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^ M_⊙) sample of galaxies from the quick data release (Q1) 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 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_̊m 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_̊m merg as a function of the AGN bolometric luminosity (L_̊m bol) and the contribution of the point-source component to the total galaxy light in the IE-band (f_ PSF) as a proxy for the relative AGN contribution fraction. We uncovered a rising f_̊m 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_̊m merg monotonically increases as a function of L_ PSF at z<0. 9, with f_ ̊m merg ≥50% for L_ PSF ≃ 2. Similarly, at 0. 9łeq z łeq 2, f_̊m merg rises as a function of L_ PSF though mergers do not dominate until L_ PSF ≃ 10^. 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 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.
Marca et al. (2025) studied this question.