Harikumar N | Searcharxiv

arXiv2026

Low-Redshift Interlopers in the SDSS DR16 $z > 5$ Quasar Catalogue

High-redshift quasars ($z>5$) are crucial for studying early black hole growth and reionisation. SDSS Data Release 16 catalogues 655 such sources with multiple redshift estimators. We systematically examined this subsample and found 480 sources (73.2\%) where pipeline redshift (Z\_DR16Q) disagrees with the derived estimate (Z\_FIT) significantly. Visual inspection confirms that 125 of these (19.1\% of the total) are actually low-redshift active galactic nuclei. In 114 cases, broad Mg\,{\sc ii} with C\,{\sc iii} and C\,{\sc iv} was misidentified as Ly$\alpha$; in 11 cases, H$\alpha$ was misidentified as Ly$\alpha$. This error is immediately revealed by the absence of Gunn--Peterson trough blueward of Ly$\alpha$. Crucially, Z\_FIT recovers correct redshift in the affected sources where emission lines are prominent, indicating the algorithm is sound; the issue lies in community preference for Z\_DR16Q or Z\_SPEC. We urge caution against uncritical use of pipeline redshifts for $z>5$ DR16Q sources and recommend Z\_FIT as the preferred estimator.

astro-ph.GA↗

arXiv2026

STARSFLUX: an all-sky catalogue of absolute spectro-photometric calibrators from 0.3 to 30 $\mu$m

Obtaining accurate fluxes for faint sources from the ground at near- and mid-infrared wavelengths is challenging because of rapidly varying atmospheric absorption, and is often limited by the lack of a nearby spectro-photometric calibrator. We present the STellar Absolute Reference Spectroscopic Flux Library (STARSFLUX), an all-sky catalogue of 64,484 calibrator spectra spanning 0.3--30 $\mu$m, based on the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC). STARSFLUX combines Gaia DR3 stellar parameters, multi-band photometry from space- and ground-based surveys, and synthetic NewEra PHOENIX atmosphere models. For each star, we fit the observed SED with an interpolated model spectrum, angular diameter, and extinction to produce an absolutely flux-calibrated spectrum. The derived angular diameters can also be used to calibrate interferometric observations. We validate the catalogue using three independent tests. First, STARSFLUX angular radii agree closely with Gaia DR3 radii, with a median $|\Delta R|/R_{\rm Gaia,DR3} \simeq 4.8$ per cent. Second, integrated L-band (2.8--4.2 $\mu$m) fluxes for 12 stars in common with the Cohen infrared standards agree with Cohen values with a mean absolute percentage difference of $4.7 \pm 2.6$ per cent. Third, comparison with Gaia DR3 BP/RP spectra shows that the near-UV/visible/near-IR absolute spectro-photometric fluxes agree to approximately 3 per cent. The FITS spectra are available at https://home.strw.leidenuniv.nl/~gamez/ and will be submitted to VizieR.

astro-ph.SR↗