The Complex Kinematics of the Neutral Hydrogen Associated with I Zw 18

We present the results of high velocity (1.3 km s channels) and high spatial (∼5, or ∼ 250 pc at the distance of I Zw 18) resolution HI synthesis observations of the blue compact dwarf galaxy I Zw 18 to investigate the link between its unique evolutionary history and the neutral gas distribution and kinematics. The HI distribution is extensive, with diffuse neutral gas extending to the northwest and south of the main component. This diffuse gas may be a remnant of the nascent HI cloud. The kinematics of the I Zw 18 system are complex, with 4 components identified: HI–A, HI–C, HI–I, and HI–SX. The gas associated with the main body, HI–A, has a steep velocity gradient; although our analysis is hindered by poor spatial resolution relative to the extent of the system, the main body appears to be undergoing solid body rotation. The optical condensation to the northwest of I Zw 18 is embedded in the common HI envelope, and is found to be kinematically separate from the main body at a velocity of 740 km s (HI–C). The interbody gas, HI–I, connects HI–A and HI–C. Finally, a large diffuse, kinematically distinct, gas component extends at least 1 to the south of the main body (HI–SX), with no known optical counterpart. The peak of the gas column density coincides with the SE HII region in the main body; two other HI peaks are associated with the NW HII region and an HII region in the optical condensation to the northwest. In many respects, the HI properties of the main body of I Zw 18 (HI–A) are not unusual for dwarf galaxies; the peak column density, gas dispersion, MH/LB, and MH/MT are remarkably similar to other low mass systems. The neutral gas associated with I Zw 18 is best described as a fragmenting HI cloud in the early stages 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 HST detected O I cloud. While significantly affected by beam smearing in the presence of a steep velocity gradient, the derived gas velocity dispersion in the main body of I Zw 18 is approximately 12–14 km s. Based on the present analysis, the O I cloud has an oxygen abundance ∼>1/60th of solar, indicating that both the neutral and ionized medium are well–mixed.