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 del(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(circle dot) released in SDSS DR15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2D del(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 del M-*/L-r similar to -0.1 (i.e. the M-*/L-r is larger at the centre) for massive galaxies, becomes flat around M-* similar to 10(10)M(circle dot) and change sign to del M-*/L-r similar to 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-disc 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 del(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 versus 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.

Automated summary

The study reveals that the gradient of stellar mass-to-light ratio varies with galaxy mass and is heavily influenced by stellar age. Galaxies with higher specific star formation rates show steeper negative del(M-*/L-r) gradients.