Evolution of the Halpha luminosity function
  The Smithsonian Hectospec Lensing Survey (SHELS) is a window on the star
formation history over the last 4 Gyr. SHELS is a spectroscopically complete
survey for Rtot < 20.3 over 4 square degrees. We use the 10k spectra to select
a sample of pure star forming galaxies based on their Halpha emission line. We
use the spectroscopy to determine extinction corrections for individual
galaxies and to remove active galaxies in order to reduce systematic
uncertainties. We use the large volume of SHELS with the depth of a narrowband
survey for Halpha galaxies at z ~ 0.24 to make a combined determination of the
Halpha luminosity function at z ~ 0.24. The large area covered by SHELS yields
a survey volume big enough to determine the bright end of the Halpha luminosity
function from redshift 0.100 to 0.377 for an assumed fixed faint-end slope
alpha = -1.20. The bright end evolves: the characteristic luminosity L*
increases by 0.84 dex over this redshift range. Similarly, the star formation
density increases by 0.11 dex. The fraction of galaxies with a close neighbor
increases by a factor of 2-5 for L(Halpha) >~ L* in each of the redshift bins.
We conclude that triggered star formation is an important influence for star
forming galaxies with Halpha emission.
