Starburst or Seyfert? Using near-infrared spectroscopy to measure the activity in composite galaxies

Abstract

We present near-infrared spectra for a sample of galaxies with ambiguous optical emission line ratios. These galaxies fall between starbursts and Seyferts in the usual optical diagnostic diagrams. We find a similar result with the near-infrared emission line ratios, which suggests that the galaxies are composite, containing both a starburst and AGN component. Furthermore, CO absorption, produced in late-type stars, is detected within the sample, although at a weaker level than is typical for starburst galaxies. We conclude that the CO feature is being diluted by a contribution from an AGN, thereby confirming the composite nature of these galaxies.