EDP Sciences
Free access
Volume 405, Number 2, July II 2003
Page(s) 607 - 616
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030610

A&A 405, 607-616 (2003)
DOI: 10.1051/0004-6361:20030610

The composition of the interstellar medium towards the Lockman Hole

$\ion{H}{I}$, UV and X-ray observations
M. Kappes1, J. Kerp1 and P. Richter2

1  Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: jkerp@astro.uni-bonn.de
2  Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Florence, Italy
    e-mail: richter@arcetri.astro.it

(Received 21 January 2003 / Accepted 18 April 2003 )

The Lockman Hole is well known as the region with the lowest neutral atomic hydrogen column 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 $\ion{H}{i}$ 21 cm-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 $\ion{H}{i}$ 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.

Key words: X-rays: diffuse background -- X-rays: ISM -- ultraviolet: ISM -- Galaxy: halo -- Galaxy: structure

Offprint request: M. Kappes, mkappes@astro.uni-bonn.de

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