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 is 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 centre 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 H β, 〈Fe〉, [MgFe], and TiO2SDSS 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 from accounting for IMF gradients is offset to smaller sizes by 0.3 dex 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 in 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.