Anti-Truncation of Disks in Early-Type Barred Galaxies

Abstract

The disks of spiral galaxies are commonly thought to be truncated: the radial surface brightness profile steepens sharply beyond a certain radius (3--5 inner-disk scale lengths). Here we present the radial brightness profiles of a number of barred S0--Sb galaxies with the opposite behavior: their outer profiles are distinctly shallower in slope than the main disk profile. We term these "anti-truncations"; they are found in at least 25% of a larger sample of barred S0--Sb galaxies. There are two distinct types of anti-truncations. About one-third show a fairly gradual transition and outer isophotes which are progressively rounder than the main disk isophotes, suggestive of a disk embedded within a more spheroidal outer zone -- either the outer extent of the bulge or a separate stellar halo. But the majority of the profiles have rather sharp surface-brightness transitions to the shallower, outer exponential profile and, crucially, outer isophotes which are not significantly rounder than the main disk; in the Sab--Sb galaxies, the outer isophotes include visible spiral arms. This suggests that the outer light is still part of the disk. A subset of these profiles are in galaxies with asymmetric outer isophotes (lopsided or one-armed spirals), suggesting that interactions may be responsible for at least some of the disklike anti-truncations.