Hα imaging of the Herschel Reference Survey The star formation properties of a volume-limited, K-band-selected sample of nearby late-type galaxies

We present new H alpha+[NII] imaging data of late-type galaxies in the Herschel Reference Survey aimed at studying the star formation properties of a K-band-selected, volume-limited sample of nearby galaxies. The H alpha+[NII] data are corrected for [NII] contamination and dust attenuation using different recipes based on the Balmer decrement and the 24 mu m luminosities. We show that the H alpha luminosities derived with different corrections give consistent results only whenever the uncertainty on the estimate of the Balmer decrement is sigma[C(H beta)] <= 0.1. We used these data to derive the star formation rate of the late-type galaxies of the sample and compare these estimates to those determined using independent monochromatic tracers (far-UV, radio continuum) or the output of spectral energy distribution (SED) fitting codes. This comparison suggests that the 24 mu m based dust extinction correction for the H alpha data might not be universal and that it should be used with caution in all objects with a low star formation activity, where dust heating can be dominated by the old stellar population. Furthermore, because of the sudden truncation of the star formation activity of cluster galaxies occurring after their interaction with the surrounding environment, the stationarity conditions required to transform monochromatic fluxes into star formation rates might not always be satisfied in tracers other than the H alpha luminosity. In a similar way, the parametrisation of the star formation history generally used in SED fitting codes might not be adequate for these recently interacting systems. We then use the derived star formation rates to study the star formation rate luminosity distribution and the typical scaling relations of the late-type galaxies of the HRS. We observe a systematic decrease of the specific star formation rate with increasing stellar mass, stellar mass surface density, and metallicity. We also observe an increase of the asymmetry and smoothness parameters measured in the H alpha-band with increasing specific star formation rate, probably induced by an increase of the contribution of giant HII regions to the H alpha luminosity function in star-forming low-luminosity galaxies.