The Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of "CO-Dark" Gas in the Perseus Arm

Abstract

We report the results from a new, highly sensitive ($\Delta T_{mb}\sim 3$mK) survey for thermal OH emission at 1665 and 1667 MHz over a dense, 9 x 9-pixel grid covering a $1°$ x $1°$ patch of sky in the direction of $l=105°,b=+2.50°$ towards the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope (GBT) 1667 MHz OH results with archival CO J=1-0 observations from the Five College Radio Astronomy Observatory (FCRAO) Outer Galaxy Survey within the velocity range of the Perseus Arm at these galactic coordinates. Out of the 81 statistically-independent pointings in our survey area, 86% show detectable OH emission at 1667 MHz, and 19% of them show detectable CO emission. We explore the possible physical conditions of the observed features using a set of diffuse molecular cloud models. In the context of these models, both OH and CO disappear at current sensitivity limits below an A${}_{\mathrm{v}}$ of 0.2, but the CO emission does not appear until the volume density exceeds 100-200 cm${}^{-3}$. These results demonstrate that a combination of low column density A${}_{\mathrm{v}}$ and low volume density $n_H$ can explain the lack of CO emission along sight lines exhibiting OH emission. The 18-cm OH main lines, with their low critical density of $n^{\ast }$ $\sim 1$ cm${}^{-3}$, are collisionally excited over a large fraction of the quiescent galactic environment and, for observations of sufficient sensitivity, provide an optically-thin radio tracer for diffuse H${}_2$.

arXiv commentAccepted to The Astrophysical Journal, 12 pages, 7 figures, 2 tables

References in corpus (34)

• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The CO-to-H2 Conversion Factor
• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic Structure, Fundamental Parameters and Non-Circular Motions
• The Dark Molecular Gas
• A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components
• 3.6 and 4.5 Micron Phase Curves and Evidence for Non-Equilibrium Chemistry in the Atmosphere of Extrasolar Planet HD 189733b
• A Parallax-based Distance Estimator for Spiral Arm Sources
• Fermi observations of Cassiopeia and Cepheus: diffuse gamma-ray emission in the outer Galaxy
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?
• Evolution of OH and CO-dark Molecular Gas Fraction Across a Molecular Cloud Boundary In Taurus
• Trigonometric Parallaxes of Star Forming Regions in the Perseus Spiral Arm
• The Five-hundred-meter Aperture Spherical Radio Telescope (FAST) Project
• Planck Early Results: The Planck View of Nearby Galaxies
• GASKAP -- The Galactic ASKAP Survey
• The local spiral structure of the Milky Way
• Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
• The cosmic-ray and gas content of the Cygnus region as measured in gamma rays by the Fermi Large Area Telescope
• Molecular gas in spiral galaxies: a new warm phase at large galactocentric distances?
• Where is OH and Does It Trace the Dark Molecular Gas (DMG)?
• [CII] observations of H${}_2$ molecular layers in transition clouds
• The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
• CO-dark gas and molecular filaments in Milky Way-type galaxies - II: The temperature distribution of the gas
• SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl - First Science from the Pilot Region
• The relation between column densities of interstellar OH and CH molecules
• The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l\approx 105^{°},b\approx +1^{°}$
• Far-infrared study of tracers of oxygen chemistry in diffuse clouds
• Faint Extended OH Emission from the Local Interstellar Medium in the Direction l \approx 108\circ, b \approx 5\circ
• OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of the OH 18-cm Main Lines in W5
• Sensitive CO(1-0) Survey in Pegasus-Pisces Reduces CO-Dark Gas Inventory by Factor of Two

Cited by in corpus (10)

• Photodissociation and X-Ray Dominated Regions
• On the Three-Dimensional Structure of Local Molecular Clouds
• SILCC-Zoom: H${}_2$ and CO-dark gas in molecular clouds -- The impact of feedback and magnetic fields
• Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy
• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1
• OH Evolution in Molecular Clouds
• GNOMES II: Analysis of the Galactic diffuse molecular ISM in all four ground state hydroxyl transitions using Amoeba
• AMOEBA: Automated Molecular Excitation Bayesian Line-Fitting Algorithm
• Shaken or stirred: the diffuse interstellar medium with exceptionally high SiO abundance
• A Search for OH 18-cm Emission from Intermediate-Velocity Gas at High Galactic Latitudes