We present the results of high velocity ( 1.3 km s ^ { -1 } channels ) and high spatial ( \sim 5 \arcsec , or \sim 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 ^ { -1 } ( 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 I Zw 18 ( 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 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 \ion O1 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 ^ { -1 } .
Based on the present analysis , the \ion O1 cloud has an oxygen abundance \gtrsim 1/60th of solar , indicating that both the neutral and ionized medium are well–mixed .