Constraints on the properties of warm ionized gas from low-frequency hydrogen radio recombination lines

Abstract

The ionized gas in the Milky Way is a major component of the interstellar medium. Observations of extinction free tracers, such as hydrogen radio recombination lines (HRRLs), have revealed the presence of a dense (electron density 1 to 100 cm${}^{-3}$) warm ionized medium. Motivated by advances in radio instrumentation, the existence of fully sampled HRRL maps, and a better knowledge about the population of discrete HII regions in our Galaxy, we have acquired new low-frequency ($\nu \lesssim 1$ GHz) observations of HRRLs to characterize the properties of this gas. We target three positions in the Galactic plane, with few or no known HII regions, using the 342 MHz and 800 MHz feeds of the Green Bank Telescope. We detect HRRL emission from all three positions. We combine these with the fully sampled HRRL 5.8 GHz cubes from the GBT Diffuse Ionized Gas Survey (GDIGS) to determine the gas properties using a forward modeling approach. From our analysis we find electron densities between 6 and 15 cm${}^{-3}$, and that to determine the gas temperature and emission measure we require informative priors or higher signal-to-noise observations.

20 pages, 8 figures, accepted for publication in AJ

References in corpus (12)

• Post-correlation radio frequency interference classification methods
• The Electron Temperature Gradient in the Galactic Disk
• Expanding bubbles in Orion A: [CII] observations of M42, M43, and NGC 1977
• Gradients of chemical abundances in the Milky Way from HII regions: distances derived from Gaia EDR3 parallaxes and temperature inhomogeneities
• Carbon and hydrogen radio recombination lines from the cold clouds towards Cassiopeia A
• Low Frequency Carbon Radio Recombination Lines I: Calculations of Departure Coefficients
• The GBT Diffuse Ionized Gas Survey (GDIGS): Survey Overview and First Data Release
• Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium
• Peering into the Milky Way by FAST: I. Exquisite HI structures in the inner Galactic disk from the piggyback line observations of the FAST GPPS survey
• The GBT Diffuse Ionized Gas Survey: Tracing the Diffuse Ionized Gas around the Giant HII Region W43
• The Dense Warm Ionized Medium in the Inner Galaxy
• Searching for the largest bound atoms in space