Introduction

NGCá 157 is a rather innocuous looking SAB(rs)bc galaxy that first came to our attention when major axis optical spectra by Zasov & Kyazumov (1981) showed an abrupt drop (of at least 50á kmá s) in projected rotation velocity, but only on the northern side. The fact that this drop occurs across a ring of HIIá regions leads us to hypothesise that this velocity discontinuity may be the signature of an expanding HIá ``superbubble''. Assuming a distance to NGCá 157 of 20.9á Mpc (Tully 1988), such a superbubble would be among the largest yet known, with a diameter of 4á kpc! In order to test this theory, we have mapped the distribution and kinematics of the neutral atomic hydrogen using a combination of C- and D-array observations with the Very Large Array. In addition, we have obtained wide-field B, I, and H images with the Kitt Peak National Observatory 0.9á m telescope, and measured the kinematics of the ionised gas in the inner disk using a Fabry-Perot spectrometer on the Special Astrophysical Observatory 6á m telescope. The reduction and analysis of these observations is described in Ryder, A. Zasov, V McIntyre etá al. (1997); in this paper, we concentrate on mass models which can account for the unusual rotation curve of NGCá 157.

á© Copyright Astronomical Society of Australia 1997