The close environment of Seyfert galaxies and its implication for unification models

Dultzin-Hacyan, D., Krongold, Y., Fuentes-Guridi, I. and Marziani, P. (1999) The close environment of Seyfert galaxies and its implication for unification models. Astrophysical Journal, 513 (2), L111-L114. (doi:10.1086/311925).

Abstract

In this Letter, we present a statistical analysis of the circumgalactic environment of nearby Seyfert galaxies based on a computer-aided search of companion galaxies on the Digitized Sky Survey. We defined a sample of 72 nearby Seyfert 1 galaxies (redshift 0.007 ≤ z ≤ 0.034) and a sample of 60 Seyfert 2 galaxies (0.007 ≤ z ≤ 0.020), which include only high galactic latitude objects. In addition, we built two control samples of nonactive galaxies that match the number of sample members, the redshift, morphological type, and diameter distribution of the Seyfert 1 and Seyfert 2 galaxy samples separately. We stress how our sample selection introduces important methodological improvements that avoid several sources of strong bias. An intrinsic difference between the environment of Seyfert 1 galaxies and that of Seyfert 2 galaxies, suggested by previous work, is confirmed as statistically significant. For Seyfert 2 galaxies, we find a significant excess of large companions (D_C ≳ 10 kpc) within a search radius ≲100 kpc of projected linear distance as well as within a search radius equal to 3 times the diameter D_S of each Seyfert galaxy. For Seyfert 1 galaxies, there is no clear evidence of any excess of companion galaxies either within 100 kpc or within 3D_S. For all samples, the number of companions that are actually counted within a search radius of 3D_S is a factor of ≈2 above the expectation values derived from the number density of galaxies over the 1 deg² fields that are centered on the sample galaxies, suggesting a markedly non-Poissonian distribution for galaxies on scales ≲100 kpc. This difference in environment is not compatible with the simplest formulation of the unification model for Seyfert galaxies: both type 1 and type 2 should be intrinsically alike, the only difference being the result of the orientation of an obscuring torus. We propose an alternative formulation.

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