The 4MOST–Gaia Purely Astrometric Quasar Survey (4G-PAQS) will carry out the first large-scale, colour-independent quasar survey selected solely on the basis of astrometry from Gaia. Our main objective is to quantify the selection effects of current colour-selected samples. These colour-selected samples bias our view of the neutral gas and its chemical enrichment because of dust obscuration and reddening of optical colours. Moreover, the broad absorption-line outflows observed in quasars are under-represented by optical colour selection. 4G-PAQS will provide the first sample to overcome these challenges and will constrain the physical and chemical properties of gas in galaxies and quasars at cosmic noon.
Published in The Messenger vol. 190, pp. 38-41, March 2023.