We study the evidence for a diversity of formation processes in early-type galaxies by presenting the first complete volume-limited sample of slow rotators with both integral-field kinematics from the ATLAS ^ { 3 D } Project and high spatial resolution photometry from the Hubble Space Telescope .
Analysing the nuclear surface brightness profiles of 12 newly imaged slow rotators , we classify their light profiles as core-less , and place an upper limit to the core size of about 10 pc .
Considering the full magnitude and volume-limited ATLAS ^ { 3 D } sample , we correlate the presence or lack of cores with stellar kinematics , including the proxy for the stellar angular momentum ( \lambda _ { Re } ) and the velocity dispersion within one half-light radius ( \sigma _ { e } ) , stellar mass , stellar age , \alpha -element abundance , and age and metallicity gradients .
More than half of the slow rotators have core-less light profiles , and they are all less massive than 10 ^ { 11 } M _ { \odot } .
Core-less slow rotators show evidence for counter-rotating flattened structures , have steeper metallicity gradients , and a larger dispersion of gradient values ( \overline { \Delta [ Z / H ] } = -0.42 \pm 0.18 ) than core slow rotators ( \overline { \Delta [ Z / H ] } = -0.23 \pm 0.07 ) .
Our results suggest that core and core-less slow rotators have different assembly processes , where the former , as previously discussed , are the relics of massive dissipation-less merging in the presence of central supermassive black holes .
Formation processes of core-less slow rotators are consistent with accretion of counter-rotating gas or gas-rich mergers of special orbital configurations , which lower the final net angular momentum of stars , but support star formation .
We also highlight core fast rotators as galaxies that share properties of core slow rotators ( i.e .
cores , ages , \sigma _ { e } , and population gradients ) and core-less slow rotators ( i.e .
kinematics , \lambda _ { Re } , mass , and larger spread in population gradients ) .
Formation processes similar to those for core-less slow rotators can be invoked to explain the assembly of core fast rotators , with the distinction that these processes form or preserve cores .