Unusual CO Line Ratios and kinematics in the N83/N84 Region of the Small Magellanic Cloud

Abstract

We present new CO 1-0 and 2-1 observations of the N83/N84 molecular cloud complex in the south-east Wing of the Small Magellanic Cloud (SMC). While the 2-1/1-0 integrated line brightness ratio (in temperature units) is uniformly 0.9 throughout most of the complex, we find two distinct regions with unusually high ratios 2-1/1-0 > 2. These regions are associated with the N84D nebula and with the inside of the 50 pc expanding molecular shell N83. This shell is spatially coincident with the NGC 456 stellar association and the HFPK2000-448 radio continuum/X-ray source tentatively classified as a supernova remnant. We explore possible causes for the high ratios observed and conclude that the CO emission probably arises from an ensemble of small (R~0.1 pc), warm (T~40 K) clumps. Analysis of the CO shell parameters suggests that it is wind--driven and has an age of slightly more than 2 million years. We have also used this dataset to determine the CO-to-H2 conversion factor in the SMC, an especially interesting measurement because of the low metallicity of this source (~1/9 solar). Surprisingly, after comparing the CO luminosities of clouds in N83/N84 with their virial masses, we find a CO-to-H2 conversion factor Xco only 1.9 times larger than what we obtain when applying the same algorithm to solar metallicity clouds in the Milky Way and M 33. This result fits into the emerging pattern that CO observations with high linear resolution suggest nearly Galactic values of Xco in a wide range of environments.

arXiv comment29 pages, aastex preprint style, 7 B&W figures (some degraded to fit the file size constraints), accepted for publication in ApJ, retrieve paper with full resolution figures from http://astro.berkeley.edu/~bolatto/ApJn83.pdf

References in corpus (15)

• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
• The Equilibrium State of Molecular Regions in the Outer Galaxy
• The Metallicity Dependence of the CO-to-H${}_2$ Conversion Factor from Observations of Local Group Galaxies
• Dust-to-gas Ratio and Metal Abundance in Dwarf Galaxies
• Molecular gas in late-type galaxies
• The Evolution of C/O in Dwarf Galaxies from HST Fos Observations
• CO Emission in Low Luminosity, HI Rich Galaxies
• A semianalytical model for the observational properties of the dominant carbon species at different metallicities
• Supernova Remnants Associated with Molecular Clouds in the Large Magellanic Cloud
• The Interacting Dwarf Galaxy NGC 3077: The Interplay of Atomic and Molecular Gas with Violent Star Formation
• Carbon in the N159/N160 Complex of the Large Magellanic Cloud
• The Interaction between the ISM and Star Formation in the Dwarf Starburst Galaxy NGC 4214
• The Distances of the Magellanic Clouds
• Molecular gas in blue compact dwarf galaxies

Cited by in corpus (48)

• The CO-to-H2 Conversion Factor
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• Bias-free Measurement of Giant Molecular Cloud Properties
• Low CO Luminosities in Dwarf Galaxies
• The molecular gas in Luminous Infrared Galaxies I: CO lines, extreme physical conditions, and their drivers
• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• The Molecular ISM of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
• The molecular gas in Luminous Infrared Galaxies II: extreme physical conditions, and their effects on the X_{co} factor
• Physical Properties of Giant Molecular Clouds in the Large Magellanic Cloud
• Giant Molecular Clouds in M64
• Low-J CO Line Ratios From Single Dish CO Mapping Surveys and PHANGS-ALMA
• The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey: A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)
• Molecular Gas in the Lensed Lyman Break Galaxy cB58
• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data
• A Total Molecular Gas Mass Census in z~2-3 Star-forming Galaxies: Low-J CO Excitation Probes of Galaxies' Evolutionary States
• A Spitzer Space Telescope far-infrared spectral atlas of compact sources in the Magellanic Clouds. I. The Large Magellanic Cloud
• The Lifetime of Grand Design
• A Spitzer Space Telescope far-infrared spectral atlas of compact sources in the Magellanic Clouds. II. The Small Magellanic Cloud
• C+ Emission from the Magellanic Clouds II. [CII] maps of star-forming regions LMC-N 11, SMC-N 66, and several others
• Deep and narrow CO absorption revealing molecular clouds in the Hydra-A brightest cluster galaxy
• The 30 Doradus Molecular Cloud at 0.4 pc Resolution with the Atacama Large Millimeter/submillimeter Array: Physical Properties and the Boundedness of CO-emitting Structures
• The Influence of Far-Ultraviolet Radiation on the Properties of Molecular Clouds in the 30 Dor Region of the Large Magellanic Cloud
• Giant Molecular Clouds in the Spiral Arm of IC 342
• Tracing molecular gas mass in extreme extragalactic environments: an observational study
• Measurement of the core-collapse progenitor mass distribution of the Small Magellanic Cloud
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• The Cosmological Significance of High-Velocity Cloud Complex H
• Bright Extragalactic ALMA Redshift Survey (BEARS) III: Detailed study of emission lines from 71 Herschel targets
• VLA Legacy Survey of Molecular Gas in Massive Star-forming Galaxies at High Redshift
• Thermal Pressures in the Interstellar Medium of the Magellanic Clouds
• A molecular absorption line survey toward the AGN of Hydra-A
• LABOCA observations of giant molecular clouds in the south west region of the Small Magellanic Cloud
• ALMA observations of N83C in the early stage of star formation in the Small Magellanic Cloud
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions
• High Excitation Molecular Gas in the Magellanic Clouds
• The Nature of Nearby Counterparts to Intermediate Redshift Luminous Compact Blue Galaxies II. CO Observations
• CO(2-1) Survey at 9 parsec resolution in the SMC
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog
• Infrared Dark Clouds in the Small Magellanic Cloud?
• Using CO line ratios to trace compressed areas in bubble N131
• ALMA resolves molecular clouds in the metal poor Magellanic Bridge A
• SuperCAM CO(3-2) APEX survey at 6 pc resolution in the Small Magellanic Clouds
• 12CO and 13CO observation of the low-metallicity dwarf galaxy DDO 154
• ALMA 0.1 pc View of Molecular Clouds Associated with High-Mass Protostellar Systems in the Small Magellanic Cloud: Are Low-Metallicity Clouds Filamentary or Not?
• Molecular Clouds at the Edge of the Galaxy I. Variation of CO J=2-1/1-0 Line Ratio
• Molecular Gas in Nearby Dwarf Galaxies: Single Dish and Interferometric Results