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tokens: magnitude is defined as QT = F814W 0 , where F555W − F814W) 0 for the ACS/WFC. We plotted QT magnitude versus (F555W–F814W) 0 diagrams of the resolved point sources in Figures 4 (e)–(h). Foreground reddenings are known to be very small: , 0.028, 0.035, and 0.007 for NGC 3021, NGC 3370, NGC 1309, and the HXDF, respectively (Schlafly Finkbeiner 2011 , NASA/IPAC Extragalactic Database). Internal reddenings of the selected outer regions are expected to be negligible so that these are assumed to be zero in this study. 3.2. Distance Estimation We determined the distances to the target galaxies using the TRGB method (Lee et al. 1993 ; Madore Freedman 1995 ). We selected resolved stars in the shaded regions of Figure 4 . The shaded regions for F814W – (F555W – F814W) CMDs were designed to sample blue RGB stars with near the TRGB level because the color (metallicity) dependence of the TRGB at the blue side is estimated to be small (Jang Lee 2017 ). In the cases of QT – (F555W – F814W) 0 CMDs, we set slightly wider shaded regions, sampling RGB stars with near the TRGB level. Figure 5 displays F814W (a)–(c) and QT (d)–(f) luminosity functions of the selected RGB stars in the CMDs. The luminosity functions for all three galaxies show abrupt increments at mag, which correspond to the TRGB. We applied an edge detection algorithm for the quantitative TRGB detection. We used a Sobel filter employing the zero-sum kernel of [−1, −2, −1, 0, +1, +2, +1] with a bin size of 0.05 mag. Output edge detection responses are shown as red lines in Figure 5 . Figure 5. (a)–(c) LFs (histograms) and edge detection responses (curved lines) of the RGB stars in NGC 3021 (a), NGC 3370 (b), and NGC 1309 (c). Estimated TRGB magnitudes are marked by vertical dashed lines in each panel. (d)–(f) Same as (a)–(c), except for the QT magnitude. Mean TRGB magnitudes and associated uncertainties were measured by running 10,000 simulations of bootstrap resampling as done for our previous studies (Lee Jang 2012 , 2013 ; Jang Lee 2015 ). In each simulation, we made a new sample of RGB stars by sampling a half size of stars randomly from the original RGB sample. We obtained TRGB magnitudes from the new samples as done for the original sample. We generated a histogram of the TRGB magnitudes obtained from the simulations and fitted a Gaussian function to the histogram, quoting the Gaussian mean as a mean TRGB magnitude and the Gaussian width as 1 σ uncertainty. Measured TRGB values are F814W TRGB = 28.179 ± 0.049, 28.252 ± 0.035, and 28.457 ± 0.044 mag for NGC 3021, NGC 3370, and NGC 1309, respectively. Similarly, QT RGB = 28.146 ± 0.033 mag for NGC 3021, 28.212 ± 0.041 mag for NGC 3370, and 28.398 ± 0.040 mag for NGC 1309 were obtained. Derived TRGB magnitudes from the QT luminosity functions are systematically brighter than those from F814W luminosity functions, because the QT system uses foreground extinction corrected F814W magnitude (F814W 0 ). We detected small photometric offsets from the