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On the behaviour of the IR CA II triplet in normal and active galaxies

1990; Oxford University Press; Volume: 242; Issue: 3 Linguagem: Inglês

10.1093/mnras/242.3.271

1365-2966

R. Terlevich, Á. I. Díaz, R. Terlevich,

Galaxies: Formation, Evolution, Phenomena

Ca II triplet in absorption at |$\lambda \lambda 8498,8542,8662$| Å is the strongest feature in the infrared spectrum of late-type stars and normal galaxies. Its strength has been found to be a good luminosity indicator for metal-rich stellar populations. We present high signal-to-noise near-IR spectroscopic data for the nuclear region of 42 normal and active galaxies. We have explored the behaviour of the Ca II triplet strength and found that it shows a small spread around a mean value of 7 Å for our sample of normal galaxies. We also found that, in all the Seyfert type 2 galaxies measured and even in some Seyfert type 1, while the optical stellar features show substantial dilution, the strength of the IR Ca II triplet is equal to and in some cases larger than that in normal elliptical galaxies. This result is most naturally explained by the presence of young stars dominating, or at least contributing heavily to, the unresolved nuclear light at near-IR wavelengths. These findings constitute strong evidence for a stellar population substantially younger than that observed in normal early-type galaxies and give support to the starburst scenario for nuclear activity. The large equivalent width of the IR Ca II triplet in Seyfert nuclei provides a unique method of probing the gravitational potential close to the central engine. This method is relatively insensitive to the presence of ionized gas in the nuclear regions. Analysis with a cross-correlation technique has produced nuclear velocity dispersions accurate to ~ 10 per cent. We find that nuclear velocity dispersions in 'active' galaxies are comparable to those in normal ones of similar luminosity. We compare the width of the [O III] and [S III;] forbidden lines with the stellar velocity dispersion and find that Seyfert type 2 galaxies and NGC 1052 show an additional broadening affecting the forbidden lines that is not seen in our starbursts or Seyfert type 1.