The relationship of galaxy morphology to nuclear star formation in non-interacting spiral galaxies

Three specific questions concerning the relationship between galactic morphology and infrared properties were addressed for noninteracting galaxies: (1) the scarcity of high infrared luminosity Sa galaxies compared to Sb and Sc; (2) the relationship between the bulge to total luminosity and the infrared properties of early type spirals; and (3) nuclear star formation processes in noninteracting galaxies. These questions were answered using IRAS data, CO (1 to 0) measurements, 2 micron, 10 micron, and visible CCD observations. Only four percent of Sa's in the Revised Shapley-Ames Catalog (RSAC) with B(gamma) is less than 12 have infrared luminosities 1/6th of the ratio for Sb's and Sc's. Less than three Sa's of 166 in the RSAC have nuclear starbursts not associated with interactions or active nuclei. A comparison of neutral hydrogen fluxes and CO fluxes with infrared fluxes implies that molecular cloud formation is inhibited in Sa's, leading to the lack of infrared activity. An investigation of the role of bulges in suppressing star formation in Sa through Sb spirals relied on the photometric observations of Kent, Kodaira, and Cornell and on IRAS infrared observatories. The results indicate that disk star formation is relatively unaffected by bulge size. The hypothesis that high far-infrared luminosities in noninteracting galaxies are dependent on material fed into the nuclei by bars was tested by near infrared imaging of a sample of 15 optically unbarred galaxies in a search for hidden bars. At least 8 of these galaxies do not appear to have bars. Strong bars therefore are not an absolute requirement for high infrared luminosity