The JCMT Nearby Galaxies Legacy Survey – VII. Hα imaging and massive star formation properties

We present Hα fluxes, star formation rates (SFRs) and equivalent widths (EWs) for a sample of 156 nearby galaxies observed in the 12CO J= 3–2 line as part of the James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. These are derived from images and values in the literature and from new Hα images for 72 galaxies which we publish here. We describe the sample, observations and procedures to extract the Hα fluxes and related quantities. We discuss the SFR properties of our sample and confirm the well-known correlation with galaxy luminosity, albeit with high dispersion. Our SFRs range from 0.1 to 11 M⊙ yr−1 with a median SFR value for the complete sample of 0.2 M⊙ yr−1. This median value is somewhat lower than similar published measurements, which we attribute, in part, to our sample being H i selected and, thus, not biased towards high SFRs as has frequently been the case in previous studies. Additionally, we calculate internal absorptions for the Hα line, A(Hα), which are lower than many of those used in previous studies. Our derived EWs, which range from 1 to 880 A with a median value of 27 A, show little dependence on luminosity but rise by a factor of 5 from early- to late-type galaxies. This paper is the first in a series aimed at comparing SFRs obtained from Hα imaging of galaxies with information derived from other tracers of star formation and atomic and molecular gas.