Morphology of 15 southern early-type disk galaxies

Abstract

Structural analysis has been performed for a sample of 15 southern early-type disk galaxies, mainly S0 galaxies, using high-resolution K(s)-band images. The galaxies are mostly barred, and many of them show multiple structures including bars and ovals, typical for S0 galaxies. The new images are of sufficient quality to reveal new detail of the morphology of the galaxies. For example, we report a hitherto undetected nuclear ring in NGC 1387, a nuclear bar in NGC 1326, and in the residual image a weak primary bar in NGC 1317. For the galaxies we (1) measure the radial profiles of the orientation parameters derived from the elliptical isophotes, (2) apply Fourier methods for calculating tangential forces, and, in particular, ( 3) apply structural decomposition methods. For galaxies with multiple structures a two-dimensional method is found to be superior to a one-dimensional method but only if in addition to the bulge and the disk at least one other component is taken into account. We find strong evidence of pseudobulges in S0 galaxies: 10 of the galaxies have a shape parameter of the bulge near n = 2, indicating that the bulges are more disklike than following the R(1/4) law. Also, six of the galaxies have either nuclear rings, nuclear bars, or nuclear disks. In all non-elliptical galaxies in our sample the bulge-to-total (B/T) flux ratio is less than 0.4, as is typically found in galaxies having pseudobulges. In two of the galaxies the B/T flux ratio is as small as in typical Sc-type spiral galaxies. This might be the hitherto undiscovered link in the scenario in which spiral galaxies are transformed into S0 galaxies. Also, bars in S0 systems are found to be shorter and less massive and to have smaller bar torques than bars in S0/a-type galaxies.