A strategy for astrometric quasar selection with Gaia DR3 and its application to discover new luminous z>2 quasars
Abstract
We aim to provide a cookbook for purely astrometric selection of quasars with Gaia DR3 and to provide an example of its use for the identification of very luminous z>2 quasars. We analyse the Gaia DR3 astrometry of confirmed quasars and stationary sources. We show that the proper motion signal to noise μ/σμ follows a Rayleigh-like distribution. We also show the importance of properly taking into account the correlation between proper motions in right ascension and declination, as well as the non-linear error propagation. We adopt the Mahalanobis distance to the null proper-motion hypothesis. A scaled χ2 distribution with k=2 and scale s=1.1485 is fitted to Gaia-CRF3; the corresponding ±2σ Gaussian-equivalent threshold is D2M(μ)<7.097. The final criteria combine this with the parallax condition and ruwe<1.4. Applying the new criteria to the SDSS DR16Q confirmed quasars, the recovery rate is 90.25%. The suggested purely astrometric selection significantly improves both completeness and purity compared to other strategies used in the literature, reaching a quasar purity of approximately 95% (∼2σ Gaussian-equivalent). It can be used to improve the selection purity and completeness for future quasar surveys. As an example, we use the suggested strategi to locate 24 very bright z>2 quasars without redshifts in the literature. Our proposed astrometric selection strategy recovers 95\% of known quasars. Its potential use for new discoveries is demonstrated by the identification of 24 very luminous z>2 quasars. Ten of these are new discoveries and for an additional seven we correct the redshifts determined by other methods.}
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BibTeX
@misc{li2026strategy,
title = {{A strategy for astrometric quasar selection with Gaia DR3 and its application to discover new luminous $z>2$ quasars}},
author = {J. Li and J. P. U. Fynbo},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.13742},
eprint = {2610.13742},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.13742},
}
RIS
TY - UNPB
TI - A strategy for astrometric quasar selection with Gaia DR3 and its application to discover new luminous z>2 quasars
AU - J. Li
AU - J. P. U. Fynbo
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2610.13742
UR - https://arxiv.org/abs/2610.13742
AB - We aim to provide a cookbook for purely astrometric selection of quasars with Gaia DR3 and to provide an example of its use for the identification of very luminous z>2 quasars. We analyse the Gaia DR3 astrometry of confirmed quasars and stationary sources. We show that the proper motion signal to noise μ/σ_μ follows a Rayleigh-like distribution. We also show the importance of properly taking into account the correlation between proper motions in right ascension and declination, as well as the non-linear error propagation. We adopt the Mahalanobis distance to the null proper-motion hypothesis. A scaled χ^2 distribution with k=2 and scale s=1.1485 is fitted to Gaia-CRF3; the corresponding ±2σ Gaussian-equivalent threshold is D^2_M(μ)<7.097. The final criteria combine this with the parallax condition and ruwe<1.4. Applying the new criteria to the SDSS DR16Q confirmed quasars, the recovery rate is 90.25%. The suggested purely astrometric selection significantly improves both completeness and purity compared to other strategies used in the literature, reaching a quasar purity of approximately 95% (∼2σ Gaussian-equivalent). It can be used to improve the selection purity and completeness for future quasar surveys. As an example, we use the suggested strategi to locate 24 very bright z>2 quasars without redshifts in the literature. Our proposed astrometric selection strategy recovers 95\% of known quasars. Its potential use for new discoveries is demonstrated by the identification of 24 very luminous z>2 quasars. Ten of these are new discoveries and for an additional seven we correct the redshifts determined by other methods.}
ER -