A treasure hunt in the pool of spectra classified as high-redshift QSOs by the spectroscopic pipeline of the SDSS DR16

Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany

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Received: 12 May 2025
Accepted: 17 September 2025

Abstract

The discovery of outsiders in the form of unusual, rare, or even unknown object types is important as they can provide useful information about otherwise hidden physical phenomena and processes. The present study takes advantage of the fact that the automated spectroscopic pipeline of the Sloan Digital Sky Survey (SDSS) occasionally assigns uncommon spectra to high-redshift quasi-stellar objects (QSOs). This paper presents an analysis of ∼4000 spectra that are QSOs with redshifts z > 4.5 according to the spectroscopic pipeline of the SDSS DR16. It turns out that, after excluding non-classifiable spectra of low quality and those from three special plates, only 26% are high-z QSOs, 50% are QSOs at lower redshifts, 16% are galaxies, and 8% are stars. A significant proportion of the last three categories prove to be unusual and are re-assigned here to a variety of rare types. The results of the re-evaluation are summarised in a catalogue.