The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. II. Comparison between observations and theoretical models

Abstract

We compare observations of HI from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO${}^{+}$ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse ($A_V\lesssim 1$) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multi-phase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic ray ionization rate (CRIR) for each structure identified in HCO${}^{+}$ and HI absorption. These structures fall into two categories. Structures with $T_s<40\mathrm{K}$, mostly with $N(\mathrm{HC}{\mathrm{O}}^{+})\lesssim 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$, are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperature $T_s>40\mathrm{K}$, mostly with $N(\mathrm{HC}{\mathrm{O}}^{+})\gtrsim 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$, are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable HI. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., non-equilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO${}^{+}$ in this direction. Similarly, while our observations and simulations of the turbulent, multi-phase ISM agree that HCO${}^{+}$ formation occurs along sightlines with $N(\mathrm{HI})\gtrsim 10^{21}\mathrm{c}{\mathrm{m}}^{-2}$, the simulated data fail to explain HCO${}^{+}$ column densities $\gtrsim \mathrm{few}\times 10^{12}\mathrm{c}{\mathrm{m}}^{-2}$. Since a majority of our sightlines with HCO${}^{+}$ had such high column densities, this likely indicates that non-equilibrium chemistry is important for these lines of sight.

arXiv comment15 pages, 3 figures, 1 table. Accepted to ApJ

References in corpus (53)

• Starburst99: Synthesis Models for Galaxies with Active Star Formation
• The Milky Way in Molecular Clouds: A New Complete CO Survey
• Planck 2013 results. XI. All-sky model of thermal dust emission
• On the Star Formation Rates in Molecular Clouds
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• Thermal condensation in a turbulent atomic hydrogen flow
• Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• The relation between gas and dust in the Taurus Molecular Cloud
• Three-phase Interstellar medium in Galaxies Resolving Evolution with Star formation and Supernova feedback (TIGRESS): Algorithms, Fiducial model, and Convergence
• Modelling CO formation in the turbulent interstellar medium
• Models of turbulent dissipation regions in the diffuse interstellar medium
• HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation
• Comparative Chemistry of Diffuse Clouds II: CN, HCN, HNC, CH3CN & N2H+
• First results from SMAUG: Characterization of Multiphase Galactic Outflows from a Suite of Local Star-Forming Galactic Disk Simulations
• The CO luminosity and CO-H2 conversion factor of diffuse ISM: does CO emission trace dense molecular gas?
• The 21-SPONGE HI Absorption Line Survey II: The temperature of Galactic HI
• The gas-to-extinction ratio and the gas distribution in the Galaxy
• Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
• A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM
• Comparative chemistry of diffuse clouds III: sulfur-bearing molecules
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• The environmental dependence of the X_CO conversion factor
• The structure of the thermally bistable and turbulent atomic gas in the local interstellar medium
• Comparative Chemistry of Diffuse Clouds IV: CH
• Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
• APEX mapping of H3O+ in the Sgr B2 region
• The X_CO conversion factor from galactic multiphase ISM simulations
• Where is OH and Does It Trace the Dark Molecular Gas (DMG)?
• The Lognormal Probability Distribution Function of the Perseus Molecular Cloud: A Comparison of HI and Dust
• UV-driven chemistry in simulations of the interstellar medium. I. Post-processed chemistry with the Meudon PDR code
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk
• Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption
• Origin of CH+ in diffuse molecular clouds warm H2 and ion-neutral drift
• On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
• HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
• Dust Polarization Maps from TIGRESS: E/B power asymmetry and TE correlation
• HP2 survey: III The California Molecular Cloud--A Sleeping Giant Revisited
• Effects of turbulent diffusion on the chemistry of diffuse clouds
• Molecular ion abundances in the diffuse ISM : CF+, HCO+, HOC+, and C3H+
• Diffuse Ionized Gas in Simulations of Multiphase, Star-Forming Galactic Disks
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• The kinematics of molecular clumps surrounding hot cores in G29.96-0.02 and G31.41+0.31
• Chemical complexity in local diffuse and translucent clouds: ubiquitous l-C3H and CH3CN, a detection of HC3N and an upper limit on the abundance of CH2CN
• Dissipative structures of diffuse molecular gas: I - Broad HCO${}^{+}$(1-0) emission
• Recovering interstellar gas properties with HI spectral lines: A comparison between synthetic spectra and 21-SPONGE
• A Dynamical Study of the Non-Star Forming Translucent Molecular Cloud MBM16: Evidence for Shear Driven Turbulence in the Interstellar Medium
• Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon
• Revealing The CO X-factor In Dark Molecular Gas through Sensitive ALMA Absorption Observations
• ALMA observations of molecular absorption in four directions toward the Galactic bulge
• Study of Interstellar Molecular Clouds using Formaldehyde Absorption toward Extragalactic Radio Sources