Noncircular Outer Disks in Unbarred S0 Galaxies: NGC 502 and NGC 5485

TL;DR

This study reveals strongly noncircular, triaxial stellar disks in two unbarred S0 galaxies, NGC 502 and NGC 5485, through spectroscopic and photometric analysis, suggesting complex formation histories involving minor mergers and intrinsic triaxiality.

Contribution

The paper provides the first evidence of noncircular outer disks in unbarred S0 galaxies, highlighting their triaxial structures and possible formation via minor mergers.

Findings

NGC 502 has elliptical rings indicating an oval distortion.

NGC 5485 exhibits a misalignment between kinematic and photometric axes.

Both galaxies show evidence of complex, non-axisymmetric structures.

Abstract

Strongly noncircular outer stellar disks have been found in two unbarred SA0 galaxies by analyzing spectroscopic data on the rotation of stars and photometric data on the shape and orientation of the isophotes. In NGC 502, the oval distortion of the disk is manifested as two elliptical rings, the inner and the outer ones, covering wide radial zones between the bulge and the disk and at the outer edge of the stellar disk. Such a structure may be a consequence of the so-called "dry" minor merger - multiple accretion of gas-free satellites. In NGC 5485, the kinematical major axis does not coincide with the orientation of isophotes in the disk-dominated region, and for this galaxy the conclusion about its global triaxial structure is unavoidable.