Description |
Studies of evolved systems like AM0644-741 and Cartwheel find the ISM in the starburst rings to be overwhelmingly atomic despite conditions (e.g., pressure and ambient far-UV fields) clearly favoring a dominant molecular component. Moreover, the local molecular fraction anti-correlates with SFR/area, yielding unusually high star formation efficiencies (SFE) and a highly peculiar star formation law. AM0644-741s starburst ring moreover appears mostly stable gravitationally (Q = 2-6). We have argued that these all follow naturally from the ISMs >100 Myr confinement time in the ring, which amplifies the destructive effects of embedded OB stars and SNe, producing an ``over-cooked ISM, i.e., one characterized by small H2 clouds, and a large photo-dissociated HI background. Due to reduced dust columns, H2 is poorly traced by CO rotational lines, and we expect a large ``dark molecular component in the ring. We will use PACS and SPIRE photometry with existing IRAC and MIPS data to (1) construct infrared-submillimeter SEDs for the rings of AM0644-741 and Cartwheel to determine the total mass and distribution of H2 by its dust emission following Israel (1996) and Leroy et al. (2009), (2) re-evaluate their peculiar SFE, non-Schmidt star formation laws, and gravitational stability. Not finding a large hidden H2 component would imply that star formation is triggered by other processes (e.g., collisions) at unusually high SFE. |