Mapping the Cosmic-Ray Ionization Rate in the Local Galaxy with H${}_3^{+}$

Abstract

Chemistry in diffuse molecular clouds relies primarily on rapid ion-molecule reactions. Formation of the initial ions, H${}^{+}$ and H${}_2^{+}$, is dominated by cosmic-ray ionization of H and H${}_2$, making the cosmic-ray ionization rate (denoted $\zeta (\mathrm{X})$ for species X) an important parameter for chemical modeling. We have made observations targeting absorption lines of H${}_3^{+}$, one of the most reliable tracers of $\zeta ({\mathrm{H}}_2)$, toward diffuse molecular cloud sight lines where the H${}_2$ column density has been directly measured in the ultraviolet, detecting H${}_3^{+}$ in 12 out of 27 sight lines. The 3D-PDR modeling method introduced by Obolentseva et al. (2024) was used to infer cosmic-ray ionization rates in the clouds along these sight lines, and our combined sample has a mean ionization rate of $5.3\times 10^{-17}$ s${}^{-1}$ with standard deviation $2.5\times 10^{-17}$ s${}^{-1}$. By associating H${}_3^{+}$ absorption with gas density peaks derived from the differential extinction maps of Edenhofer et al. (2024) we have constructed a sparsely sampled 3D map of the cosmic-ray ionization rate in targeted regions within about 1~kpc of the Sun. Specific regions show reasonably uniform ionization rates over length scales of tens of parsecs, with the average ionization rate in each region being different. Large differences (factor of 5) in $\zeta ({\mathrm{H}}_2)$ are found over length scales of about 100 pc. This supports a picture where the cosmic-ray ionization rate varies smoothly over small size scales, but is not uniform everywhere in the Galactic disk, likely being controlled by proximity to particle acceleration sites.

58 pages, 37 figures, 6 table; accepted for publication in ApJ

References in corpus (29)

• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• Ultraviolet Survey of CO and H_2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Updated Gaia-2MASS 3D maps of Galactic interstellar dust
• One Relation for All Wavelengths: The Far-Ultraviolet to Mid-Infrared Milky Way Spectroscopic R(V) Dependent Dust Extinction Relationship
• A parsec-scale Galactic 3D dust map out to 1.25 kpc from the Sun
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Parameters of 220 million stars from Gaia BP/RP spectra
• SOLIS II. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4
• Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant
• Cosmic Ray Transport, Energy Loss, and Influence in the Multiphase Interstellar Medium
• The Per-Tau Shell: A Giant Star-Forming Spherical Shell Revealed by 3D Dust Observations
• A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk
• Diffusive Versus Free-Streaming Cosmic Ray Transport in Molecular Clouds
• Magnetically regulated collapse in the B335 protostar? II. Observational constraints on gas ionization and magnetic field coupling
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• Probing the physics of star formation (ProPStar): I. First resolved maps of the electron fraction and cosmic-ray ionization rate in NGC 1333
• MELCHIORS: The Mercator Library of High Resolution Stellar Spectroscopy
• Evidence of Cosmic-Ray Excess from Local Giant Molecular Clouds
• Extending the view of ArH+ chemistry in diffuse clouds
• Parsec-scale cosmic-ray ionisation rate in Orion
• The EDIBLES Survey. VII. A survey of C2 and C3 in interstellar clouds
• Far-ultraviolet Dust Extinction and Molecular Hydrogen in the Diffuse Milky Way Interstellar Medium
• Absorption Line Observations of H${}_3^{+}$ and CO in Sight Lines Toward the Vela and W28 Supernova Remnants
• Cosmic-ray ionisation rate in low-mass cores: the role of the environment
• Exclusion of cosmic rays from molecular clouds by self-generated electric fields

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