The deep2 galaxy redshift survey: Spectral classification of galaxies at z ∼ 1

We present a principal component analysis (PCA)-based spectral classification, η, for the first 5600 galaxies observed in the DEEP2 Redshift Survey. This parameter provides a very pronounced separation between absorption- and emission-dominated galaxy spectra - corresponding to passively evolving and actively star-forming galaxies in the survey, respectively. In addition it is shown that, despite the high resolution of the observed spectra, this parameter alone can be used to quite accurately reconstruct any given galaxy spectrum, suggesting there are not many "degrees of freedom" in the observed spectra of this galaxy population. It is argued that this form of classification, η, will be particularly valuable in making future comparisons between high- and low-redshift galaxy surveys for which very large spectroscopic samples are now readily available, particularly when used in conjunction with high-resolution spectral synthesis models, which will be made public in the near future. We also discuss the relative advantages of this approach to distant galaxy classification compared to other methods such as colors and morphologies. Finally, we compare the classification derived here with that adopted for the 2dF Galaxy Redshift Survey and in so doing show that the two systems are very similar. This will be particularly useful in subsequent analyses when making comparisons between results from each of these surveys to study evolution in the galaxy populations and large-scale structure.