Shells and isophotal distortions in elliptical galaxies

Abstract

We describe an improved shell-finding algorithm and present images of eight previously unknown shell galaxies visible from the northern hemisphere. The data were obtained in the R-band using the 1-m Jacobus Kapteyn Telescope, located in the Canary Islands. Most of the galaxies were selected on the basis of independent evidence for a merger or interaction, such as extensive dust or kinematically decoupled cores. The detection of shells and other fine structures is achieved by subtracting a smooth elliptical model of the underlying galaxy and then inspecting the residual map. The isophotal distortions resulting from these structures are quantified using the fourth cosine term of the isophotal intensity. In particular, we suggest that shells may be responsible for boxy isophotes in some cases