Estimated completeness of the four passbands for both on-galaxy, including NGC 3640 and NGC 3641, and three off-galaxy regions in a box with a size of ∼6 × 6 arcmin². Dark solid lines and gray dashed lines are the fitted Fermi function (Alamo-Martínez et al. 2013) for the on-galaxy and off-galaxy, respectively. Red dots are completeness measurements per magnitude bin for the on-galaxy region. Blue vertical dashed lines are the expected turn-over magnitudes in each passband (see Sect. 3.1). Upper row: estimated completeness of the u- and g-band, respectively. Lower row: estimated completeness of the r- and i-band, respectively.