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

Being one of the most abundant atomic/molecular species observed in dense molecular gas, atomic carbon (${\mathrm{C}}^0$) is a potential good tracer of molecular gas mass in many chemical/physical environments, though the ${\mathrm{C}}^0$ abundance variation outside the Galactic disk region is yet to be fully known. This paper presents a wide-field 500 GHz [CI] 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 the [CI] emission predominantly originates from the low-excitation gas component with a 20-50 K temperature and $\sim 10^3{\mathrm{cm}}^{-3}$ density, whereas ${\mathrm{C}}^0$ abundance is likely suppressed in the high-excitation gas component. The average $N({\mathrm{C}}^0)$/$N(\mathrm{CO})$ abundance ratio in the CMZ is 0.3-0.4, which is 2-3 times that in the Galactic disk. The $N({\mathrm{C}}^0)$/$N(\mathrm{CO})$ ratio increases to 0.7 in the innermost 10 pc region and to $\sim 2$ in the circumnuclear disk. We discovered ${\mathrm{C}}^0$-rich regions distributed in a ring-shape encircling the supernova remnant (SNR) Sgr A east, indicative that the ${\mathrm{C}}^0$-enrichment in the central 10 pc region is a consequence of a molecular cloud-SNR interaction. In the 15 atom/molecules included in principal component analysis (PCA), CN is the only other species that increases in the [CI]-bright ring. The origin of the [CI]-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 ${\mathrm{C}}^0$-to-CO conversion has not reached the equilibrium.

23 pages, 16 figures, accepted for publication in Astrophysical Journal

References in corpus (22)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Diagnostics of irradiated dense gas in galaxy nuclei. II. A grid of XDR and PDR models
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline
• Effective destruction of CO by cosmic rays: implications for tracing H${}_2$ gas in the Universe
• HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
• The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone
• Absorption Line Survey of H3+ toward the Galactic Center Sources II. Eight Infrared Sources within 30 pc of the Galactic Center
• The Central 300 pc of the Galaxy probed by infrared spectra of H${}_3^{+}$ and CO: part I. Predominance of warm and diffuse gas and high H${}_2$ ionization rate
• APEX mapping of H3O+ in the Sgr B2 region
• Seeding the Galactic Centre gas stream: gravitational instabilities set the initial conditions for the formation of protocluster clouds
• Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant
• GMC Collisions as Triggers of Star Formation. V. Observational Signatures
• ALMA Observations of Multiple-CO and C Lines Toward the Active Galactic Nucleus of NGC 7469: X-Ray-dominated Region Caught in the Act
• Very Large Array Observations of Galactic Center OH 1720 MHz Masers in Sagittarius A East and in the Circumnuclear Disk
• 12CO 4-3 and [CI] 1-0 at the centers of NGC 4945 and Circinus
• ALMA Observations of Atomic Carbon [C I] (${{}^3\mathrm{P}}_1\to {{}^3\mathrm{P}}_0$) and Low-$J$ CO Lines in the Starburst Galaxy NGC 1808
• Discovery of a [CI]-faint, CO-bright Galaxy: ALMA Observations of the Merging Galaxy NGC 6052
• [CI] 492 GHz mapping toward Cas A
• Indications of M-dwarf Deficits in the Halo and Thick Disk of the Galaxy
• A Multi-Line Ammonia Survey of the Galactic Center Region with the Tsukuba 32-m Telescope - I. Observations and Data

Cited by in corpus (3)

• Cosmic Ray Interstellar Propagation Tool using Itô Calculus (criptic): software for simultaneous calculation of cosmic ray transport and observational signatures
• Cosmic-ray-driven enhancement of the C${}^0$/CO abundance ratio in W51C
• The Discovery of an Active Wind from the Milky Way's Black Hole