NGC 5775: Anatomy of a disk-halo interface

¹ Astronomy Department, University of Toronto, 60 St. George Street, Toronto, Ontario, Canada M5S 3H8
² Department of Physics, Queen's University, Kingston, Ontario, Canada K7L 3N6 e-mail: irwin@astro.queensu.ca
³ Astronomisches Institut der Ruhr-Universitt Bochum, Universittsstr. 150/NA 7, 44780 Bochum, Germany e-mail: dettmar@astro.ruhr-uni-bochum.de
⁴ National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, Canada V9E 2E7 e-mail: charles.cunningham@nrc.ca
⁵ Department of Astronomy, University of Massachusetts, B-524 Lederle Graduate Research Tower, Amherst, MA, 01003, USA e-mail: wqd@astro.umass.edu

Corresponding author: S.-W. Lee, swlee@astro.utoronto.ca

Received: 10 October 2000
Accepted: 12 July 2001

Abstract

We present the first high-resolution study of the disk-halo interface in an edge-on galaxy (NGC 5775) in which every component of the interstellar medium is represented and resolved (though not all to the same resolution). New single-dish CO and CO data, ROSAT X-ray data, and HIRES IRAS data are presented along with HI data which emphasizes the high latitude features. In conjunction with previously published radio continuum (6 and 20 cm) and Hα data, we find spatial correlations between various ISM components in that all components of the ISM are present in the disk-halo features (except for CO for which there is insufficient spatial coverage). The HI features extend to ~7 kpc above the plane, form loops in position-position space, in one case, form a loop in position-velocity space, and are also observed over a large velocity range. This implies that the disk-halo features represent expanding supershells. However, the shells may be incomplete and partially open-topped, suggesting that we are observing the breakup of the supershells as they traverse the disk-halo interface. There is some evidence for acceleration with z and both redshifted and blueshifted velocities are present, although the gas which is lagging with respect to galactic rotation dominates. The radio continuum spectral index is flatter around the shell rims and we show that this cannot be due to a contribution from thermal gas but rather is due to intrinsic flattening of the non-thermal spectral index, suggesting that shocks may be important in these regions. The Hα emission is located interior to the HI. For feature F3, the Hα emission forms the interior "skin" of the HI shell, yet there appears to be a minimum of in-disk star formation immediately below the feature. We present a picture of a "typical" HI supershell which accelerates and breaks up through the disk-halo interface. Such a feature is likely internally generated via an energetic event in the disk.