THE COMPLEX KINEMATICS OF THE NEUTRAL HYDROGEN ASSOCIATED WITH I-ZW-18

We present the results of high velocity (1.3 km s(-1) channels) and hi gh spatial (similar to 5'', or similar to 250 pc at the distance of I Zw 18) resolution H I synthesis observations of the blue compact dwarf galaxy I Zw 18 to investigate the link between its unique evolutionar y history and the neutral gas distribution and kinematics. The H I dis tribution is extensive, with diffuse neutral gas extending to the nort hwest and south of the main component. This diffuse gas may be a remna nt of the nascent H I cloud. The kinematics of the I Zw 18 system are complex, with four components identified: H I-A, H I-C, H I-I, and H I -SX. The gas associated with the main body, H I-A, has a steep velocit y gradient; although our analysis is hindered by poor spatial resoluti on relative to the extent of the system, the main body appears to be u ndergoing solid-body rotation. The optical condensation to the northwe st of I Zw 18 is embedded in the common H I envelope and is found to b e kinematically separate from the main body at a velocity of 740 km s( -1) (H I-C). The interbody gas, H I-I, connects H I-A and H I-C. Final ly, a large, diffuse, kinematically distinct gas component extends at least 1' to the south of the main body (H I-SX), with no known optical counterpart. The peak of the gas column density coincides with the so utheast H II region in the main body; two other H I peaks are associat ed with the northwest H II region and an H It region in the optical co ndensation to the northwest. In many respects, the H I properties of t he main body of I ZW 18 (H I-A) are not unusual for dwarf galaxies; th e peak column density, gas dispersion, M-H/L-B, and M-H/M-T are remark ably similar to other low-mass systems. The neutral gas associated wit h I Zw 18 is best described as a fragmenting H I cloud in the early st ages of galaxy evolution. The derived gas distribution and kinematics are placed in the context of the known star formation history of I Zw 18. In particular, the neutral gas velocity dispersion is critical for calculating the abundance of the O I cloud detected by the Hubble Spa ce Telescope. While significantly affected by beam smearing in the pre sence of a steep velocity gradient, the derived gas velocity dispersio n in the main body of I Zw 18 is approximately 12-14 km s(-1). Based o n the present analysis, the O I cloud has an oxygen abundance greater than or similar to 1/60 of solar, indicating that both the neutral and ionized mediums are well mixed.