Astron. Astrophys. 342, 213-232 (1999)

1. Introduction

High-velocity clouds are H I structures characterized by radial velocities which deviate typically by several hundred km s^-1 from conventional galactic rotation (see Wakker & van Woerden (1997) for a recent review). Distances remain uncertain for most of the clouds. Distance limits have been constrained for only two lines of sight by optical absorption lines found by Danly et al. (1993) and by van Woerden et al. (1998) toward complexes M and A . There is consensus that the HVCs comprising the Magellanic Stream are at Magellanic Cloud distances , based on positional and kinematic coincidences and the ability of tidal models to account for these coincidences. But the matter of distances remains largely unresolved for the majority of the HVCs. Blitz et al. (1998) suggest that some HVCs are scattered throughout the Local Group, excepting the principal northern complexes and the Magellanic Stream. Morphological arguments have led several authors (e.g. Hirth et al. 1985) to suggest that some HVCs interact with the galactic disk. This scenario is supported by the detection of soft X-ray enhancements close to HVC complexes M (Herbstmeier et al. 1995) and C (Hirth et al. 1985; Kerp et al. 1994, 1995, 1996). In addition, evidence of a physical connection of some HVCs with the galactic disk has been found in the H I "velocity bridges" which seem to link the HVC gas with gas at conventional velocities (Pietz et al. 1996).

We extend the SXRB investigations of Herbstmeier et al. (1995) and Kerp et al. (1996) to other HVC complexes, different in location, velocity, and possibly in origin, we use correlations of ROSAT keV X-ray data (see Snowden et al. 1997) with data from the Leiden/Dwingeloo H I survey (Hartmann & Burton 1997). The selected fields are at high latitudes, widely distributed over the sky, which encompass readily identifiable (see Wakker & van Woerden 1997) parts of HVC complexes. The complexes C, A, D, WA, and GCN fit those criteria; the detailed shapes of the selected fields were partly determined by the polar-grid projection of the ROSAT data.

We evaluate the transmission of keV photons through the X-ray absorbing interstellar medium, and demonstrate that the transmission is quantitatively traced by H I . Our approach aims at distinguishing fluctuations in the soft X-ray intensities caused by photoelectric absorption effects from those signifying true excess soft X-ray emission. To this end, we first model the SXRB distribution modulated by the photoelectric absorption caused by Milky Way gas at conventional and intermediate velocities. We then subtract the modelled SXRB distribution from the observed one, and identify regions where the modelled distribution deviates from what is observed.

In Sect. 2, we describe the X-ray and H I data used. In Sect. 3, we evaluate the soft X-ray radiation-transfer equation with the goal of finding HVC signatures in the SXRB distribution. In Sect. 4, we show the results of the correlation analysis towards individual HVC complexes. In Sect. 5, we discuss the implications for the origin and distribution of the SXRB sources. The results are summarized in Sect. 6.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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