a: total stellar halo mass fraction as a function of total galaxy stellar mass. b: stellar halo metallicity as a function of the total stellar mass of the galaxy. Observed galaxies are marked by symbols with error bars: six GHOSTS galaxies (red circles), the MW and M 31 (yellow circles), and M 101 in this study (starlet symbol). Shaded regions indicate model predictions in the literature: Bullock & Johnston (2005, red area), Purcell et al. (2007, 2008, light green area with a green line), Cooper et al. (2010, blue circles), Cooper et al. (2013, pink area), Deason et al. (2016, light yellow area).