Eso 138 g1 : a high excitation seyfert 2 nucleus in a low luminosity early-type galaxy

Abstract

We analyze in detail the spectral properties of the active nucleus ESQ 138 G1. We find that this nucleus lies at the heart of a low luminosity elliptical, making it a particularly interesting and rare object. We have been able to model the continuous energy distribution over the wavelength range λλ3400–9700, by an old stellar population which has reached at most a solar metallicity and contributes 60% of the flux at λ5870, together with a younger population at age less than 1 Gyr. After subtraction of the stellar population contribution, we have obtained the pure emission line spectrum from the active nucleus. It displays a wide range of ionization species, which for iron in particular goes from [Fe II] up to [Fe XI] and possibly [Fe XIV]. The main line ratios observed in ESO 138 G1 lead us to classify this active nucleus as a Seyfert 2 nucleus. A detailed analysis of the emission lines reveals that there exists in this object, a stratification of the emitting gas clouds according to their density and dominant ionization stage. The width of the lines is observed to correlate with their critical density. The N/O abundance ratio in the gas is about three times larger than the solar value; this result is consistent with the emitting gas being intrinsic material at the center of an elliptical galaxy. Compared to the other two active nuclei which are known so far to exhibit such a high excitation line spectrum, IIIZw77 and Tol 0109–383, ESO 138 G1 builds up a sequence of spectral properties which can be understood if the mean electron density and the excitation of the gas clouds decrease from IIIZw77 to ESQ 138 G1. As the spectral properties of the nucleus in IIIZw77 appear to bridge it with radiogalaxies, we might be seeing, along the sequence, ESO 138 G1→Tol 0109–383→IIIZw77→radiogalaxies, an increasing efficiency of matter accretion in systems with increasing mass, from dwarf to giant galaxies.