Li-Wen Liao (NTHU); Andrew Cooper (NTHU)
Color gradients in galaxies trace radial variations of age and metallicity, and may help to distinguish between different models for how and when their stellar mass was assembled. Previous work on the correlation of color gradients with average color and magnitude has focused on early-type galaxies and dense environments. Only a few studies have discussed the color gradient in spiral galaxies. To improve our understanding of color gradients across the bulk of the galaxy population, we use photometry from DR8 of the DESI Legacy Imaging Survey (LS), which reaches a depth of r ~ 23.5 over ~19,700 deg2. Here we present a study of g-r and r-z color gradients and their correlations with other galaxy properties for 158,000 bright LS galaxies with SDSS spectra and 1.2 million galaxies with LS photometric redshift. We found that most are redder at the center (negative color gradient), and there is a clear trend for galaxies with redder average g-r colors to have relatively redder cores (steeper gradients). The same trend is seen with increasing absolute magnitude (and stellar mass), but reverses at Mr >~ -21 such that the brightest galaxies have shallower gradients. The correlations between color gradients and these observables indicate that these data could improve constraints on models of galaxy evolution. We also compare our results to the predictions of the Illustris TNG-100 simulation and show that Illustris fails to reproduce the trends we observe.