Wheels of Fire. II. Neutral Hydrogen in the Cartwheel Ring Galaxy

The VLA was used to investigate the distribution and kinematics of neutral hydrogen, and the relation between massive star formation (MSF) and gas surface density, in the Cartwheel ring galaxy. Over 85% of the Cartwheel's H I resides in the outer ring, with a low surface density component filling much of the interior. Analysis of the H I velocity field indicates that the outer ring is expanding at V_exp_ = 53 +/- 9 km s^-1^, and that ~300 Myr have elapsed since the intruder's passage. Changes in V_exp_ with radius show that gas is beginning to leave the outer ring and is infalling for R <~ 8 kpc. H I is accumulating just beyond the inner ring, though optical (V - R) maps show gas and dust crossing the ring and flowing into the nucleus along two streams. No H I analogs of the optical "spokes" are found. The ring's surface brightness in Hα ({SIGMA}_Hα_) and 20 cm continuum ({SIGMA}_20 cm_) are identical, showing that peculiar extinction is not responsible for the observed crescent of MSF. Only the two most luminous H II complexes are detected at 6 cm and are characterized by nonthermal spectra (α^bar^ = - 0.65) and low 6 cm- Hα extinction (A^bar^_V_ = 1.7 mag). The 20 cm continuum-derived Type II SN rate (0.1 +/- 0.02 yr^-1^) is consistent with the lower bound of optical estimates. H I and Hα are both concentrated and anticorrelated in the outer ring. MSF appears to take place on the H I ring's leading edge throughout the starburst quadrant, where a disturbed H I component is found. The Cartwheel's gas consumption time-scale is 290 Myr. If the inner ring triggers a second sustained starburst, a significant fraction of the remaining gas supply may be converted into stars. The global distribution of MSF can be understood in terms of a critical surface density ({SIGMA}_crit_; reported by Kennicutt in 1989): Only in the outer ring does the atomic gas surface density ({SIGMA}_ag_) exceed {SIGMA}_crit_ when averaged over a full range in azimuth. At smaller radii, {SIGMA}_ag_/{SIGMA}_crit_ <= 0.5. The absence of strong shocks in the outer ring argue against MSF induced by high-velocity cloud collisions (as discussed by Olson & Kwan in 1990). The ring starburst appears to be the result of very massive cloud complexes formed through agglomeration at low velocities. Feedback and systematic variations in collision rates/speeds likely influence MSF on kiloparsec scales in the ring itself. Large departures from Schmidt law behavior are found. H I is detected in two companion galaxies. Mass estimates using their H I line widths yield values <~ 6% of the Cartwheel's disk + halo. A broad and massive (2 x 10^9^ M_sun_) H I plume projects 82 kpc from the ring to the northeast. It is argued that this H I "splash" resulted from the passage of G3 through the Cartwheel's disk ~300 Myr ago. Much of this material will escape the ring galaxy.