Galaxy properties as revealed by MaNGA - I. Constraints on IMF and M*/L gradients in ellipticals

We estimate ages, metallicities, α-element abundance ratios, and stellar initial mass functions (IMFs) of elliptical (E) and S0 galaxies from the MaNGA-DR15 survey. We stack spectra and use a variety of single stellar population synthesis models to interpret the absorption line strengths in these spectra. We quantify how these properties vary across the population, as well as with galactocentric distance. This paper is the first of a series and is based on a sample of pure elliptical galaxies at z ≤ 0.08. We confirm previous work showing that IMFs in Es with the largest luminosity (L_r) and central velocity dispersion (σ₀) appear to be increasingly bottom heavy towards their centres. For these galaxies the stellar mass-to-light ratio decreases at most by a factor of 2 from the central regions to R_e. In contrast, for lower L_r and σ₀ galaxies, the IMF is shallower and M_*/L_r in the central regions is similar to the outskirts, although quantitative estimates depend on assumptions about element abundance gradients. Accounting self-consistently for these gradients when estimating both M_* and M_dyn brings the two into good agreement: gradients reduce M_dyn by ∼0.2 dex while only slightly increasing the M_* inferred using a Kroupa IMF. This is a different resolution of the M_*-M_dyn discrepancy than has been followed in the recent literature where M_* of massive galaxies is increased by adopting a Salpeter IMF throughout the galaxy while leaving M_dyn unchanged. A companion paper discusses how stellar population differences are even more pronounced if one separates slow from fast rotators.