2023ApJ...944...75M

We use the DR9 of the DESI legacy imaging survey and SDSS galaxy groups to measure the conditional luminosity function (CLF) for groups with halo mass M_h ≥ 10¹² M_⊙ and redshift 0.01 ≤ z ≤ 0.08, down to a limiting r-band magnitude of M_r = -10 to -12. For given halo masses we measure the CLF for the total populations and for the red and blue populations classified using the (g - z) color. We find a clear faint-end upturn in the CLF of red satellites, with a slope α ≈ -1.8, which is almost independent of halo mass. This faint-end upturn is not seen for the blue and total populations. Our stellar population synthesis modeling shows that (g - z) provides a clean red/blue division and that red group galaxies defined by (g - z) are all dominated by old stellar populations. The fraction of old galaxies as a function of galaxy luminosity shows a minimum at M_r ∼ -18, corresponding to M_* ∼ 10^9.5 M_⊙. This scale is independent of halo mass and is comparable to the characteristic luminosity at which galaxies show a dichotomy in surface brightness and size, suggesting that the dichotomy in the old fraction and in galaxy structure may have a common origin. The rising of the old fraction at the faint end for Milky Way (MW)-sized halos is in good agreement with the quenched fraction measured for the MW/M31 system and from the ELVES survey. We discuss the implications of our results for the formation and evolution of low-mass galaxies and for the stellar mass functions of low-mass galaxies to be observed at high redshift.