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Astron. Astrophys. 364, L36-L41 (2000)

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1. Introduction

There is increasing evidence that halos of normal spiral galaxies contain a complex multiphase interstellar medium (ISM) (for a review see, e.g., Dahlem 1997). Besides a hot component as observed by X-ray emission ([FORMULA]106 K) or UV absorption ([FORMULA]0.5[FORMULA]105 K), Diffuse Ionized Gas (DIG) constitutes another important phase of typically 104 K (Dettmar 1992). This component is easily observed by its [FORMULA] region like line emission and is found to be widespread in galaxies with actively star forming disks (Rand 1996, Dettmar 1999, Rossa & Dettmar 2000).

In the last couple of years two important problems have been discussed with regard to the physics of this interstellar gas phase. The observed line ratios are not explained in a straight forward way by pure photoionization models and require additional heating mechansims of the halo gas (e.g., Golla et al. 1996, Rand 1997, Tüllmann & Dettmar 2000) while first kinematic observations hint at a slower rotation of halo gas if compared to the underlying disk (Rand 1998). Both kinds of observations could help to constrain models of the origin of halo gas which in turn are important for the understanding of galaxy evolution since large scale mass flows are of influence for star formation and chemical evolution.

Only for a few objects, such as NGC 891 (Dahlem et al. 1994), NGC 4631 (Golla & Hummel 1994), and NGC 5775, the various components of the halo ISM are observed with sufficient sensitivity and resolution to study possible physical processes at their interfaces such as turbulent mixing or thermal conduction. For such processes yet another ISM component can be of importance, namely the non-thermal component consisting of magnetic fields and cosmic rays as observed by its radio-continuum radiation. Any large scale gas motion and in particular its z-component will also be of importance for dynamo theories which try to explain the large scale distribution of magnetic fields in galaxies. However, various approaches to describe galactic dynamos are still very controversial and more detailed information on galactic gas flows in halos as well as magnetic field structures could help constraining such models.

For all these questions, the case of the Sc edge-on galaxy NGC 5775 has received some attention recently, since marked radio-continuum (Duric et al. 1998) and [FORMULA] (Irwin 1994) halo components are accompanied by a very extended DIG halo (Dettmar 1992, Collins et al. 2000) for which a strong decline of the rotational velocity with height above the plane (z) has been found (Rand 2000).

In this letter we report on first spectroscopic results for the DIG halo of NGC 5775 with the VLT, confirming the strong z-dependence of the halo rotation presented by Rand (2000), along with a first detailed determination of the magnetic field structure in the halo from VLA radio-continuum observations.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000