The half mass radius of MaNGA galaxies: Effect of IMF gradients

Abstract

Gradients in the stellar populations (SP) of galaxies -- e.g., in age, metallicity, stellar Initial Mass Function (IMF) -- can result in gradients in the stellar mass to light ratio, M*/L. Such gradients imply that the distribution of the stellar mass and light are different. For old SPs, e.g., in early-type galaxies at z 0, the M*/L gradients are weak if driven by variations in age and metallicity, but significantly larger if driven by the IMF. A gradient which has larger M*/L in the center increases the estimated total stellar mass (M*) and reduces the scale which contains half this mass (Re,*), compared to when the gradient is ignored. For the IMF gradients inferred from fitting MILES simple SP models to the Hβ, , [MgFe] and TiO 2SDSS absorption lines measured in spatially resolved spectra of early-type galaxies in the MaNGA survey, the fractional change in Re,* can be significantly larger than that in M*, especially when the light is more centrally concentrated. The Re,*-M* correlation which results is offset by 0.3 dex to smaller sizes compared to when these gradients are ignored. Comparisons with `quiescent' galaxies at higher-z must account for evolution in SP gradients (especially age and IMF) and the light profile before drawing conclusions about how Re,* and M* evolve. The implied merging between higher-z and the present is less contrived if Re,*/Re at z 0 is closer to our IMF-driven gradient calibration than to unity.