SDSS-IV MaNGA: How the stellar populations of passive central galaxies depend on stellar and halo mass

We analyze spatially resolved and co-added SDSS-IV MaNGA spectra with signal-to-noise ratio ∼100 from 2200 passive central galaxies (z ∼ 0.05) to understand how central galaxy assembly depends on stellar mass (M_*) and halo mass (M_h). We control for systematic errors in M_h by employing a new group catalog from Tinker and the widely used Yang et al. catalog. At fixed M_*, the strengths of several stellar absorption features vary systematically with M_h. 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 3.5σ 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 M_h , high-M_* centrals are older, show lower [Fe/H], and have higher [Mg/Fe] for M_h > 10¹² h⁻¹ M⊙ with confidence > 4σ. While massive passive galaxies are thought to form early and rapidly, our results are among the first to distinguish these trends at fixed M_h . 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.