Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies

摘要

To understand the nuclear stellar populations and star formation histories ofthe nuclei of spiral galaxies, we have obtained K-band nuclear spectra for 41galaxies and H-band spectra for 20 galaxies in the ISO Atlas of Bright SpiralGalaxies. In the vast majority of the subsample (80%), the near-infraredspectra suggest that evolved red stars completely dominate the nuclear stellarpopulations and that hot young stars are virtually non-existent. The signaturesof recent star formation activity are only found in 20% of the subsample, eventhough older red stars still dominate the stellar populations in thesegalaxies. Given the dominance of evolved stars in most galaxy nuclei and thenature of the emission lines in the galaxies where they were detected, wesuggest that nuclear star formation proceeds in the form of instantaneousbursts. The stars produced by these bursts comprise only ~2% of the totalnuclear stellar mass in these galaxies, but we demonstrate how the nuclearstellar populations of normal spiral galaxies can be built up through a seriesof these bursts. The bursts were detected only in Sbc galaxies and later, andboth bars and interactions appeared to be sufficient but not necessary triggersfor the nuclear star formation activity. The vast majority of galaxies withnuclear star formation were classified as HII galaxies. With one exception,LINERs and transition objects were dominated by older red stars, whichsuggested that star formation was not responsible for generating thesegalaxies' optical line emission.