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

Abstract

We report sensitive [C i ]  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 the Atacama Large Millimeter/submillimeter Array (ALMA). We detect 12 CO  J  = 4–3 toward the northern part of NGC 6052, but [C i ]  3 P 1 – 3 P 0 is not detected with a [C i ]  3 P 1 – 3 P 0 to 12 CO  J  = 4–3 line luminosity ratio of ≲0.07. According to models of photodissociation regions, the unusual weakness of [C i ]  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 , conditions that might arise naturally in the ongoing merging process in NGC 6052. Its [C I]  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 [C I] 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.