X-ray shadowing experiments toward infrared dark clouds

Abstract

We searched for X-ray shadowing toward two infrared dark clouds (IRDCs) using the MOS detectors on XMMNewton to learn about the Galactic distribution of X-ray emitting plasma. IRDCs make ideal X-ray shadowing targets of 34 keV photons due to their high column densities, relatively large angular sizes, and known kinematic distances. Here we focus on two clouds near 30° Galactic longitude at distances of 2 and 5 kpc from the Sun. We derive the foreground and background column densities of molecular and atomic gas in the direction of the clouds. We find that the 34 keV emission must be distributed throughout the Galactic disk. It is therefore linked to the structure of the cooler material of the interstellar medium and to the birth of stars. © 2010. The American Astronomical Society. All rights reserved.