THE NUCLEAR-ENERGY SOURCES POWERING BRIGHT INFRARED-SELECTED GALAXIES

We present the results of a multiwavelength investigation of energy ge neration mechanisms in 33 objects from the IRAS Bright Galaxy Sample. This study is based on long-slit optical spectroscopy and near-infrare d broadband imaging at J, H, and K. We used established emission-line criteria, involving [O m] lambda 5007/H beta, [N II] lambda 6583/H alp ha, [S II] (lambda 6716 + lambda 6731)/H alpha, and [O I] lambda 6300/ H alpha intensity ratios, to classify the optical spectra of all 36 sa mple nuclei (three sources were binary systems). The line ratios indic ate that our sample contains seven active galactic nuclei (AGNs) and 2 4 starburst galaxies. We also find that five galaxies (straddlers) can not be straightforwardly classified by this simple scheme. Follow-up n ear-infrared imaging of the straddler galaxies yields near-infrared co lors consistent with emission from relatively weak AGNs. Considered to gether with the fact that their narrow optical emission-line ratios re semble those of NGC 7469, which is known to contain an unresolved AGN inside an H II legion-like galaxy disk or torus, the evidence seems to indicate that NGC 7469 may be typical of a pattern exhibited by infra red luminous galaxies, i.e., that low-luminosity AGNs and starbursts o ften coexist in infrared-selected galaxies.