Host Galaxies and Circumgalactic Environment of “Narrow Line” Seyfert 1 Nuclei

Abstract

This paper presents a statistical study of the host galaxies and environment for a sample of 27 narrow line Seyfert 1 (NLSy1) nuclei, based on a computer-aided search of companions in the Digitized Sky Survey. The sample includes all objects listed as NLSy1 in the Veron-Cetty & Veron catalog (8th edition) with redshift in the range 0.010 ≤ z ≤ 0.061 and Galactic latitude |b| ≥ 30°. We compared the environments of NLSy1's with those of Seyfert 1 and Seyfert 2 galaxies using the method and the Seyfert samples employed in 1999 by Dultzin-Hacyan et al. The frequency of physical companions was measured within a search radius equal to 3 times the diameter DS of each NLSy1 and up to a projected linear distance of ≈100 kpc. No statistical difference was found between the environments of NLSy1's and those of Seyfert 1 galaxies. Our results further suggest that NLSy1 hosts may show fewer companions and may be preferentially farther away from bright companion galaxies than normal galaxies with similar morphological type, diameter, and redshift distribution. The results for host galaxies and environments help to disprove the hypothesis that NLSy1 objects have characteristics similar to those of Seyfert 2 galaxies: whereas Seyfert 2 galaxies show a highly significant excess of bright companions, Seyfert 1's and NLSy1's do not. The morphological distribution of NLSy1's and Seyfert 1 objects is similar, but NLSy1 nuclei are hosted in galaxies smaller than Seyfert 1 and nonactive galaxies, to a statistical confidence level of 95%. These results support the interpretation of NLSy1 nuclei (suggested by X-ray observations) as Seyfert 1 nuclei with higher Eddington ratio, since smaller diameter galaxies may host central black holes of lower mass.