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 3/4 kev
photons due to their high column densities, relatively large angular sizes, and
known kinematic distances. Here we focus on two clouds near 30 deg. 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 3/4 kev emission must be distributed throughout
the Galactic disk. It is therefore linked to the structure of the cooler
material of the ISM, and to the birth of stars.
