A MaNGA about the Legacy. I. Connecting the Assembly of Stellar Halo with the Average Star Formation History in Low-redshift Massive Galaxies

We investigate the connection between stellar mass distribution, assembly history, and star formation timescales in low-redshift massive early-type galaxies (ETGs) by combining deep LegacySurvey imaging with MaNGA’s spatially resolved spectroscopy. Focusing on stellar population properties, especially the [Mg/Fe] abundance ratio, we analyze stacked spectra using both absorption line indices and full-spectrum fitting. We find that, among massive ETGs with identical average stellar mass distributions beyond 20 kpc, those with higher central velocity dispersion ( σ ⋆,cen ) are older and more α -enhanced, suggesting a connection between the in situ star formation in the past and the central gravitational potential today for massive ETGs with a similar stellar accretion history. Conversely, at fixed σ ⋆,cen and total stellar mass, galaxies with more extended stellar halos show lower [Fe/H], higher [Mg/Fe], and older ages, indicating an intriguing link between early starburst and quenching and later ex situ assembly. These results demonstrate that the evolution of massive galaxies cannot be fully described by simple scaling relations alone, as the interplay between in situ star formation and ex situ accretion leaves distinct imprints in both their inner and outer stellar populations. Our findings highlight the importance of extending stellar population studies to large radii and underscore the scientific potential of next-generation integral-field-unit surveys and deep, high-resolution spectroscopy for probing the galaxy–halo connection.