The formation sequence of S0 galaxies

The opposite is true for lower mass galaxies. We conclude that we see two separate populations of lenticular galaxies. The old, massive, and metal-rich population possess bulges that are predominantly older than their discs, which we speculate may have been caused by morphological or inside-out quenching.

In contrast, the less massive and more metal-poor population have bulges with more recent star formation than their discs. We postulate they may be undergoing bulge rejuvenation (or disc fading), or compaction. Environment does not play a distinct role in the properties of either population.

Our findings give weight to the notion that while the faded spiral scenario likely formed low-mass S0s, other processes, such as mergers, may be responsible for high-mass S0s.