Cosmic rays cannot explain the high ionisation rates in the Galactic centre

Abstract

The H${}_2$ ionisation rate in the central molecular zone, located in the Galactic centre, is estimated to be $\zeta \sim 2\times 10^{-14}{\mathrm{s}}^{-1}$, based on observations of H${}_3^{+}$ lines. This value is two to three orders of magnitude larger than that measured anywhere else in the Galaxy. Due to the high density of the gas in the central molecular zone, UV and X-ray photons do not penetrate this region. Hence, cosmic rays are expected to be the exclusive agents of ionisation. A high cosmic-ray density has been invoked to explain the unusually high ionisation rate. However, this excess is not seen in the $\gamma$-ray emission from this region, which is produced by high-energy cosmic rays. Therefore, an excess is expected only in the low-energy cosmic-ray spectrum. Here, we derive constraints on this hypothetical low-energy component in the cosmic-ray spectra, and we question its plausibility. To do so, we numerically solved the cosmic-ray transport equation in the central molecular zone. We derived stationary solutions under the assumption that cosmic rays are continuously injected by a source located in the Galactic centre. The high-energy component in the cosmic-ray spectrum was then fitted to available $\gamma$-ray and radio data, and a steep low-energy component was added to the cosmic-ray spectrum to explain the large ionisation rates. We find that injection spectra of $p^{-7}$ for protons below $p_{\mathrm{enh},\mathrm{p}}c\simeq 780\mathrm{MeV}$ and $p^{-5.2}$ for electrons below $p_{\mathrm{enh},\mathrm{e}}c=1.5\mathrm{GeV}$ are needed to reach the observed ionisation rates. This corresponds to a cosmic-ray power of the order of $\sim 10^{40-41}\mathrm{erg}{\mathrm{s}}^{-1}$ injected at the Galactic centre. We conclude that cosmic rays alone cannot explain the high ionisation rates in the Galactic centre.

References in corpus (56)

• The Neutral Atomic Phases of the ISM in the Galaxy
• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Acceleration of petaelectronvolt protons in the Galactic Centre
• Discovery of Very-High-Energy Gamma-Rays from the Galactic Centre Ridge
• Cosmic-ray ionization of molecular clouds
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• Parametrization of gamma-ray production cross-sections for pp interactions in a broad proton energy range from the kinematic threshold to PeV energies
• Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Global cosmic-ray related luminosity and energy budget of the Milky Way
• Dense gas in the Galactic central molecular zone is warm and heated by turbulence
• Detection of large-scale X-ray bubbles in the Milky Way halo
• Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
• Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+
• Cosmic-ray propagation with DRAGON2: I. numerical solver and astrophysical ingredients
• Simple analytical approximations for treatment of inverse Compton scattering of relativistic electrons in the black-body radiation field
• Cosmic-ray ionisation in circumstellar discs
• A lower limit of 50 microgauss for the magnetic field near the Galactic Centre
• Galactic synchrotron emission with cosmic ray propagation models
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• Interacting Cosmic Rays with Molecular Clouds: A Bremsstrahlung Origin of Diffuse High Energy Emission from the Inner 2deg by 1deg of the Galactic Center
• Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2
• The 1.28 GHz MeerKAT Galactic Center Mosaic
• The Galactic Centre Chimney
• PICARD: A novel code for the Galactic Cosmic Ray propagation problem
• The Origin of Diffuse X-ray and $\gamma$-ray Emission from the Galactic Center Region: Cosmic Ray Particles
• Wild at Heart:-The Particle Astrophysics of the Galactic Centre
• Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission
• INTEGRAL SPI All-Sky View in Soft Gamma Rays: Study of Point Source and Galactic Diffuse Emissions
• Physical conditions in the central molecular zone inferred by H3+
• Diffuse cosmic rays shining in the Galactic center: A novel interpretation of H.E.S.S. and Fermi-LAT gamma-ray data
• The Central 300 pc of the Galaxy probed by infrared spectra of H${}_3^{+}$ and CO: part I. Predominance of warm and diffuse gas and high H${}_2$ ionization rate
• APEX mapping of H3O+ in the Sgr B2 region
• Inverse Compton Scenarios for the TeV Gamma-Ray Emission of the Galactic Centre
• Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region
• Characterising the VHE diffuse emission in the central 200 parsecs of our Galaxy with H.E.S.S
• The X-ray lightcurve of Sgr A* over the past 150 years inferred from Fe-Ka line reverberation in Galactic Centre molecular clouds
• GBT Multiwavelength Survey of the Galactic Center Region
• MAGIC observations of the diffuse $\gamma$-ray emission in the vicinity of the Galactic Centre
• VERITAS Observations of the Galactic Center Region at Multi-TeV Gamma-Ray Energies
• Infrared H3+ and CO Studies of the Galactic Core: GCIRS 3 and GCIRS 1W
• Low energy cosmic rays
• Two New and Remarkable Sightlines through the Galactic Center's Molecular Gas
• Power requirements for cosmic ray propagation models involving re-acceleration and a comment on second order Fermi acceleration theory
• Stochastic Fluctuations of Low-Energy Cosmic Rays and the Interpretation of Voyager Data
• Absorption Line Survey of H3+ toward the Galactic Center Sources III. Extent of the Warm and Diffuse Clouds
• An X-ray survey of the central molecular zone: variability of the Fe Ka emission line
• Spectrum of Relativistic and Subrelativistic Cosmic Rays in the 100 pc Central Region
• Effective Shielding of $\lesssim$ 10 GeV Cosmic Rays from Dense Molecular Clumps
• Stochasticity of Cosmic Rays from Supernova Remnants and the Ionization Rates in Molecular Clouds
• The origin of the 6.4 keV line emission and H${}_2$ ionization in the diffuse molecular gas of the Galactic center region
• Implications on Spatial Models of Interstellar Gamma-Ray Inverse-Compton Emission from Synchrotron Emission Studies in Radio and Microwaves
• Can a cosmic ray carrot explain the ionization level in diffuse molecular clouds?
• Does the SN rate explain the very high energy cosmic rays in the central 200 pc of our Galaxy?
• Sgr B2 hard X-ray emission with INTEGRAL after 2009: still detectable?

Cited by in corpus (4)

• Anomalous Ionization in the Central Molecular Zone by sub-GeV Dark Matter
• Sub-keV dark matter can strongly ionize molecular clouds
• High Energy Emission from the Galactic Center
• Modelling the non-equilibrium chemistry of the Milky Way's cold nuclear wind