The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also traces diffuse gas

TL;DR

This paper demonstrates that the CO-to-H2 conversion factor is similar in both dense and diffuse interstellar medium, challenging previous assumptions and impacting how molecular gas is traced in space.

Contribution

It reveals that bright 12CO emission can trace diffuse gas, showing the XCO factor's consistency across different ISM conditions, which is a novel insight.

Findings

XCO factor is similar in dense and diffuse gas

Bright 12CO emission traces diffuse molecular gas

Implications for ISM studies and molecular gas estimation

Abstract

We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium.