SDSS low-z quasar companion galaxies

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, (PD), Italy
² INAF – IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy

^⋆ Corresponding authors: daniela.bettoni@inaf.it; renato.falomo@inaf.it; simona.paiano@inaf.it

Received: 28 April 2025
Accepted: 11 September 2025

Abstract

We investigate the relationship between quasars (QSOs) and close companion galaxies using photometry and spectroscopy from the large dataset of the SDSS DR16 survey. Using the SDSS-QSO catalogue of quasars, we selected objects with 0.1 < z ≤ 0.35 and absolute magnitude M(r) <  − 21.3. For all these targets, we searched for candidate companion galaxies located at a projected distance from the target of < 700 kpc and a radial velocity difference from the QSO of ΔV < 1000 km/s. We find that in 447 QSOs at least one companion galaxy was found to be associated. A total of 691 associated galaxies were found in these QSO fields. In the majority there was just one associated galaxy however in several cases a number of companion were also discovered and in two cases the QSO was found in a rich galaxy environments. We note that the possible contamination attributed to a chance projection of the companion galaxies is less than 5%. Based on the available data we expected to find on average ∼2 associated companion galaxies for each QSO. A small fraction (13%) of the companion galaxies exhibit [OII] emission lines as signature of recent star formation. However a similar fraction (16%) of unassociated galaxies in the same QSO fields exhibit [OII] emission. Overall this study suggests that there is no significant link between the presence of these close companion galaxies or the signature of recent star formation and QSO nuclear activity.