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.

arXiv comment20 pages, 8 figures, accepted for publication in AJ

References in corpus (34)

• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• The WISE Catalog of Galactic HII Regions
• An Accurate Flux Density Scale from 50 MHz to 50 GHz
• The warm ionized medium in spiral galaxies
• Post-correlation radio frequency interference classification methods
• Morphology of the Interstellar Cooling Lines Detected by COBE
• Star Formation in Massive Clusters via the Wilkinson Microwave Anisotropy Probe and the Spitzer Glimpse Survey
• A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy
• The Electron Temperature Gradient in the Galactic Disk
• The Parallax of W43: a Massive Star Forming Complex near the Galactic Bar
• Kinematic Distances: A Monte Carlo Method
• Planck 2018 results. II. Low Frequency Instrument data processing
• The HIPASS survey of the Galactic plane in Radio Recombination Lines
• The Radio Sky at Meter Wavelengths: m-Mode Analysis Imaging with the Owens Valley Long Wavelength Array
• A New 1.4GHz Radio Continuum Map of the Sky South of Declination +25 deg
• S-band Polarization All Sky Survey (S-PASS): survey description and maps
• Expanding bubbles in Orion A: [CII] observations of M42, M43, and NGC 1977
• Filling factors and scale heights of the DIG in the Milky Way
• Gradients of chemical abundances in the Milky Way from HII regions: distances derived from Gaia EDR3 parallaxes and temperature inhomogeneities
• Herschel Galactic plane survey of [NII] fine structure emission
• A Green Bank Telescope Survey of Large Galactic HII Regions
• Super Star Clusters in the Central Starburst of NGC 4945
• Carbon and hydrogen radio recombination lines from the cold clouds towards Cassiopeia A
• Unbiased flux calibration methods for spectral-line radio observations
• 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
• Electron Densities and Nitrogen Abundances in Ionized Gas Derived Using [NII] Fine-structure and Hydrogen Recombination lines
• 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