Determination of astrophysical parameters of quasars within the Gaia mission

Abstract

We describe methods designed to determine the astrophysical parameters of quasars based on spectra coming from the red and blue spectrophotometers of the Gaia satellite. These methods principally rely on two already published algorithms that are the weighted principal component analysis and the weighted phase correlation. The presented approach benefits from a fast implementation; an intuitive interpretation as well as strong diagnostic tools on the potential errors that may arise during predictions. The production of a semi-empirical library of spectra as they will be observed by Gaia is also covered and subsequently used for validation purpose. We detail the pre-processing that is necessary in order for these spectra to be fully exploitable by our algorithms along with the procedures that are used in order to predict the redshifts of the quasars; their continuum slopes; the total equivalent width of their emission lines and whether these are broad absorption line (BAL) quasars or not. Performances of these procedures were assessed in comparison with the Extremely Randomized Trees learning method and were proven to provide better results on the redshift predictions and on the ratio of correctly classified observations though the probability of detection of BAL quasars remains restricted by the low resolution of these spectra as well as by their limited signal-to-noise ratio. Finally, the triggering of some warning flags allows us to obtain an extremely pure subset of redshift predictions where approximately 99% of the observations come along with absolute errors that are below 0.1.