P. N. Appleton

Models of ring galaxies. I. The growth and disruption of clouds in the expanding density wave

Cloud-fluid models of the growth and evolution of density waves in a colliding ring galaxy with a low mass companion are studied. It is found that, if the ring expands and grows on a time scale comparable with the collision time between interstellar clouds in the disk galaxy, both enhancement and suppression of star formation can occur. A second ring should form after the first wave has become well developed. This ring is likely to be more strongly compressed than the first, leading to generally stronger star formation. The infall of gas onto a nucleus capable of nonthermal activity at the ring center appears to be inevitable during most of the lifetime of the ring phenomenon. 38 references.

Star formation rates in ring galaxies from IRAS observations

IRAS and optical data for a sample of 26 ring galaxies are analyzed, and it is found that: (1) relatively high average values of far-infrared luminosity L(FIR), infrared to blue luminosity ratio L(FIR)/L(B), and color temperature compared to normal galaxies, implying a high recent star formation rate; (2) evidence that a large fraction of the young stars are located in the rings, indicating a very extended, coherent starburst; and (3) a possible trend of the dispersion of L(FIR) among rings as a function of ring diameter. Thus, within the uncertainties inherent in the study of this relatively small sample, it appears that ring galaxies represent a unique class of nonnuclear coherent starbursts. 45 references.

Models of ring galaxies. II - Extended starbursts

Numerical models of the development of star-formation bursts in collisional ring galaxies are presented. To extend the work of Appleton and Struck-Marcell (1987) target disks which have relatively high mean cloud mass and gas density are emphasized. In such cases, even relatively low mass intruder galaxies are capable of triggering intense star-formation bursts in the density waves. Although the bursts are very short-lived in any individual gas element, pressure effects stimulate neighboring gas elements to burst, which can result in a sustained enhancement in the net star-formation rate. The results are capable of explaining the high far-infrared fluxes observed in righ galaxies and provide clues to the development of starburst activity in other colliding galaxies. 26 references.