The Dense Gas Fraction in Galactic Center Clouds

Abstract

We present an analysis of gas densities in the central R=300 parsecs of the Milky Way, focusing on three clouds: GCM-0.02-0.07 (the 50 km/s cloud), GCM-0.13-0.08 (the 20 km/s cloud), and GCM0.25+0.01 (the "Brick"). Densities are determined using observations of the J=(3-2), (4-3), (5-4), (10-9), (18-17), (19-18), (21-20), and (24-23) transitions of the molecule HC3N. We find evidence of at least two excitation regimes for HC3N and constrain the low-excitation component to have a density less than 10^4 cm^-3 and the high-excitation component to have a density between 10^5 and 10^6 cm^-3. This is much less than densities of 10^7 cm^-3 that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of 'dense' (n>10^5 cm^-3) gas for these clouds. We find that this is ~15% for all three clouds. Applying the results of our models to ratios of the (10-9) and (3-2) line across the entire central R=300 pc, we find that the fraction of gas with n>10^4 cm^-3 increases inward of a radius of ~140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC3N is an excellent molecule for probing the density structure of clouds in the Galactic center.

arXiv comment20 pages, 8 figures, 3 tables. ApJ accepted

References in corpus (69)

• A computer program for fast non-LTE analysis of interstellar line spectra
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• A 100-parsec elliptical and twisted ring of cold and dense molecular clouds revealed by Herschel around the Galactic Center
• Variations in the Galactic star formation rate and density thresholds for star formation
• Toward a Unification of Star Formation Rate Determinations in the Milky Way and Other Galaxies
• What controls star formation in the central 500 pc of the Galaxy?
• An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis
• The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk
• Dense gas in the Galactic central molecular zone is warm and heated by turbulence
• Molecular gas kinematics within the central 250 pc of the Milky Way
• Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs
• Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342
• The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016
• Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
• The H_2O southern Galactic Plane Survey (HOPS): I. Techniques and H_2O maser data
• Spectral imaging of the Central Molecular Zone in multiple 3-mm molecular lines
• Characterizing Precursors to Stellar Clusters with Herschel
• The thermal state of molecular clouds in the Galactic Center: evidence for non-photon-driven heating
• G0.253+0.016: a molecular cloud progenitor of an Arches-like cluster
• Candidate super star cluster progenitor gas clouds possibly triggered by close passage to Sgr A*
• A Dynamical Model for the Formation of Gas Rings and Episodic Starbursts Near Galactic Centres
• The linewidth-size relationship in the dense ISM of the Central Molecular Zone
• The Galactic Center Molecular Cloud Survey. I. A Steep Linewidth-Size Relation & Suppression of Star Formation
• ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
• A cluster in the making: ALMA reveals the initial conditions for high-mass cluster formation
• The AST/RO Survey of the Galactic Center Region. I. The Inner 3 Degrees
• A Dynamical Model for Gas Flows, Star Formation, and Nuclear Winds in Galactic Centres
• Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl
• The Galactic Center Cloud G0.253+0.016: A Massive Dense Cloud with low Star Formation Potential
• Schmidt's Conjecture and Star Formation in Molecular Clouds
• Gas infall towards Sgr A* from the clumpy circumnuclear disk
• Herschel/HIFI discovery of interstellar chloronium (H${}_2$Cl${}^{+}$)
• 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
• A theoretical explanation for the Central Molecular Zone asymmetry
• SEPIA - a new single pixel receiver at the APEX Telescope
• The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253
• Detection of Widespread Hot Ammonia in the Galactic Center
• The Excitation of HCN and HCO+ in the Galactic Center Circumnuclear Disk
• GREAT confirms transient nature of the circumnuclear disk
• Spatially Resolved Chemistry in Nearby Galaxies II. The Nuclear Bar in Maffei 2
• On the temperature structure of the Galactic Centre cloud G0.253+0.016
• Collisional excitation of HC3N by para- and ortho-H2
• A multi-wavelength view of the Galactic center dust ridge reveals little star formation
• Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?
• Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C
• The Galactic Centre in the Far Infrared
• Spectral imaging of the Central Molecular Zone in multiple 7-mm molecular lines
• Main-sequence stars masquerading as Young Stellar Objects in the central molecular zone
• Herschel/HIFI Observations of Hydrogen Fluoride Toward Sagittarius B2(M)
• The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC4945
• The Molecular Gas Environment in the 20 km s${}^{-1}$ Cloud in the Central Molecular Zone
• Tracing gas accretion in the Galactic center using isotopic ratios
• Two New and Remarkable Sightlines through the Galactic Center's Molecular Gas
• Herschel observations of ortho- and para-oxidaniumyl (H2O+) in spiral arm clouds toward Sgr B2(M)
• Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology
• Physical Conditions of Molecular Gas in the Galactic Center
• Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia Thermometry, Water and Methanol Masers in the Nuclear Starburst of NGC 253
• Absorption Line Survey of H3+ toward the Galactic Center Sources III. Extent of the Warm and Diffuse Clouds
• The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core
• Origins of Scatter in the Relationship Between HCN 1-0 and Dense Gas Mass in the Galactic Center
• Precise observations of the 12C/13C ratios of HC3N in the low-mass star-forming region L1527
• HCN $J$=4-3, HNC $J$=1-0, ${\mathrm{H}}^{13}\mathrm{CN}$ $J$=1-0, and $\mathrm{H}{\mathrm{C}}_3\mathrm{N}$ $J$=10-9 Maps of Galactic Center Region I: Spatially-Resolved Measurements of Physical Conditions and Chemical Composition
• Survey of Water and Ammonia in Nearby galaxies (SWAN): Resolved Ammonia Thermometry, and Water and Methanol Masers in IC 342, NGC 6946 and NGC 2146
• An analysis of HCN observations of the Circumnuclear Disk at the galactic centre
• The diffuse molecular component in the nuclear bulge of the Milky Way
• Rethinking a Mysterious Molecular Cloud
• CH 3 GHz Observations of the Galactic Center

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