Spatial distribution of dust in the shell elliptical NGC 5982

¹ School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin 2, Ireland e-mail: cburgo@cp.dias.ie
² Astrophysics Research Institute, Liverpool John Moores University, 12 Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 22 May 2007
Accepted: 8 October 2007

Abstract

Aims.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 NGC 5982, and compare the spatial distributions of the dust and gas phases, in order to investigate the origin of shells and dust in NGC 5982.

Methods.We use Spitzer data in the wavelength range from 3.6 to 160 μm 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 , and of the underlying galaxy and of the excess emission are obtained using techniques similar to those of Pahre et al. The and colours of a well-resolved shell are determined using techniques of Sikkema et al.

Results.Shells are visible in the 3.6 μm image, fainter at 4.5 μm and undetected at 5.8 μm. We find two new shells, the outermost identified so far in NGC 5982. 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 and bluer than the underlying galaxy. Excess emission at 4.5, 5.8 and 8 μm 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 colour indicates a major contribution from late-type (K2-M0III) stars towards the centre. Dust traced by 24 μm emission follows the stellar spatial distribution, supporting a circumnuclear origin. We find extended 160 μm emission from cold dust, possibly forming a disk inclined to the principal axes, with a mass of a few . The warmer big grains and/or smaller grains traced by the 70 μm emission are confined to a small region elongated along the minor axis and shifted a few kpc from the centre.

Conclusions.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 NGC 5982.