Activity of the Seyfert galaxy neighbours^⋆,⋆⋆

¹ Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli 152 36, Athens, Greece
e-mail: ekoulour@noa.gr
² Physics Department of Aristotle, University of Thessaloniki, University Campus, 54124 Thessaloniki, Greece
³ Instituto Nacional de Astrofísica Optica y Electrónica, Puebla, C.P. 72840 México, Mexico
⁴ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, D. F. 04510 México, Mexico
⁵ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁶ Aalto University, Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland

Received: 15 May 2012
Accepted: 23 January 2013

Abstract

We present a follow-up study of a series of papers concerning the role of close interactions as a possible triggering mechanism of AGN activity. We have already studied the close (≤100 h^-1 kpc) and the large-scale (≤1 h^-1 Mpc) environment of a local sample of Sy1, Sy2, and bright IRAS galaxies (BIRG) and of their respective control samples. The results led us to the conclusion that a close encounter appears capable of activating a sequence where an absorption line galaxy (ALG) galaxy first becomes a starburst, then a Sy2, and finally a Sy1. Here we investigate the activity of neighbouring galaxies of different types of AGN, since both galaxies of an interacting pair should be affected. To this end we present the optical spectroscopy and X-ray imaging of 30 neighbouring galaxies around two local (z ≲ 0.034) samples of 10 Sy1 and 13 Sy2 galaxies. Although this is a pilot study of a small sample, various interesting trends have been discovered that imply physical mechanisms that may lead to different Seyfert types. Based on the optical spectroscopy, we find that more than 70% of all neighbouring galaxies exhibit star forming and/or nuclear activity (namely recent star formation and/or AGN), while an additional X-ray analysis showed that this percentage might be significantly higher. Furthermore, we find a statistically significant correlation, at a 99.9% level, between the value of the neighbour’s [OIII]/Hβ ratio and the activity type of the central active galaxy, i.e. the neighbours of Sy2 galaxies are systematically more ionized than the neighbours of Sy1s. This result, in combination with trends found using the Equivalent Width of the Hα emission line and the stellar population synthesis code STARLIGHT, indicate differences in the stellar mass, metallicity, and star formation history between the samples. Our results point towards a link between close galaxy interactions and activity and also provide more clues regarding the possible evolutionary sequence inferred by our previous studies.