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 $\nabla (m_*/l_r)$ 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 $10^{12}m_\odot$ released in sdss dr15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2d $\nabla (m_*/l_r)$ 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 $\nabla m_*/l_r\sim -0.1$ (i.e., the $m_*/l_r$ is larger at the centre) for massive galaxies, becomes flat around $m_*\sim 10^{10} m_{\odot}$ and change sign to $\nabla m_*/l_r\sim 0.1$ 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 $10^{-3} {\rm gyr}^{-1}$. with increased ssfr gradient, one can obtain a steeper negative $\nabla (m_*/l_r)$. 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.