2008A&A...477..105D

Shells in Ellipticals are peculiar faint sharp edged features that are thought to be formed by galaxy mergers. We determine the shell and dust distributions, and colours of a well-resolved shell and the underlying galaxy in NGC5982, and compare the spatial distributions of the dust and gas phases, in order to investigate the origin of shells and dust in NGC5982. We use Spitzer data in the wavelength range from 3.6 to 160µm and HST/ACS optical data. Surface photometry, galaxy models and residual images are determined using IRAF task ELLIPSE and GALPHOT task ELLIPFIT. After subtracting the galaxy models, residual images are used to identify the shells. Excess emission is identified by subtracting a scaled stellar photosphere emission template. Colours [3.6]-[4.5], [3.6]-[8.0] and [5.8]-[8.0] of the underlying galaxy and [5.8]-[8.0] of the excess emission are obtained using techniques similar to those of Pahre et al. (2004ApJS..154..229P). The [3.6]-[4.5] and V-I colours of a well-resolved shell are determined using techniques of Sikkema et al. Shells are visible in the 3.6µm image, fainter at 4.5µm and undetected at 5.8µm. We find two new shells, the outermost identified so far in NGC5982. All shells apart from shell 24 are on the photometric major axis. Shell 24 is the only one for which reliable colours are obtained. It has colours [3.6]-[4.5] and V-I bluer than the underlying galaxy. Excess emission at 4.5, 5.8 and 8µm is widely extended. The ratio of excess to total emission decreases towards the centre, which could be related to a lower mass loss rate from AGB stars. The [3.6]-[4.5] colour indicates a major contribution from late-type (K2-M0III) stars towards the centre. Dust traced by 24µm emission follows the stellar spatial distribution, supporting a circumnuclear origin. We find extended 160µm emission from cold dust, possibly forming a disk inclined to the principal axes, with a mass of a few 10⁵M_☉. The warmer big grains and/or smaller grains traced by the 70µm emission are confined to a small region elongated along the minor axis and shifted a few kpc from the centre. We detect for the first time shells from mid-infrared data. The very different distributions of dust, warm gas and HI gas together with the presence of shells and a kinematically decoupled core suggest a minor merger in NGC5982.