Galaxy And Mass Assembly (GAMA) : spectroscopic analysis

Abstract

The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to similar to 300 000 galaxies over 280 deg(2), to a limiting magnitude of r(pet) < 19.8 mag. The target galaxies are distributed over 0 < z less than or similar to 0.5 with a median redshift of z approximate to 0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z = 1. The redshift accuracy ranges from sigma(v) approximate to 50 km s(-1) to sigma(v) approximate to 100 km s(-1) depending on the signal-to-noise ratio of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750 less than or similar to lambda less than or similar to 8850 angstrom at a resolution of R approximate to 1300. The final flux calibration is typically accurate to 10-20 per cent, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterized through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [N II]/H alpha versus [O III]/H beta spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies.