The thermal and non-thermal gaseous halo of NGC5775

Abstract

In this letter we present first results from spectroscopic observations of Diffuse Ionized Gas (DIG) in the halo of NGC5775 obtained with FORS1 attached to UT1/Antu of the Very Large Telescope (VLT). At our slit position perpendicular to the disk (41" SE of the nucleus) the emission of [NII]6583, [OIII]5007, and [OII]3727 is detected out to 9 kpc into the halo, allowing possible ionization mechanisms of the DIG to be examined. Photoionization models which assume a dilute radiation field are able to fit the data for the disk, but they cannot account for the line ratios measured in the halo (e.g., [OI]/Halpha or [OII]/Halpha). In particular they fail to predict the observed increase of [OIII]5007/Halpha with increasing z. The most striking result concerns the kinematics of the halo gas. Velocities at high galactic latitudes drop from the midplane value to reach the systemic velocity at z ~ 9 kpc. An analysis of VLA archive data of the polarized radio-continuum emission at 4.86 GHz and 1.49 GHz reveals that magnetic fields in the halo have a strong component perpendicular to the disk and are aligned with the Halpha and radio-continuum spurs in the halo. This can result either from a strong wind action or, more likely, from the generation of dipolar magnetic fields. We briefly discuss the interrelation of the magnetic field structure and gas dynamics, in particular the role of magnetic fields in gas outflows, as well as the possible heating of DIG by magnetic reconnection.