The H alpha galaxy survey. I. The galaxy sample, H alpha narrow-band observations and star formation parameters for 334 galaxies

We discuss the selection and observations of a large sample of nearby galaxies, which we are using to quantify the star formation activity in the local Universe. The sample consists of 334 galaxies across all Hubble types from S0/a to Im and with recession velocities of between 0 and 3000 km s−1. The basic data for each galaxy are narrow band Hα+[N] and R-band imaging, from which we derive star formation rates, Hα+[N] equivalent widths and surface brightnesses, and R-band total magnitudes. A strong correlation is found between total star formation rate and Hubble type, with the strongest star formation in isolated galaxies occurring in Sc and Sbc types. More surprisingly, no significant trend is found between Hα+[N] equivalent width and galaxy R-band luminosity. More detailed analyses of the data set presented here will be described in subsequent papers.