Properties of the Hα3 sample, compared to that of the entire ALFALFA catalogue (dotted lines) and the subset restricted to galaxies with optical counterparts (dashed lines). Panel a) 1σ limiting surface brightness (erg cm^-2 s^-1 arcsec^-2) in the Hα NET images. Panel b) morphological types. Panel c) stellar masses from i-band photometry. Panel d) color (g − i) magnitude (i band) diagram (color coded by morphology: red = early, blue = disk; green = bulge + disk) (SDSS magnitudes are uncorrected for internal extinction). Hα3 is a homogeneous survey, complete down to a SFR density of 3 × 10^-9 M_⊙ yr^-1 pc^-2 (1σ) and HI masses of 10⁸ M_⊙. This sample spans a wide range in color, morphological type, colors and stellar masses, thereby allowing a comparison of the SFR over a broad parameter space.