Imaging galactic diffuse gas: Bright, turbulent CO surrounding the line of sight to NRAO150

TL;DR

This study uses high-resolution imaging to explore the structure and chemistry of galactic diffuse gas around NRAO150, revealing that CO emission variations are likely due to chemical processes rather than large-scale molecular components.

Contribution

The paper provides detailed imaging of galactic diffuse gas at multiple resolutions, linking CO emission variations to chemical processes rather than large-scale molecular structures.

Findings

Absence of large-scale molecular components in emission despite absorption features.

Bright CO lines are likely due to chemical variations, specifically C+-CO conversion.

Emission features do not reveal hidden large-scale molecular gas.

Abstract

To understand the environment and extended structure of the host galactic gas whose molecular absorption line chemistry, we previously observed along the microscopic line of sight to the blazar/radiocontinuum source NRAO150 (aka B0355+508), we used the IRAM 30m Telescope and Plateau de Bure Interferometer to make two series of images of the host gas: i) 22.5 arcsec resolution single-dish maps of 12CO J=1-0 and 2-1 emission over a 220 arcsec by 220 arcsec field; ii) a hybrid (interferometer+singledish) aperture synthesis mosaic of 12CO J=1-0 emission at 5.8 arcsec resolution over a 90 arcsec-diameter region. CO components that are observed in absorption at a moderate optical depth (0.5) and are undetected in emission at 1 arcmin resolution toward NRAO 150 remain undetected at 6 arcsec resolution. This implies that they are not a previously-hidden large-scale molecular component revealed in absorption, but they do highlight the robustness of the chemistry into regions where the density and column density are too low to produce much rotational excitation, even in CO. Bright CO lines around NRAO150 most probably reflect the variation of a chemical process, i.e. the C+-CO conversion. However, the ultimate cause of the variations of this chemical process in such a limited field of view remains uncertain.