The relationship of galaxy morphology to nuclear star formation in noninteracting spiral galaxies.

Abstract

Three specific questions concerning the relationship between galaxian morphology and infrared properties were addressed for non-interacting galaxies: (1) Why are high infrared luminosity (L > 10¹⁰ L(⊙)) Sa galaxies scarce compared to Sb and Sc galaxies; (2) Is there a relationship between the bulge to total luminosity and the infrared properties of early type spirals; and (3) Are bars essential to nuclear star formation processes in non-interacting galaxies? These questions were answered using IRAS data, CO (1-0) measurements, 2 μm, 10 μm, and visible CCD observations. Only 4% of Sa's in the Revised Shapley-Ames Catalog (RSAC) with B(Γ) < 12 have infrared luminosities > 10¹⁰ L(⊙), 1/6 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 observations. The bulge to total luminosity is uncorrelated with the ratio of infrared to blue flux, 60 μm/100 μm fluxes, or infrared luminosity. These results indicate that disk star formation is relatively unaffected by bulge size. The hypothesis that high far-infrared luminosities in non-interacting galaxies are dependent on material fed into their nuclei by bars was tested by near infrared imaging of a sample of 15 optically unbarred galaxies in a search for hidden bars. These galaxies were non-interacting, non-Seyfert galaxies with far infrared luminosities > 10¹⁰ L(⊙) and hot colors between 60 and 100 μm (S₆₀/S₁₀₀ > 0.5, indicative of nuclear starbursts). At least 8 of these galaxies do not appear to have bars. Strong bars therefore are not an absolute requirement for high infrared luminosity.