Cosmic rays, gas and dust in nearby anticentre clouds

Abstract

Aims. We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ-ray observations with the Fermi Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H 2 conversion factor, X CO , in different environments. Methods. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H i 21-cm and 12 CO 2.6-mm line emission. We have jointly modelled the γ-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ 353 at 353 GHz as a combination of H i -bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H i , free-free, and 12 CO emissions, namely (i) the opaque H i and diffuse H 2 present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H 2 to be added where 12 CO lines saturate. Results. The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomic envelopes to the 12 CO-bright cores, and with a small ±9% cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H ii region NGC1499 in the dust and γ-ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the X CO factor derived from dust is systematically larger by 30% to 130% than the γ-ray estimate. We also evaluate the average γ-ray X CO factor for each cloud, and find that X CO tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find X CO factors in the anti-centre clouds close to or below 10 20 cm −2 K −1 km −1 s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean X CO values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in 12 CO intensities between simulations and observations at low molecular gas densities.

Central molecular zones in galaxies:12CO-to-13CO ratios, carbon budget, andXfactors

“…Instead, its empirically estimated value is rather sensitive to assumptions made in the process, and it varies depending on author and method. The most reliable method uses gamma-ray observations to trace hydrogen nuclei, and a useful overview of X values thus obtained can be found in Table E.1 of Remy et al (2017). The empirically determined X values implicitly include both H 2 gas mixed with CO and H 2 gas that contains no Article number, page 1 of 41 (2) (3) (4) (5) (1) (2) (3) (4) (5) (1) (2) (3) (4) (5) N 134 21.…”

Cosmic rays, gas, and dust in local clouds

“…1 based on Grenier et al 2015;Remy et al 2017, and reference therein). In the 0.4-100 GeV energy band we find no significant spectral variations except in Perseus for which we observe opposite trends for decreasing H i and increasing CO emissivities with energy.…”

“…The results presented in the following are based on the same dust and γ-ray analyses procedures as applied to two broad regions in the anticentre (Remy et al 2017) and in the Chamaeleon directions (Planck and Fermi Collaborations 2015) in the sky. The analysis method and multiwavelength data used to build the γ-ray and dust models are detailed in these papers; we summarize below their main features.…”

Fermi-LAT γ-Ray Study of the Interstellar Medium and Cosmic Rays in the Chamaeleon Molecular Cloud Complex: A Look at the Dark Gas as Optically Thick H i

“…Study of local CRs and the ISM using γ rays from molecular clouds in the vicinity of the solar system (within ∼1 kpc) started in the COS-B era (e.g., Bloemen et al 1984), and was significantly advanced by EGRET on board the Compton Gamma-Ray Observatory (e.g., Hunter et al 1994;Digel et al 1999). Recently, the Large Area Telescope (LAT) (Atwood et al 2009) on board the Fermi Gamma-ray Space Telescope, launched in 2008, detected diffuse γ rays from nearby molecular clouds with unprecedented sensitivity, and allowed us to investigate local CRs and interstellar gas with better precision (e.g., Ackermann et al 2012b,c;Planck and Fermi Collaboration 2015;Remy et al 2017Remy et al , 2018.…”

“…To estimate the H i column density (N H i ), the optically thin approximation has often been adopted (e.g., Boulanger & Perault 1988). Most Fermi-LAT γ-ray studies have used the N H i map based on a uniform spin temperature T s ( 100 K) or the optically thin approximation (e.g., Ackermann et al 2012c;Planck and Fermi Collaboration 2015;Tibaldo et al 2015;Remy et al 2017). On the other hand, dust optical depths derived from the Planck all-sky survey have a large scatter in the correlation with the H i integrated intensity for the local ISM.…”