Balmer decrements for α.btfr galaxies. The blue thick histogram represents the distribution of Balmer decrements for the galaxies in the α.btfr sample. Balmer decrements have been computed from the Hα and Hβ line fluxes reported in the NSA catalog (refer to Sect. 2.2). The red histogram is the Balmer decrement distribution for a broad sample of galaxies that are high quality ALFALFA detections. This parent sample spans a broad range in galaxy inclination, stellar mass, and gas fraction. The green histogram represents the distribution of the subsample of galaxies in the parent ALFALFA sample that are oriented edge-on. The vertical dashed line denotes an Hα/ Hβ ratio of 2.86, which is adopted here as the fiducial value for HII regions in the absence of extinction. The upper y-axis represents the implied dimming of the r-band luminosity due to internal extinction (Eq. (A.2)).
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