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

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Abstract
We present the results of high velocity and high spatial resolution HI synthesis observations of the blue compact dwarf galaxy IZw18 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 IZw18 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 IZw18 is embedded in the common HI envelope, and is found to be kinematically separate from the main body (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 arcmin 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 IZw18 (HI-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 IZw18 is best described as a fragmenting HI cloud in the early stages of galaxy evolution.Comment: 25 pages, 12 figures. using AASTex and psfig. Accepted to the Astronomical Journal, March 199
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