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

Kunihiko Tanaka, Makoto Nagai, Kazuhisa Kamegai

Research output: Contribution to journalArticlepeer-review

Abstract

Atomic carbon (C0), 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 C0 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 ∼103 cm-3, whereas C0 abundance is likely suppressed in the high-excitation gas component. The average N(C0)/N(CO) abundance ratio in the CMZ is 0.3-0.4, which is 2-3 times that in the Galactic disk. The N(C0)/N(CO) ratio increases to 0.7 in the innermost 10 pc region and to ∼2 in the circumnuclear disk. We discovered C0-rich regions distributed in a ring shape encircling the supernova remnant (SNR) Sgr A East, indicating that the C0 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 C0 to CO has not reached equilibrium.

Original languageEnglish
Article number79
JournalAstrophysical Journal
Volume915
Issue number2
DOIs
Publication statusPublished - 2021 Jul 10

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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