Gaia Data Release 3: The extragalactic content

TL;DR

Gaia DR3 has identified millions of extragalactic objects, including quasars and galaxies, using machine learning and spectral analysis, providing valuable data for extragalactic astronomy.

Contribution

This paper presents the processing, data products, and analysis of extragalactic objects in Gaia DR3, including candidate selection and spectral composite construction.

Findings

6.6 million candidate quasars and 4.8 million candidate galaxies identified.

High-quality spectra of 43,000 quasars used to create a composite spectrum.

Purer sub-samples with 95% purity contain 1.9 million quasars and 2.9 million galaxies.

Abstract

The Gaia Galactic survey mission is designed and optimized to obtain astrometry, photometry, and spectroscopy of nearly two billion stars in our Galaxy. Yet as an all-sky multi-epoch survey, Gaia also observes several million extragalactic objects down to a magnitude of G~21 mag. Due to the nature of the Gaia onboard selection algorithms, these are mostly point-source-like objects. Using data provided by the satellite, we have identified quasar and galaxy candidates via supervised machine learning methods, and estimate their redshifts using the low resolution BP/RP spectra. We further characterise the surface brightness profiles of host galaxies of quasars and of galaxies from pre-defined input lists. Here we give an overview of the processing of extragalactic objects, describe the data products in Gaia DR3, and analyse their properties. Two integrated tables contain the main results for a high completeness, but low purity (50-70%), set of 6.6 million candidate quasars and 4.8 million candidate galaxies. We provide queries that select purer sub-samples of these containing 1.9 million probable quasars and 2.9 million probable galaxies (both 95% purity). We also use high quality BP/RP spectra of 43 thousand high probability quasars over the redshift range 0.05-4.36 to construct a composite quasar spectrum spanning restframe wavelengths from 72-100 nm.