Is the taxonomy of early-type galaxies more complicated than we thought?

Early-type galaxies (ETGs) are often classified into two distinct groups based on their photometry and kinematics. More luminous ETGs are expected to have boxy-shaped isophotes and show a slow or no rotation pattern. On the other hand, fast rotator ETGs are fainter and disky-shaped. Although this dichotomy leads to enormous simplifications in the description of the galaxy assembly process, this idea is not a consensus, and there is still an intense debate in the literature.

To go one step further in this discussion, we analyzed a large sample of almost two thousand ETGs classified via deep learning in the MaNGA survey or visual inspection by the Galaxy Zoo DECaLS. Next, we extracted a diverse set of parameters, including chemical composition, isophote shape, stellar rotation pattern, and velocity dispersion, to which we applied the principal component analysis to check whether the ETGs show any statistical distinction in the multi-parameter space.

Although some parameters showed a mild correlation (for example, those already mentioned, isophotal shape, and stellar kinematics), our analysis found no evidence of a dichotomy since the galaxies' properties vary smoothly in the parameter space and do not present any discontinuity. This suggests that the formation process of early-type galaxies may be more complex than previously assumed.