Long-Slit Optical Spectroscopy of Powerful Far-Infrared Galaxies: The Nature of the Nuclear Energy Source

December 1989 • 1989ApJ...347..727A

Authors • Armus, Lee • Heckman, Timothy M. • Miley, George K.

Abstract • Optical spectroscopic data are presented for a sample of 47 powerful far-infrared galaxies (FIRGs) chosen with regard to IR spectral shape, and for six other IR-bright galaxies. The stellar absorption lines expected from a population of old stars (e.g., the Mg I "b" band) are generally very weak in the nuclei (< 1 kpc) of these IR color-selected galaxies. In the cases for which we have adequate signal-to-noise ratio, we still find very weak Mg I absorption in regions well off the nucleus, implying that the visible spectrum is dominated by young stars and not by an AGN. At least one, and probably five, of the galaxies have detectable Wolf-Rayet emission features providing additional evidence for a young stellar population. About 20% of the galaxies have strong Balmer absorption lines, indicating the presence of a substantial intermediate-age (10^8^-10^9^ yr) stellar population. The equivalent width of the Hα emission line can be modeled as arising from a mixture of a large young population, and an intermediate-age population of stars. Several pieces of evidence demonstrate that the spectra are usually observed through large column densities of cool gas and dust (steep Balmer decrements, strong Na D interstellar absorption lines, heavily reddened continua). The extinction-corrected optical continuum suggests that the young stellar population can typically provide at least 25% of the energy needed to power the far-infrared emission. We find that the emission-line spectra scatter between the loci of giant H II regions and LINERs in standard two-dimensional line ratio diagrams published by Baldwin, Phillips, and Terlevich in 1981. Examination of the spectrum of M82, the closest member of the sample, suggests that the primary mechanism for ionizing the emission-line gas changes from hot stars to shocks as the distance from the nucleus increases. This might explain the intermediate location of the more distant galaxies in the line ratio diagrams.

Links

View on ADS

IPAC Authors
(alphabetical)

Lee Armus

Senior Scientist