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Register a new userThe cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
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David A. Neufeld
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We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously-published measurements of the abundance of four molecular ions: ArH$^+$, OH$^+$, H$_2$O$^+$ and H$_3^+$. For diffuse $atomic$ clouds at Galactocentric distances in the range $R_g sim 4 - 9$ kpc, observations of ArH$^+$, OH$^+$, and H$_2$O$^+$ imply a mean primary CRIR of $(2.2 pm 0.3) exp [(R_0-R_g)/4.7,rm{kpc}] times 10^{-16} rm , s^{-1}$ per hydrogen atom, where $R_0=8.5$ kpc. Within diffuse $molecular$ clouds observed toward stars in the solar neighborhood, measurements of H$_3^+$ and H$_2$ imply a primary CRIR of $(2.3 pm 0.6) times 10^{-16},,rm s^{-1}$ per H atom, corresponding to a total ionization rate per H$_2$ molecule of $(5.3 pm 1.1) times 10^{-16},,rm s^{-1},$ in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight-line to Cas A. Here, our best-fit estimate for the primary CRIR is $2.9 times 10^{-16},,rm s^{-1}$ per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, $A_{rm V}({rm tot})$, with a best-fit dependence $propto A_{rm V}({rm tot})^{-1}$ for $A_{rm V}({rm tot}) ge 0.5$.
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We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady state photodissociation region code that treats the freeze-out of gas species, grain surface chemistry, and desorption of ices from grains. The code includes PAHs, which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far ultraviolet flux on the cloud ({chi}<~ 1000; {chi}= 1= local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic ray primary ionization rate {zeta}(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {zeta}(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate {zeta}(crp)/n, {chi}/n and A_V in them. We compare our models to Herschel observations of these two ions, and estimate {zeta}(crp) ~ 4-6 x 10^-16 (n/100 cm^-3) s^-1 and chi/n = 0.03 cm^3 for diffuse foreground clouds towards W49N.
In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042+-0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-16 s^-1), and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H3+ observations. Ionization rates in the Galactic center tend to be 10--100 times larger than found in the Galactic disk, also in accord with prior studies.
We consider the rate of ionization of diffuse and molecular clouds in the interstellar medium by Galactic cosmic rays (GCR) in order to constrain its low energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high energies ($ge 200$ GeV/ nucleon) and a recently derived GCR proton flux at $1hbox{--}200$ GeV from observations of gamma rays from molecular clouds, and find that the observed average Galactic ionization rate can be reconciled with this GCR spectrum if there is a low energy cutoff for protons at $10hbox{--}100$ MeV. We also identify the flattening below a few GeV as being due to (a) decrease of the diffusion coefficient and dominance of convective loss at low energy and (b) the expected break in energy spectrum for a constant spectral index in momentum. We show that the inferred CR proton spectrum of $Phi propto E_{kin}^{-1.7pm0.2}$ for $E_{kin} le$ few GeV, is consistent with a power-law spectrum in momentum $p^{-2.45pm0.4}$, which we identify as the spectrum at source. Diffusion loss at higher energies then introduces a steepening by $E^{-alpha}$ with $alpha sim 1/3$, making it consistent with high energy measurements.
The cosmic-ray ionization rate ($zeta$, s$^{-1}$) plays an important role in the interstellar medium. It controls ion-molecular chemistry and provides a source of heating. Here we perform a grid of calculations using the spectral synthesis code CLOUDY along nine sightlines towards, HD 169454, HD 110432, HD 204827, $lambda$ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, Cyg OB2 12. The value of $zeta$ is determined by matching the observed column densities of H$_3^+$ and H$_2$. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H$_3^+$ density and the derived ionization rate. PAHs are ubiquitous in the Galaxy, but there are also regions where PAHs do not exist. Hence, we consider clouds with a range of PAH abundances and show their effects on the H$_3^+$ abundance. We predict an average cosmic-ray ionization rate for H$_2$ ($zeta$(H$_2$))= (7.88 $pm$ 2.89) $times$ 10$^{-16}$ s$^{-1}$ for models with average Galactic PAHs abundances, (PAH/H =10$^{-6.52}$), except Cyg OB2 5 and Cyg OB2 12. The value of $zeta$ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We estimate the average value of $zeta$(H$_2$)= (95.69 $pm$ 46.56) $times$ 10$^{-16}$ s$^{-1}$ for models without PAHs.
(Abridged) We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115 GHz lines toward five nearby prototypical starburst/AGN systems, and OH+ 971 GHz spectra toward three of these. The beam size of 20 corresponds to resolutions between 0.35 and 7 kpc. The observed line profiles range from pure absorption (NGC 4945, M82) to P-Cygni indicating outflow (NGC 253, Arp 220) and inverse P-Cygni indicating infall (Cen A). The similarity of the H2O, OH+, and H2O+ profiles to each other and to HI indicates that diffuse and dense gas phases are well mixed. We estimate column densities assuming negligible excitation (for absorption features) and using a non-LTE model (for emission features), adopting calculated collision data for H2O and OH+, and rough estimates for H2O+. Column densities range from ~10^13 to ~10^15 cm^-2 for each species, and are similar between absorption and emission components, indicating that the nuclear region does not contribute much to the emission in these ground-state lines. The N(H2O)/N(H2O+) ratios of 1.4-5.6 indicate an origin of the lines in diffuse gas, and the N(OH+)/N(H2O+) ratios of 1.6-3.1 indicate a low H2 fraction (~11%) in the gas. Adopting recent Galactic values for the average gas density and the ionization efficiency, we find ionization rates for our sample galaxies of ~3x10^-16 s^-1 which are similar to the value for the Galactic disk, but ~10x below that of the Galactic Center and ~100x below estimates for AGN from excited-state H3O+ lines. We conclude that the ground-state lines of water and its associated ions probe primarily non-nuclear gas in the disks of these centrally active galaxies. Our data thus provide evidence for a decrease in ionization rate by a factor of ~10 from the nuclei to the disks of galaxies, as found before for the Milky Way.
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