SDSS-IV MaNGA: Stellar M/L gradients and the M/L-colour relation in galaxies

The stellar mass-to-light ratio gradient in SDSS r−band ∇(M ∗ /L r ) of a galaxy depends on its mass assembly history, which is imprinted in its morphology and gradients of age, metallicity, and stellar initial mass function (IMF). Taking a MaNGA sample of 2051 galaxies with stellar masses ranging from 10 9 to 10 12 M released in SDSS DR15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2D ∇(M ∗ /L r ) on other galaxy properties, including M ∗ /L r -colour relationships by assuming a fixed Salpeter IMF as the mass normalization reference. The median gradient is ∇M ∗ /L r ∼ −0.1 (i.e., the M ∗ /L r is larger in the centre) for massive galaxies, becomes flat around M ∗ ∼ 10 10 M and change sign to ∇M ∗ /L r ∼ 0.1 at the lowest masses. The M ∗ /L r inside a half light radius increases with increasing galaxy stellar mass; in each mass bin, early-type galaxies have the highest value, while pure-disk late-type galaxies have the smallest. Correlation analyses suggest that the mass-weighted stellar age is the dominant parameter influencing the M ∗ /L r profile, since a luminosity-weighted age is easily affected by star formation when the specific star formation rate (sSFR) inside the half light radius is higher than 10 −3 Gyr −1 . With increased sSFR gradient, one can obtain a steeper negative ∇(M ∗ /L r ). The scatter in the slopes of M ∗ /L-colour relations increases with increasing sSFR, for example, the slope for post-starburst galaxies can be flattened to 0.45 from the global value 0.87 in the M ∗ /L vs. g − r diagram. Hence converting galaxy colours to M ∗ /L should be done carefully, especially for those galaxies with young luminosity-weighted stellar ages, which can have quite different star formation histories.