Dynamically close galaxy pairs from the unWISE survey

Testing the merger-AGN-star formation connection

¹ Department of Physics, School of Mathematics and Natural Sciences, Copperbelt University P.O. Box 21692 Kitwe, Zambia
² Department of Physics and Astronomy, Botswana International University of Science and Technology Private Bag 16 Palapye, Botswana
³ Finnish Centre for Astronomy with ESO, University of Turku Vesilinnantie 5 Turku 20014, Finland
⁴ Instituto de Astrofísica de Andalucía (IAA), Spanish National Research Council (CSIC) 18008 Granada, Spain
⁵ Department of Astronomy and Astrophysics, Space Science and Geospatial Institute (SSGI) P.O. Box 33679 Addis Ababa, Ethiopia
⁶ Department of Physics, Faculty of Science, Mbarara University of Science and Technology (MUST) P.O. Box 1410 Mbarara, Uganda

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 August 2025
Accepted: 17 December 2025

Abstract

Context. Galaxy mergers are expected to have a profound influence on the star formation histories of galaxies. It is generally expected that mergers are the main drivers of galaxy mass growth through the accretion of mass and the triggering of new star formation episodes, while the shocks and torques induced by the merger may drive gas and dust to central supermassive black holes and fuel active galactic nucleus (AGN) activity and produce both positive and negative feedback.

Aims. We test whether a merger-AGN-star formation connection exists by selecting samples of galaxy pairs of stellar masses log(M/M_⊙)∼10.2 and ∼11.4 within redshift z < 0.25 at various projected separation and velocity differences in an increasing order, and would therefore have a decreasing probability of being truly bound and interacting.

Methods. We identified galaxies in close pairs and then measured their star formation rates (SFRs; via their NUV − r colours) and the degree of AGN activity (from X-rays, radio emission at 20 cm, WISE infrared colours, and emission line ratios) as a function of their projected separation and velocity difference.

Results. We find only weak evidence that galaxies in pairs have higher SFRs as galaxies become closer in projected and velocity separation. This trend occurs for pairs at closest separation of r_p < 20 kpc and ΔV < 500 km/s. Similarly, we observe no strong evidence that AGNs are more common for galaxies in closer pairs, irrespective of the method used to detect AGNs.

Conclusions. For this sample, we did not find any clear evidence that mergers and interactions may play a significant role in triggering star formation and AGN activity, opposite to expectations from theoretical models invoking feedback episodes. Secular processes may be more important, although this may depend on the selection of galaxies and indicators for star formation and AGN activity.