We analyze spatially resolved and co-added SDSS-IV MaNGA spectra with signal-to-noise ratio �1/4100 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.

The Sloan Digital Sky Survey IV extended Baryonic Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) will observe 195 000 emission-line galaxies (ELGs) to measure the baryonic acoustic oscillation (BAO) standard ruler at redshift 0.9. To test different ELG selection algorithms, 9000 spectra were observed with the SDSS spectrograph as a pilot survey based on data from several imaging surveys. First, using visual inspection and redshift quality flags, we show that the automated spectroscopic redshifts assigned by the pipeline meet the quality requirements for a reliable BAO measurement. We also show the correlations between sky emission, signal-to-noise ratio in the emission lines, and redshift error. Then we provide a detailed description of each target selection algorithm we tested and compare them with the requirements of the eBOSS experiment. As a result, we provide reliable redshift distributions for the different target selection schemes we tested. Finally, we determine an target selection algorithms that is best suited to be applied on DECam photometry because they fulfill the eBOSS survey efficiency requirements.