Diffuse Warm and Hot Gas in I Zw 18

Abstract

Using HST WFPC2 archival data we resolved the faint diffuse Hα emission in the halo of I Zw 18, first noted by Dufour & Hester (1990), into several large loops and filaments. Using new long-slit echelle data we have showed that these filaments and shells have complex kinematics:- the central high surface brightness bubble shows an expansion signature. The lower part of the echellogram, corresponding to the large wiggly loop around the west of I Zw 18 does not show a large scale expansion, but broad profiles, possibly indicating turbulent motions. The upper part of the echellogram shows a well defined expansion ellipse with a split of 75 km s^-1. The Hα HST image shows a large bubble in this region with a size of about 1 kpc. Taking our results together with the shell at the south end of IZw 18, detected by Martin (1996), the kinematic data strongly supports the presence of large shells expanding away from I Zw 18.