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Astron. Astrophys. 332, L61-L64 (1998)

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1. Introduction

The detection of neutral interstellar clouds at large z distances (Münch 1957) led to the hypothesis of a gaseous galactic halo (Spitzer 1956). A rarefied high-temperature ionized gas ([FORMULA] K) was assumed to be in pressure equilibrium with normal interstellar clouds. A relatively cool neutral halo ([FORMULA] K) was postulated by Pikelner & Shklovsky (1958). Their model predicted emission lines of neutral species with velocity dispersions of 70 [FORMULA] due to turbulent motions, but was abandoned a few years later because such lines were not observed.

Observations of faint, wide lines originating from a galactic halo are plagued by considerable instrumental difficulties. Instrumental improvements in recent years have led to increasing evidence for emission from a neutral galactic halo. HI gas with dispersions of up to 35 [FORMULA] has been found in the directions of the galactic poles by Kulkarni & Fich (1985). Lockman & Gehman (1991) pointed out that the turbulent energy of these HI clouds can support layers up to distances z [FORMULA] 1 kpc. Evidence for an extended neutral galactic halo was presented by Albert et al. (1994). Diffuse high-dispersion HI gas was found in sensitive observations of extragalactic objects by Schulman et al. (1994). In 10 out of 14 deep integrations of face-on galaxies HI profile wings were detected and interpreted as due to gas with dispersions of 30 to 50 [FORMULA].

This interpretation implies galactic halo gas of modest ([FORMULA] K) temperatures. However, spectral line observations of highly ionized atoms indicate temperatures of [FORMULA] K (Savage et al. 1997), while a plasma with T [FORMULA] K is required to explain the soft X-ray background (Kerp 1994, Pietz et al. 1998). This implies that the galactic halo gas cannot be assigned a single temperature.

We have analyzed the Leiden/Dwingeloo 21-cm Survey (LDS) and found high velocity dispersion (HVD) HI emission widely distributed over the sky. In Sect. 2 we describe the observations and present our findings. In Sect. 3 we discuss instrumental difficulties and demonstrate that our results seem unaffected by such problems. In Sect. 4 we reproduce our observations by a model HI distribution. A discussion is given in Sect. 5.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998