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

We report sensitive [C i] ³ P ₁–³ P ₀ and ¹²CO 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 ¹²CO 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 ¹²CO 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 ¹²CO J = 4–3 can be explained if the interstellar medium has a hydrogen density larger than ${10}^5{cm}^{-3}$, 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 ¹²CO J = 1–0 and ¹²CO 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.