Stellar populations from H-band VLT spectroscopy in a sample of seven active galaxies

TL;DR

This study uses H-band VLT spectroscopy to analyze stellar populations in seven active galaxies, revealing diverse star formation histories and linking stellar properties to nuclear activity.

Contribution

It provides new insights into the stellar ages and metallicities of AGN host galaxies using infrared spectral synthesis, a novel approach in this context.

Findings

Three galaxies show recent star formation within 100 pc.

No star formation detected in three Seyfert 2 galaxies.

Inner nucleus star formation history is highly heterogeneous.

Abstract

The relationship between an active galactic nucleus (AGN) and its host galaxy is still far from being understood. Properties of the host galaxies of Seyfert nuclei, such as luminosity concentration, morphological type, metallicity, and age of the stellar population are expected to be related with nuclear activity -- either at the epoch of galaxy formation or in the present days via feeding of the central black-hole. In this paper we investigate whether stellar ages and metallicities are linked to the activity within the nucleus in a sample of AGN of various types. Our sample includes seven AGN, from Seyfert 1 to LINERs, observed with VLT/ISAAC and VLT/SINFONI. Based on an inverse method using a stellar library, we analyse H band infrared spectra, in a wavelength region devoid of emission lines, at a spectral resolution of about 3000, in the central few 100 pc. HST images are used to visualise the regions defined in each galaxy. For each galaxy, we give the results of the spectral synthesis, in particular the percentages of the stellar, power law and blackbody continua, and the percentages of various stellar types that account for the stellar lines. Out of the seven galaxies, three show strong and recent star formation in the inner 100 pc, while no star formation is detected in the three genuine Seyfert 2 galaxies. Beyond a radius of 100 pc, all show more or less recent star formation. Moreover we can conclude that the star formation history of the inner nucleus is highly heterogeneous.