First Results from the $Herschel$ and ALMA Spectroscopic Surveys of the SMC: The Relationship Between [CII]-bright Gas and CO-bright Gas at Low Metallicity

Abstract

The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the Herschel Spectroscopic Survey of the SMC (HS${}^3$), which mapped the key far-IR cooling lines [CII], [OI], [NII], and [OIII] in five star-forming regions, and new ALMA 7m-array maps of ${}^{12}$CO and ${}^{13}$CO $(2-1)$ with coverage overlapping four of the five HS${}^3$ regions. We detect [CII] and [OI] throughout all of the regions mapped. The data allow us to compare the structure of the molecular clouds and surrounding photodissociation regions using ${}^{13}$CO, CO, [CII], and [OI] emission at $<10$" ($<3$ pc) scales. We estimate Av using far-IR thermal continuum emission from dust and find the CO/[CII] ratios reach the Milky Way value at high A${}_V$ in the centers of the clouds and fall to $\sim 1/5-1/10\times$ the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright CO emission. We estimate the amount of molecular gas traced by bright [CII] emission at low A${}_V$ and bright CO emission at high A${}_V$. We find that most of the molecular gas is at low A${}_V$ and traced by bright [CII] emission, but that faint CO emission appears to extend to where we estimate the H${}_2$-to-HI transition occurs. By converting our H${}_2$ gas estimates to a CO-to-H${}_2$ conversion factor ($X_{CO}$), we show that $X_{CO}$ is primarily a function of A${}_V$, consistent with simulations and models of low metallicity molecular clouds.

Accepted for publication in ApJ

References in corpus (19)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• GREAT: the SOFIA high-frequency heterodyne instrument
• A Compendium of Far-Infrared Line and Continuum Emission for 227 Galaxies Observed by the Infrared Space Observatory
• The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy
• The Spitzer Survey of the Small Magellanic Cloud: FIR Emission and Cold Gas in the SMC
• [CII] 158 $\mu$m Emission as a Star Formation Tracer
• [CII] 158$\mu$m Emission and Metallicity in PDRs
• Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-Dust Ratio Variations across ISM Phases
• The Spatially Resolved [CII] Cooling Line Deficit in Galaxies
• The Origins of [CII] Emission in Local Star-forming Galaxies
• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• Modelling [CI] emission from turbulent molecular clouds
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• The SOFIA Observatory at the Start of Routine Science Operations : Mission capabilities and performance
• ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud
• The Survey of Lines in M31 (SLIM): Investigating the Origins of [CII] Emission
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• ALMA observations of N83C in the early stage of star formation in the Small Magellanic Cloud

Cited by in corpus (36)

• Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
• Photodissociation and X-Ray Dominated Regions
• The Herschel Dwarf Galaxy Survey: II. Physical conditions, origin of [CII] emission, and porosity of the multiphase low-metallicity ISM
• The physics of gas phase metallicity gradients in galaxies
• Low star formation efficiency in typical galaxies at z=5-6
• PHANGS-JWST First Results: Tracing the Diffuse ISM with JWST Imaging of Polycyclic Aromatic Hydrocarbon Emission in Nearby Galaxies
• The PhotoDissociation Region Toolbox: Software and Models for Astrophysical Analysis
• The [CII]--SFR correlation in dwarf galaxies across cosmic time
• 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
• A spectral stacking analysis to search for faint outflow signatures in z~6 quasars
• Characterizing the Multi-Phase Origin of [CII] Emission in M101 and NGC 6946 with Velocity Resolved Spectroscopy
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. I. Overview: CO Cloud Distributions
• Molecular gas in the outflow of the Small Magellanic Cloud
• CO(2-1) Survey at 9 parsec resolution in the SMC
• Herschel spectroscopy of Massive Young Stellar Objects in the Magellanic Clouds
• Multi-Phase ISM in the z = 5.7 Hyperluminous Starburst SPT0346-52
• An Unbiased CO Survey Toward the Northern Region of the Small Magellanic Cloud with the Atacama Compact Array. II. CO Cloud Catalog
• The First Detection of a Protostellar CO Outflow in the Small Magellanic Cloud with ALMA
• The impact of carbon and oxygen abundances on the metal-poor initial mass function
• Resolved star formation in the metal poor star-forming region Magellanic Bridge C
• $\text{Herschel}$ Photometric Observations of $\mathrm{L}\mathrm{ITTLE}$ $\mathrm{T}\mathrm{HINGS}$ Dwarf Galaxies
• Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA
• Dwarf Galaxies: Their Low Metallicity Interstellar Medium
• Does the HCN/CO ratio trace the star-forming fraction of gas? I. A comparison with analytical models of star formation
• [CII] and CO Emission Along the Bar and Counter-Arms of NGC 7479
• Inadequate turbulent support in low-metallicity molecular clouds
• ALMA resolves molecular clouds in the metal poor Magellanic Bridge A
• Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship
• SuperCAM CO(3-2) APEX survey at 6 pc resolution in the Small Magellanic Clouds
• Disentangling emission from star-forming regions in the Magellanic Clouds: Linking [OIII]88 micron and 24 micron
• 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?
• First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H${}_2$ ratio
• When did the initial mass function become bottom-heavy?
• SILCC -- IX. The multi-phase interstellar medium at low metallicity
• CO Observations of the SMC-N66 Hii Region with ALMA: Properties of Clumps along Filamentary Molecular Clouds and Possible Expansion Motion
• The Interstellar Medium and Star Formation in Dwarf Galaxies