Atomic Carbon in the Central Molecular Zone of the Milky Way: Possible Cosmic-Ray Induced Chemistry or Time-dependent Chemistry Associated with SNR Sagittarius A East

Abstract

Atomic carbon (C 0 ), being one of the most abundant atomic/molecular species observed in dense molecular gas, is potentially a good tracer of molecular gas mass in many chemical/physical environments, though the variation in C 0 abundance outside the Galactic disk region is not yet fully known. This paper presents a wide-field 500 GHz [C i ] map of the Galactic central molecular zone (CMZ) obtained with the ASTE 10 m telescope. Principal component analysis and non-LTE multi-transition analysis have shown that [C i ] emission predominantly originates from the low-excitation gas component with a temperature of 20–50 K and density of ∼10 3 cm −3 , whereas C 0 abundance is likely suppressed in the high-excitation gas component. The average N (C 0 )/ N (CO) abundance ratio in the CMZ is 0.3–0.4, which is 2–3 times that in the Galactic disk. The N (C 0 )/ N (CO) ratio increases to 0.7 in the innermost 10 pc region and to ∼2 in the circumnuclear disk. We discovered C 0 -rich regions distributed in a ring shape encircling the supernova remnant (SNR) Sgr A East, indicating that the C 0 enrichment in the central 10 pc region is a consequence of a molecular cloud–SNR interaction. In the 15 atoms/molecules included in principal component analysis, CN is the only other species that increases in the [C i ]-bright ring. The origin of the [C i ]-bright ring is likely a cosmic-ray-dominated region created by low-energy cosmic-ray particles accelerated by Sgr A East or primitive molecular gas collected by the SNR in which the conversion from C 0 to CO has not reached equilibrium.