Cosmic sieve: Gaia sifts through billions of celestial bodies ⚡ экспресс
The Discrete Source Classifier (DSC) provides probabilistic classification of objects from the fourth Gaia data release (GDR4) into quasars, galaxies, and stars, relying on astrometry, photometry, and low-resolution spectra. It includes three neural networks and three schemes for aggregating their predictions. On the test sample (excluding the Magellanic Clouds), DSC purity in GDR4 increased with a small drop in completeness. For extragalactic classes at G<20, average completeness ≥88%, purity ~96%; at 20≤G<20.5, completeness falls to 55%, purity to 71%; and at G>20.5, it's even lower—especially when XP spectra are used. Adding CatWISE2020 infrared photometry boosts completeness for faint objects (G>20) by 9–29 pp, at the cost of a 1–9 pp drop in purity. The final combined classifiers, oriented toward completeness, find 3 million quasars and 2 million galaxies (with high contamination among faint ones), while the purity-priority one yields about 2 million and 1.3 million galaxies with low contamination. Filters are recommended to enhance purity.
The Gaia telescope works like a cosmic sieve. It measures photometric data (brightness in different colors) and spectra (the rainbow breakdown of light). A neural network is trained to distinguish patterns: which ones belong to stars, and which to galaxies. For bright objects, accuracy exceeds 96%.
But faint distant sources, like grains of sand, slip through the sieve. That's where the infrared telescope CatWISE2020 comes in — it sees thermal radiation that penetrates even dust clouds. It acts as a finer-mesh sieve, allowing capture of nearly a third more hidden quasars and galaxies. As a result, the Gaia DR4 catalog gained three million quasar candidates and two million galaxies, though among faint sources there may be some false positives.
This cosmic census helps us understand how the Universe evolves in its farthest reaches.
🎯 A quasar is the searing hot core of a distant galaxy, where a supermassive black hole devours matter, emitting more light than all the galaxy's stars combined.