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

Abstract

We present the results of high velocity (1.3 km s^-1 channels) and high spatial (~5", or ~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 evolutionary history and the neutral gas distribution and kinematics. The H i 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 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 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 H i envelope and is found to be 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. Finally, 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 southeast H ii region in the main body; two other H i peaks are associated with the northwest H ii region and an H ii region in the optical condensation to the northwest. In many respects, the H i properties of the main body of I Zw 18 (H i-A) are not unusual for dwarf galaxies; the peak column density, gas dispersion, M_H/L_B, and M_H/M_T are remarkably similar to other low-mass systems. The neutral gas associated with I Zw 18 is best described as a fragmenting H i 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 O i cloud detected by the Hubble Space Telescope. 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^-1. Based on the present analysis, the O i cloud has an oxygen abundance >~1/60 of solar, indicating that both the neutral and ionized mediums are well mixed.