the ongoing merging process in NGC 6052. Its
[\ion{C}{1}]~$^3P_1$--$^3P_0$ 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}]~$^3P_1$--$^3P_0$ 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.
