The Taurus Tunable Filter Field Galaxy Survey: Sample selection and narrowband number counts

Recent evidence suggests a decline in the volume-averaged star formation rate (SFR) with the advance of cosmic time since z ∼ 1. It is not clear, however, the extent to which the selection of such samples influences the measurement of this quantity. Using the Taurus Tunable Filter (TTF) we have obtained an emission-line sample of faint star-forming galaxies over comparable look-back times: the TTF Field Galaxy Survey. By selecting through emission lines, we are screening galaxies through a quantity that scales directly with star formation activity for a given choice of initial mass function. The scanning narrowband technique furnishes a galaxy sample that differs from traditional broadband-selected surveys in both its volume-limited nature and selection of galaxies through emission-line flux. Three discrete wavelength intervals are covered, centered at Hα redshifts z = 0.08, 0.24, and 0.39. Galaxy characteristics are presented and comparisons made with existing surveys of both broadband and emission-line selection. Little overlap is found in a direct comparison between the TTF Field Galaxy Survey and a traditional galaxy redshift survey, as a result of the respective volume and flux limitations of each. When the number counts of emission-line objects are compared with those expected on the basis of existing Hα surveys, we find an excess of ∼3 times at the faintest limits. While these detections are yet to be confirmed independently, inspection of the stronger subsample of galaxies detected in both the line and continuum (line-on-continuum subsample; 13%) is sufficient to support an excess population. The faintest objects are galaxies with little or no continuum, rendering them undetectable by conventional redshift surveys. This increase in the emission-line field population implies higher star formation densities over z ≲ 0.4. However, further study in the form of multiobject spectroscopic follow-up is necessary to quantify this and confirm the faintest detections in the sample.