The Composition of the Interstellar Medium towards the Lockman Hole. HI, UV and X-ray observations

Abstract

The Lockman Hole is well known as the region with the lowest neutral atomic hydrogen colum density on the entire sky. We present an analysis of the soft X-ray background radiation towards the Lockman Hole using ROSAT all-sky survey data. This data is correlated with the Leiden/Dwingeloo survey (Galactic HI 21cm-line emission) in order to model the soft X-ray background by using radiative transfer calculations for four ROSAT energy bands simultaneously. It turns out, that an important gas fraction, ranging between 20-50%,of the X-ray absorbing material is not entirely traced by the HI but is in the form of ionized hydrogen. Far-ultraviolet absorption line measurements by FUSE are consistent with this finding and support an ionized hydrogen component towards the Lockman Hole.

11 pages, 11 figures, Accepted for Publication in Astronomy & Astrophysics, For full resolution images, see http://www.astro.uni-bonn.de/~mkappes/pub/ms3506.pdf