Star formation along the Hubble sequence : results from a new H α Galaxy Survey

We are conducting a large survey of star formation within ∼ 400 nearby spiral and irregular galaxies through imaging of the Hα emission. We present here some of our first results from 104 of the galaxies in our sample, investigating the variation of SFR and Hα EWs along the Hubble sequence for these galaxies. We find a strong dependence of SFR on Hubble type, peaking for Sbc galaxies, but with large dispersions within each type. There is a possible dependence of SFR on bar presence, but none on group membership. There is an increase in EWs for later Hubble types, but with large dispersions within each type. We find no dependence of EW on bar presence, group membership or on absolute magnitude. The Hα Galaxy Survey (HαGS) is a major survey of star formation in the local Universe, using the 1m Jacobus Kapteyn Telescope (JKT) at La Palma, and redshifted Hα filters to determine the quantity and spatial distribution of ionized gas in ∼ 400 spiral and irregular galaxies within 30 h −1 Mpc. We are using broad-and intermediate-band R filters for continuum subtraction. Our sample was selected, using the Uppsala Galaxy Catalogue (Nilson 1973) as the parent catalogue, within 5 redshift shells, but with the angular area of the selection region decreasing such that all shells have equal volumes. The well-defined selection criteria, and the large total number of galaxies, mean that it will be possible to combine the data for all shells such that galaxies with a wide range of luminosities, diameters and surface brightness are represented. Previous studies, e.g. those of Kennicutt & Kent (1983) and Young et al. (1996), have been weighted heavily towards large, luminous and relatively rare Sa-Sc spirals. The low luminosity, dwarf galaxies in our sample are the most populous in the Universe, and our survey will determine whether they are in fact responsible of a significant fraction of the total star formation locally. We have been awarded 14 weeks of observing time over 2000-2002 (long-term status) on the JKT to complete the Hα observations. We are aiming to address such questions as:-What is the total star formation rate (SFR) in the local Universe?-Which galaxy types and morphologies dominate?-Does nuclear activity (star formation or AGN) depend on galaxy properties 1