X-ray Shadowing Experiments Toward Infrared Dark Clouds

We searched for X-ray shadowing toward two infrared dark clouds (IRDCs) using the MOS detectors on XMM-Newton to learn about the Galactic distribution of X-ray emitting plasma. IRDCs make ideal X-ray shadowing targets of 3\over 4 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 3\over 4 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.