A heterogeneous set of unusual quasars selected from astrometry and broad-band photometry

The aim of this study is to explore methods for how to identify new quasars with different properties with regard to reddening, redshift, the presence of broad absorption lines from outflows, or other rare properties affecting the broad-band colours. We use Gaia-based astrometric pre-selection to study only sources that have zero parallax and zero proper motion within errors. We subsequently match these sources with information from wide-area photometric surveys and build colour-colour diagrams. Based on these diagrams we select targets for follow-up spectroscopy. In the subsequent analysis we also apply observations from SPHEREx to firmly establish redshifts. We secure spectroscopic observations for 24 sources. Of these, three are found to be stars and the remaining 21 are classified as quasars spanning a redshift range from z=0.139 to z=3.732. The majority of the sources located away from the normal quasar locus in colour-colour diagrams are found to be broad-absorption-line quasars. A single source, the lowest redshift source at z=0.139, is reminiscent of the so-called Little Red Dots found at high redshifts with JWST. Using the simple procedure followed here it is possible to locate previously undiscovered quasars with interesting properties, in particular Broad Absorption Line quasars and quasars with unusual extinction.