The CaT strength in Seyfert nuclei revisited: analyzing young stars and non-stellar light contributions to the spectra

TL;DR

This study revisits the CaT spectral feature in Seyfert nuclei, analyzing the influence of young stars and non-stellar light on spectral measurements, and confirms the connection between stellar and gas kinematics in active galaxies.

Contribution

It provides new insights into the sources of spectral dilution in Seyfert galaxies and confirms the correlation between stellar and gas velocity dispersions using near-IR spectral lines.

Findings

Seyfert 2s show no CaT dilution despite weak optical lines.

Optical line dilution in Seyfert 2s is due to young stars, not AGN continuum.

In Seyfert 1s, spectral dilution is caused by non-stellar components.

Abstract

In a former paper (Garcia-Rissmann et al. 2005; hereafter Paper I), we have presented spectra of 64 active, 9 normal and 5 Starburst galaxies in the region around the near-IR Calcium triplet absorption lines and the [SIII]9069 line. In the present paper we analyze the CaT strength (WCaT), and kinematical products derived in that study, namely stellar and ionized gas velocity dispersions. Our main results may be summarized as follows: (1) Seyfert 2s show no sign of dilution in WCaT with respect to the values spanned by normal galaxies, even when optical absorption lines such as the CaII K band at 3933 A are much weaker than in old, bulge-like stellar populations. (2) The location of Seyfert 2s in the WCaT-WCaK plane is consistent with evolutionary synthesis models. The implication is that the source responsible for the dilution of optical lines in these AGN is a young stellar population, rather than an AGN featureless continuum, confirming the conclusion of the pioneer study of Terlevich, Diaz & Terlevich. (3) In Seyfert 1s, both W[SIII] and WCaT tend to be diluted due to the presence of a non-stellar component, in agreement with the unification paradigm. (4) A comparison of stellar and gas velocity dispersions confirms the existence of a correlation between the typical velocities of stars and clouds of the Narrow Line Region. The strength and scatter around this correlation are similar to those previously obtained from the [OIII]5007 line width.