Host galaxies and circumgalactic environment of "narrow line" Seyfert 1 nuclei

This paper presents a statistical study of the host galaxies and environmen t for a sample of 27 "narrow line" Seyfert 1 (NLSy1) nuclei, based on a com puter-aided search of companions in the Digitized Sky Survey. The sample in cludes all objects listed as NLSy1 in the catalog Veron-Cetty & Veron (8th edition) with redshift in the range 0.010 less than or equal to z less than or equal to 10.061 and Galactic latitude \b\ greater than or equal to 30 d egrees. 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 19 99 by Dultzin-Hacyan et al. The frequency of "physical" companions was meas ured within a search radius equal to 3 times the diameter D-S of each NLSy1 and up to a projected linear distance of approximate to 100 kpc. No statis tical 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 compani on 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 sim ilar to those of Seyfert 2 galaxies : whereas Seyfert 2 galaxies show a hig hly significant excess of bright companions, Seyfert 1's and NLSy1's do not . The morphological distribution of NLSy1's and Seyfert 1 objects is simila r, but NLSy1 nuclei are hosted in galaxies smaller than Seyfert 1 and nonac tive galaxies, to a statistical confidence level of 95%. These results supp ort the interpretation of NLSy1 nuclei (suggested by X-ray observations) as Seyfert 1 nuclei with higher Eddington ratio, since smaller diameter galax ies may host central black holes of lower mass.