Euclid Quick Data Release (Q1)

Abstract

Galaxy major mergers are indicated as one of the principal pathways to trigger active galactic nuclei (AGN). We present the first statistical analysis of the major merger and AGN connection in the Euclid Deep Fields, and showcase the statistical power of the Euclid data. We constructed a stellar-mass-complete ( M ★ > 10 9.8 M ⊙ ) sample of galaxies from the quick data release (Euclid Quick Release Q1 2025) in the redshift range z = 0.5–2. We selected AGN using X-ray detections, optical spectroscopy, and mid-infrared (MIR) colours, and by processing I E observations with an image decomposition algorithm. We used convolutional neural networks trained on cosmological hydrodynamic simulations to classify galaxies as mergers and non-mergers. We found a larger fraction of AGN in mergers compared to the non-merger controls for all AGN selections, with AGN excess factors ranging from two to six. The largest excess we observed was in the MIR AGN. Likewise, a generally larger merger fraction ( f merg ) was seen in active galaxies than in the non-active controls, with the excess depending on the AGN selection method. Furthermore, we analysed f merg as a function of the AGN bolometric luminosity ( L bol ) and the contribution of the point-source component to the total galaxy light in the I E -band ( f PSF ) as a proxy for the relative AGN contribution fraction. We uncovered a rising f merg , with an increasing f PSF up to f PSF ≃ 0.55, after which we observed a decreasing trend. In the range f PSF = 0.3–0.7, mergers appear to be the dominant AGN fuelling mechanism. We then derived the point-source luminosity ( L PSF ) and showed that f merg monotonically increases as a function of L PSF at z < 0.9, with f merg ≥ 50% for L PSF ≃ 2 × 10 43 erg s −1 . Similarly, at 0.9 ≤ z ≤ 2, f merg rises as a function of L PSF , though mergers do not dominate until L PSF ≃ 10 45 erg s −1 . For the X-ray and spectroscopically detected AGN, we derived the bolometric luminosity, L bol , which has a positive correlation with f merg for X-ray AGN, while there is a less pronounced trend for spectroscopically selected AGN due to the smaller sample size. At L bol > 10 45 erg s −1 , AGN mostly reside in mergers. We conclude that mergers are most strongly associated with the most powerful and dust-obscured AGN, which are typically linked to a fast-growing phase of the supermassive black hole, while other mechanisms, such as secular processes, might be the trigger of less luminous and dominant AGN.