Environments of Seyfert galaxies. I. Construction of the sample and selection effects.
LAURIKAINEN E., SALO H., TEERIKORPI P. and PETROV G.
Abstract (from CDS):
In order to build a satisfactory picture of Seyferts in relation to normal galaxies we have started a series of papers first establishing some of their basic environmental properties, especially the difference in the number of close companions between Seyfert 1 and Seyfert 2 galaxies. Here we report sample selections and discuss selection effects involved for the samples of 104 Seyferts and 138 control galaxies. The results of the statistical analyses are presented by Laurikainen & Salo (1994, Paper II). The neighbouring galaxies are counted on the Palomar Sky Survey Plates to the limiting magnitude ≃19mpg within the circles of 1.5Mpc in diameter (H0=100km/s/Mpc), large measuring circles enabling good elimination of the background galaxies. Subsamples are selected in order to test the methods by Dahari (1984) and by Fuentes-Williams & Stocke (1988). Elimination of background galaxies and problems related to comparison galaxy sample selection are discussed. The most important problem in our control galaxy sample selection is that the redshifts for most of the control galaxies are unknown. The size of this uncertainty is estimated by determining the redshifts by two ways: (1) by assuming that the selected comparison galaxy has the same redshift as the nearby Seyfert and (2) by Monte-Carlo simulations for Holmberg (1975) galaxy size distribution in space, taking into account the Malmquist bias. The mean redshifts of the comparison sample are estimated with these methods to be 0.028 and 0.029, respectively, compared with 0.026 for the Seyfert sample. Distribution of Seyferts in Zwicky's clusters is also addressed, and compared with previous studies. Taking into account optical projections, about 3/4 of the Seyferts are found to be field galaxies. Galaxies in clusters lie preferentially at the cluster borders.