THE ENERGY GENERATION MECHANISMS POWERING INFRARED-SELECTED GALAXIES

Abstract

ABSTRACT I have applied newly available optical emission line diagnostics to two samples of infrared-luminous galaxies in a program to discriminate between possible energy generation mechanisms, i.e., active nuclei and starbursts. The first sample was composed of very faint (F(60 microns) < 180 mJy, z < 0.3) galaxies drawn from coadded IRAS scans of the North Ecliptic Pole Region (NEPR). The second sample consisted of 37 much brighter sources drawn from the IRAS Bright Galaxy Sample (BGS). Analysis of the redshift distribution of the NEPR sample revealed clustering is present on scales ~10 Mpc. In addition, a very strong cluster was identified at z=0.088 that has a surface density roughly three times greater than the Great Wall. Out of the 76 galaxies observed from the NEPR sample 17 had strong spectra suitable for the analysis, which was based on the relative intensities of forbidden and permitted emission lines. While 3 sources could be classified as AGNs and another 7 as starburst galaxies, the remaining 7 eluded classification. Longslit optical emission-line spectra, which effectively separated nuclear and disk contributions to the emission, indicated the BGS galaxies contained 7 AGNs and 24 starbursts. In a minority of 6 cases, however, the line diagnostics failed to unambiguously classify the luminosity source. Follow-up near-infrared imaging showed that the 'straddlers' have redder near-infrared colors than the other BGS galaxies. The optical spectra and near-infrared colors taken together with morphological peculiarities imply that the straddlers are composite objects harboring low-luminosity active nuclei residing within starburst disks. Finally, the optical spectrum of a high redshift IRAS galaxy was analysed. IRAS 18003+6652 is the third most distant (z=0.088) galaxy known to have been observed by IRAS and has an unusually strong emission line spectrum in combination with low-ionization absorption features.