GBT26A-482

Tracing Dark Molecular Gas in the Eos Cloud with 18 cm OH

Abstract

The recently discovered Eos molecular cloud is the largest molecular structure on the sky, yet it is almost entirely invisible in CO, the standard tracer of molecular gas in the Milky Way. Eos is unique in that it was detected via ultraviolet fluorescence from H2, providing rare, direct evidence for a vast reservoir of CO-dark molecular gas. However, these UV observations suffer from coarse spatial resolution (~2deg), lack velocity information, and provide only loose constraints on the total H2 column density.

We propose a grid of targeted sightlines with the Green Bank Telescope to observe the 18 cm OH main lines, a proven tracer of CO-dark molecular gas. OH traces the HI-to-H2 transition in environments where CO is photodissociated but H2 survives, offering access to molecular gas that is otherwise invisible. The GBT's unparalleled surface brightness sensitivity allows us to reach detection limits corresponding to N(H2) ~ 7e18 cm-2 in just two hours per pointing, which is approximately the average conditions across the cloud. These observations will provide the first velocity-resolved view of the molecular gas in Eos and enable new constraints on the distribution, kinematics, and chemistry of this almost entirely CO-dark molecular cloud.

Investigators

Name	Institution
Michael Busch *	National Radio Astronomy Observatory
Isabelle Grenier	Commissariat à l'Energie Atomique
Joanne Dawson	Macquarie University; Commonwealth Scientific and Industrial Research Organisation
Blakesley Burkhart	Center for Astrophysics | Harvard & Smithsonian
Timothy Robishaw	Herzberg Astronomy & Astrophysics Research Centre, National Research Council Canada
Daniel Rybarczyk	Wisconsin at Madison, University of
Juan Soler	Istituto Nazionale di Astrofisica
Frances Buckland-Willis	École Normale Supérieure
Thavisha Dharmawardena	New York University

* indicates the PI