Tunable Filter Surveys of Star‐forming Galaxies

Abstract

This thesis describes the development of a tunable filter and its use in a survey for star-forming galaxies in the field at different redshifts. The Taurus Tunable Filter (TTF) is a narrow-gap Fabry-Perot interferometer that has been implemented as a fully tunable narrowband filter on the AngloAustralian Telescope. Characteristics of the device are presented in the thesis, including the use of a CCD charge-shuffling technique. This charge shuffling is used in conjunction with TTF to obtain multiple narrowband images on a single CCD frame and has use in either science or calibration exposures. Alternative methods of tunable imaging are discussed and compared to TTF. A technique is presented for assessing the parallel nature of the glass plates central to TTF use. This is essential in ensuring correct use of the instrument. The plate spacings used by TTF are much smaller than those of conventional Fabry-Perot devices and, as such, cannot make use of standard techniques for obtaining parallel plates. A survey for star-forming galaxies making use of the tunable filter is described. This TTF Field Galaxy Survey is novel in its use of the scanning TTF narrow band in the search for faint Ha emission indicative of star formation in the field. Three discrete wavelength intervals are covered, spanning Ha galaxies with redshifts –0.4. The narrowband technique furnishes a z ∼ 0.1 galaxy sample that differs from traditional broadband-selected surveys in both its volume-limited nature and selection of galaxies through emission-line flux. Methods of candidate selection from a set of narrowband images are discussed. Particular attention is given to the selection of a homogeneous subsample from data such as these. Galaxy characteristics are presented and comparisons made with existing surveys of both broadband and emission-line origin. Small overlap is found in a direct comparison between the TTF Field Galaxy Survey and a traditional galaxy redshift survey (The Autofib Galaxy Redshift Survey; R. S. Ellis et al. 1996, MNRAS, 280, 235), due to the respective volume and flux constraints of each. The star formation rates from the galaxies inferred to be Ha are at least ∼2 times higher than previous measurements in all redshift intervals. This excess is primarily due to the significant numbers of TTF galaxies with little or no continuum, many of which are beyond the broadband selection limit of conventional redshift surveys. While the detections are yet to be independently confirmed, inspection of the stronger subsample of galaxies detected in both line and continuum (13%) also supports an excess population. Further study in the form of multiobject spectroscopic follow-up is suggested to test this hypothesis and to confirm the estimates of contamination by background emission-line galaxies. Closing remarks are made regarding the future directions of both the technical and astronomical aspects of the thesis.