Fermi Gamma-ray Space Telescope

Calibrating column density tracers with gamma-ray observations of the $\rho$ Ophiuchi molecular cloud

R. D. Abrahams
A. Teachey, T. A. D. Paglione

Abstract:

Likelihood analyses of gamma-ray counts maps require modeling a variety of presumed emission sources including their spatial extents and spectral shapes. The differences between the observed counts maps and these models often result in significant, spatially coherent residuals. These residuals are distinct from the 'dark gas' (presumed molecular gas poorly traced by CO), and persist despite accounting for other gas phases using dust maps or various measures. Given the goal to understand the underlying cosmic ray (CR) density, spectrum, and its spatial variation through the Galactic disk, the distribution and column density of the gas with which the CRs interact must be sensitively constrained. We present a study of the gamma-ray emission from the $\rho$ Ophiuchi molecular cloud seen by Fermi, and compare this emission to a number of column density tracers, including near IR stellar extinction and dust emission. This nearby molecular cloud exhibits a broad dynamic range in extinction, notably atypical dust properties, and a number of embedded B stars which heat the dust and may also act as local CR sources.