Mapping low frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54 and 43 MHz

Abstract

Quantitative understanding of the interstellar medium requires knowledge of its physical conditions. Low frequency carbon radio recombination lines (CRRLs) trace cold interstellar gas, and can be used to determine its physical conditions (e.g., electron temperature and density). In this work we present spatially resolved observations of the low frequency ($\le 390$ MHz) CRRLs centered around C$268\alpha$, C$357\alpha$, C$494\alpha$ and C$539\alpha$ towards Cassiopeia A on scales of $\le 1.2$ pc. We compare the spatial distribution of CRRLs with other ISM tracers. This comparison reveals a spatial offset between the peak of the CRRLs and other tracers, which is very characteristic for photodissociation regions and that we take as evidence for CRRLs being preferentially detected from the surfaces of molecular clouds. Using the CRRLs we constrain the gas electron temperature and density. These constraints on the gas conditions suggest variations of less than a factor of two in pressure over $\sim 1$ pc scales, and an average hydrogen density of $200$-$470$ cm${}^{-3}$. From the electron temperature and density maps we also constrain the ionized carbon emission measure, column density and path length. Based on these, the hydrogen column density is larger than $10^{22}$ cm${}^{-2}$, with a peak of $\sim 4\times 10^{22}$ cm${}^{-2}$ towards the South of Cassiopeia A. Towards the southern peak the line of sight length is $\sim 40$ pc over a $\sim 2$ pc wide structure, which implies that the gas is a thin surface layer on a large (molecular) cloud that is only partially intersected by Cassiopeia A. These observations highlight the utility of CRRLs as tracers of low density extended HI and CO-dark gas halo's around molecular clouds.

18 pages, 15 figures, 4 tables, accepted by MNRAS

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• Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
• Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
• The first detection of radio recombination lines at cosmological distances
• X-ray Absorption in Young Core-Collapse Supernova Remnants
• Searching for the largest bound atoms in space
• A High Resolution Study of Carbon Radio Recombination Lines towards Cassiopeia A
• Spectroscopy with the Engineering Development Array: cold H${}^{+}$ at 63 MHz towards the Galactic Centre
• Low-Frequency Radio Recombination Lines Away From the Inner Galactic Plane
• Efficient computation of collisional $\ell$-mixing rate coefficients in astrophysical plasmas
• Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A