https://njuastrometry.github.io/en/tag/celestial-reference-frames/Celestial Reference FramesHugo Blox Builder (https://hugoblox.com)en-USSat, 23 Aug 2025 00:00:00 +0000https://njuastrometry.github.io/media/icon_hu1909758375025618910.pnghttps://njuastrometry.github.io/en/tag/celestial-reference-frames/https://njuastrometry.github.io/en/publication/2025-mnras-542-2389/Sat, 23 Aug 2025 00:00:00 +0000https://njuastrometry.github.io/en/publication/2025-mnras-542-2389/<h2 id="quasar-populations-and-celestial-reference-frames">Quasar populations and celestial reference frames</h2> <p>Quasars provide the distant reference sources used to realize celestial reference frames. Their usefulness depends on both the accuracy of their astrometric parameters and the uniformity of their sky distribution.</p> <p>The sixth Large Quasar Astrometric Catalogue (LQAC-6) compiles 2,073,099 objects from 33 catalogues, bringing together astrometry, multiband photometry, radio fluxes, and redshifts. Approximately 83.9% of its objects have Gaia DR3 counterparts. In this work, we investigate the statistical properties of this sample and assess their implications for reference-frame construction.</p> <h2 id="main-results">Main results</h2> <ul> <li><strong>Uneven sky coverage:</strong> The catalogue contains more sources in the northern hemisphere than in the southern hemisphere, together with a strong depletion near the Galactic plane. Dense survey regions, such as those covered by AGES and parts of SDSS, produce pronounced local concentrations.</li> <li><strong>Magnitude-dependent astrometric precision:</strong> Position and proper-motion uncertainties increase strongly toward fainter magnitudes. Their dispersion also grows, indicating increasingly heterogeneous astrometric quality.</li> <li><strong>Parallax quality:</strong> After applying the adopted parallax zero-point correction, normalized parallaxes of Gaia-CRF3 sources approximately follow a Gaussian distribution. The non-Gaia-CRF3 sample contains more extreme normalized-parallax values.</li> <li><strong>Redshift discrepancies:</strong> Gaia QSOC redshifts disagree substantially with ground-based spectroscopic measurements for a significant fraction of common sources. Misidentification of emission lines explains recognizable patterns in these discrepancies. Selecting sources with <code>flags_qsoc = 0</code> improves agreement but greatly reduces the available sample.</li> <li><strong>Optical luminosity and radio properties:</strong> In the selected sample, higher-redshift quasars tend to have brighter absolute magnitudes. Radio-loud quasars are generally optically brighter than the comparison population at similar redshifts.</li> </ul> <h2 id="implications">Implications</h2> <p>The analysis highlights the importance of jointly considering sky coverage, astrometric quality, and ancillary source information when selecting reference-frame objects. Dense surveys illustrate the potential for a more closely spaced reference grid, while also revealing substantial differences in survey depth and source selection.</p> <p>The observed luminosity–redshift relation can reflect both selection effects and intrinsic quasar evolution. Separating these contributions is necessary before drawing conclusions about the underlying quasar population.</p>