[C II] absorption and emission in the diffuse interstellar medium across the Galactic plane

Abstract

Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 μm fine structure transition [C ii] is the most important cooling line of the diffuse ISM. We combine [C ii] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM.Methods. We present high-resolution [C ii] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 μm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the [C ii] and [O i] emission and the [C ii]/FIR ratio. This phenomenon may explain part of the [C ii]/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C + and C excitation in the diffuse gas is consistent with a median pressure of ∼5900 K cm −3 for a mean kinetic temperature of ∼100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C + in the Galactic plane : 9.5 × 10 −26 erg −1 H −1 . Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10 −4 .

Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38

“…Gerin et al (2015) found no Galactic gradient in N(C + )/N(H) when N(H 2 ) was derived from N(CH), but this could also be the case when the molecular fraction is high and the abundances of CH and C + scale in the same way with the local metallicity. The discussion is further complicated by the fact that the relevant quantities are the free gas phase abundances after depletion, whereas the Galactic gradient is measured on the metallicity itself.…”

“…The in situ number densities and kinetic temperatures measured in C II by Gerin et al (2015) toward W31 were n(H) = 40−45 cm −3 and T K = 90−105 K, presumably a weighted average of the molecular component at n(H) ≈ 135 cm −3 and a less dense, somewhat warmer atomic component. The thermal pressure toward W 31 fell in a narrow range about p/k = n T K = 4000 cm −3 K, implying n(H I) ≈ 30 cm −3 at the temperature T K = 135 K that we used to derive the mean H I density in Fig.…”

“…Because it ignores other contributions to the electron fraction that causes most of the excitation of HCO + , this is an upper limit. At this density C + should contribute at least 80% of the total electron abundance (Gerin et al 2015) with most of the remainder from H + and a smaller contribution from He + , implying n(H) ≈ 0.8 × 160 = 128 cm −3 . However, the electron fraction contributed by C + decreases with density as the gas supports a higher level of cosmic-ray ionization, allowing the density to be even smaller.…”

“…There is a 6% overall correction for the contribution of N(C I) ≈ 4 ± 1 × 10 17 cm −2 (Gerin et al 2015), i.e., N(H) = 4.9 × 10 22 and n(H) = 4.9 cm −3 in total overall. The value of N(H) derived from carbon is only very slightly smaller than the sum N(H I) + 2N(H 2 ) = 5.28 × 10 22 cm −2 derived from H I and CH separately.…”

“…Gerin et al (2015) found an overall Galactic median total volume filling factor f vol = 2.4% based on their interpretation of C II absorption toward the wide sample of sightlines in the Galactic plane sampled in the PRISMAS project, but with much higher filling factors 6%, 13%, and 8% over the velocity intervals 10−23 km s −1 , 23−34 km s −1 , and 34−61 km s −1 toward W31, which are in overall agreement with the total filling factors derived here from other considerations.…”