The Discrete Source Classifier in Gaia-Apsis

Abstract

The Discrete Source Classifier in the Gaia Astrophysical parameters processing chain has the task of sorting the 10⁹ detected Gaia sources into broad astrophysical classes. These classifications will form part of the final database and are used to trigger various parameterizing algorithms further down the chain. Available input information consists of low-resolution spectra from the Gaia photometers, sky position and apparent magnitude, proper motion and parallax measurements, possible variability information and, for the bright sources, high-resolution spectra from the region 8470–8740 Å, although not all of this information is currently used. Since the classification scheme must deal with several different types of input data, we use a modular approach to classify probabilistically each type of input data and then combine these probabilities. Specific problems facing the DSC include the imbalance in the class fractions, with the vast majority of the sources being single or multiple stars and a small minority falling into classes such as quasar-stellar objects or galaxies. The classifier must deal with a wide range of noise characteristics and also be robust against missing or damaged data. I will describe the system in overview and then go through some of these problems, and our responses to them, in more detail.