Nuclear activity in galaxy pairs: a spectroscopic analysis of 48 UZC-BGPs^*

¹ Dipartimento di Astronomia, Universitá di Bologna, Italy e-mail: paola.focardi@unibo.it
² INAF - OABO, Via Ranzani 1, 40127 Bologna, Italy
³ Fundacion Galileo Galilei & TNG, PO Box 565 S/C de la Palma, Tenerife, Spain

Received: 26 July 2007
Accepted: 8 March 2008

Abstract

Context. The role played by interaction on galaxy formation and evolution continues to be debated. Several questions remain open, among them whether, and to what extent, galaxy interaction induce nuclear activity, as theoretical predictions, so far, have not been adequately supported by observations. Part of the uncertainty affecting the observational results is likely to be due to the limited sizes and the inhomogeneity of the samples.

Aims. Galaxy pairs are ideal sites in which to investigate the role of interaction on nuclear activity, since the proximity, in redshift and in projected separation, between members make interaction and encounters highly probable. For this reason we have undertaken a spectroscopic survey of a large homogeneous sample of galaxy pairs (UZC-BGP) selected applying an objective neighbour search algorithm to a 3D galaxy catalog (UZC).

Methods. We present the results of the nuclear spectral classification, performed using standard diagnostic diagrams, of 48 UZC-BGPs, which represents more than half of the whole sample and has an excellent morphological match with it.

Results. The fraction of emission line galaxies in our pair sample is large, especially among spirals where it reaches 84% and 95%, for early and late spirals. Star Burst (SB) is the most frequent type of nuclear activity encountered (30% of galaxies), while AGNs (Active Galactic Nuclei) make only 19%. The fractions increase to 45% and 22% when considering only spirals. Late spirals are characterized by both an unusual increase (35%) of AGN activity and high luminosity (44% have M_B < -20.0 + 5 log h). LLAGNs (Low Luminosity AGNs) are only 8% of the total number of galaxies, but this kind of activity could be present in another 10% of the galaxies (LLAGN candidates). If confirmed, these candidates would make LLAGNs constitute a significant fraction of the whole AGN (LLAGN + AGN) population, and raise the AGN population as a whole to 37%. Absorption line galaxies reside mostly (61%) in S0 galaxies and display the lowest B luminosity in the sample; only 18% of them have M_B < -20 + 5 log h, but together with LLAGNs (candidates included) they are the most massive galaxies in the sample. Intense-SB nuclei are found in galaxy pairs with galaxy-galaxy projected separation of up to 160 h^-1 kpc suggesting that in bright isolated galaxy pairs interaction may be at work and effective up to that distance.

Conclusions. AGNs are characterized by an advanced morphological type while the SB phenomenon occurs with the same frequency in early and late spirals. Whether and how these unusual characteristics relate to the pair environment needs to be further investigated. LLAGNs and LLAGN candidates do not always show similar properties; the former are more luminous in B, richer in early-type (E-S0s) galaxies, and half of them are hosted in galaxies showing visible signs of interaction with fainter companions. This last finding suggests that minor interactions might be a driving mechanism for a fraction of LLAGNs. The differences between LLAGNs and LLAGN candidates might confirm the heterogeneous nature of this class of objects.