The SLUGGS survey: Trails of SLUGGS galaxies in a modified spin-ellipticity diagram

We present radial tracks for four early-type galaxies with embedded intermediate-scale discs in a modified spin-ellipticity diagram. Here, each galaxy's spin and ellipticity profiles are shown as a radial track, as opposed to a single, flux-weighted aperture-dependent value as is common in the literature. The use of a single ellipticity and spin parameter is inadequate to capture the basic nature of these galaxies, which transition from fast to slow rotation as one moves to larger radii where the disc ceases to dominate. After peaking, the four galaxy's radial tracks feature a downturn in both ellipticity and spin with increasing radius, differentiating them from elliptical galaxies, and from lenticular galaxies whose discs dominate at large radii. These galaxies are examples of so-called discy elliptical galaxies, which are a morphological hybrid between elliptical (E) and lenticular (S0) galaxies and have been designated ES galaxies. The use of spin-ellipticity tracks provides extra structural information about individual galaxies over a single aperture measure. Such tracks provide a key diagnostic for classifying early-type galaxies, particularly in the era of 2D kinematic (and photometric) data beyond one effective radius.