Wheels of Fire. III. Massive Star Formation in the ``Double-Ringed'' Ring Galaxy Am 0644-741

We have used Hα CCD imaging to map the distribution and intensity of massive star formation (MSF) in AM 0644-741, a large southern ring galaxy. MSF is restricted to a pair of interlocked 30 kpc diameter rings, with less than 1% contributed by a nuclear point source. Fifty percent of the total Hα emission originates in a crescent-shaped region defining the southwest quadrant of the most prominent ring (``A-ring''). North of the nucleus, MSF bifurcates into closely spaced rings of H II complexes. Particularly strong MSF occurs where they intersect. A broad continuum ring spans the disk south of the nucleus (``B-ring''). No diffuse MSF is found over the enclosed disk above an SFR per unit area of 0.15 M_⊙ Gyr^-1 pc^-2 (3 σ), which is similar to limits set in the disks of S0 galaxies. However, low Hα surface brightness (Σ_Hα) complexes are found beyond the southwest A-ring. We derive a total Hα luminosity (L_Hα) of 3.0 × 10⁴¹ ergs s^-1 (H₀ = 100 km s^-1 Mpc^-1), and a total SFR of 3 M_⊙ yr^-1. Both values are significantly larger than those of field spirals and comparable to those found in interacting disk systems. The integrated Hα equivalent width (EW_Hα) is 14 Å, which is similar to values derived for noninteracting spirals. Systematic changes in L_Hα and EW_Hα with ring position angle were found in the A-ring, although much less pronounced than the Cartwheel's (Higdon 1995). Both A-rings and B-rings show only small azimuthal changes in red continuum. We have measured L_Hα and EW_Hα for 54 ring H II complexes. While their L_Hα tend to be higher than those commonly found in spiral galaxies, their EW_Hα are very similar. The H II luminosity function possesses a flat slope and large peak L_Hα, similar to those of irregular galaxies.

We conclude that MSF in AM 0644-741 has been enhanced (~3×) and redistributed as a result of the intruder's passage and that the ring galaxy was originally an Sa-Sab spiral. We interpret the distribution of Σ_Hα around the rings and faint Hα emission beyond the southwest A-ring in terms of MSF triggered on the outer edge of a primarily gaseous ring density wave. We suggest that the unique double-ring structure reflects strong caustics along the inner and outer edges of a single-ring density wave. The much less extreme MSF properties in AM 0644 relative to the Cartwheel likely arise from a combination of a weaker orbit crowding in the ring, higher metallicity, and a thicker precollision disk.

We compare our results with ring galaxy models, where we find mixed results: the basic distribution and kinematics of material in ring galaxies appear to be described reasonably well. However, recent models fail to reproduce the observed distribution of star forming regions.