  We analyze spatially resolved and co-added SDSS-IV MaNGA spectra with
signal-to-noise ~100 from 2200 passive central galaxies (z~0.05) to understand
how central galaxy assembly depends on stellar mass (M*) and halo mass (Mh). We
control for systematic errors in Mh by employing a new group catalog from
Tinker (2020a,b) and the widely-used Yang et al. (2007) catalog. At fixed M*,
the strength of several stellar absorption features varies systematically with
Mh. Completely model-free, this is one of the first indications that the
stellar populations of centrals with identical M* are affected by the
properties of their host halos. To interpret these variations, we applied full
spectral fitting with the code alf. At fixed M*, centrals in more massive halos
are older, show lower [Fe/H], and have higher [Mg/Fe] with 3.5 sigma
confidence. We conclude that halos not only dictate how much M* galaxies
assemble, but also modulate their chemical enrichment histories. Turning to our
analysis at fixed Mh, high-M* centrals are older, show lower [Fe/H], and have
higher [Mg/Fe] for Mh>10^{12}Msun/h with confidence > 4 sigma. While massive
passive galaxies are thought to form early and rapidly, our results are among
the first to distinguish these trends at fixed Mh. They suggest that high-M*
centrals experienced unique early formation histories, either through enhanced
collapse and gas fueling, or because their halos were early-forming and highly
concentrated, a possible signal of galaxy assembly bias.
