The Taurus Tunable Filter Field Galaxy Survey: Sample Selection and Narrowband Number Counts

Recent evidence suggests a falling volume-averaged star-formation rate (SFR)over z ~ 1. It is not clear, however, the extent to which the selection of suchsamples influences the measurement of this quantity. Using the Taurus TunableFilter (TTF) we have obtained an emission-line sample of faint star-forminggalaxies over comparable lookback times: the TTF Field Galaxy Survey. Byselecting through emission-lines, we are screening galaxies through a quantitythat scales directly with star-formation activity for a given choice of initialmass function. The scanning narrowband technique furnishes a galaxy sample thatdiffers from traditional broadband-selected surveys in both its volume-limitednature and selection of galaxies through emission-line flux. Three discretewavelength intervals are covered, centered at H-alpha redshifts z = 0.08, 0.24and 0.39. Galaxy characteristics are presented and comparisons made with existingsurveys of both broadband and emission-line selection. When the number-countsof emission-line objects are compared with those expected on the basis ofexisting H-alpha surveys, we find an excess of ~ 3 times at the faintestlimits. While these detections are yet to be independently confirmed,inspection of the stronger subsample of galaxies detected in both the line andcontinuum (line-on-continuum subsample; 13 %) is sufficient to support anexcess population. This increase in the emission-line field population implieshigher star-formation densities over z ~ 0.4. However, further study in theform of multi-object spectroscopic follow-up is necessary to quantify this andconfirm the faintest detections in the sample.Comment: 48 pages, 12 figures. To appear in the Astrophysical Journal. An abridged version of the Abstract is shown her