Supermassive black hole binaries and galaxy mergers in the multi-messenger sky

In this talk, I will present a multi-messenger study of galaxy
mergers and supermassive black hole binaries (SMBHBs) as theoretically
well-motivated environments for high-energy particle acceleration. I begin
with an IceCube search for neutrinos from galaxy mergers using six merger
catalogs and 10 years of muon-track data. No significant association is
found for individual mergers or in stacking, leading to strong upper
limits and implying that considered mergers do not dominate the diffuse
IceCube neutrino flux. I then show that the same merger population is
clearly γ-ray bright: with 16.7 years of Fermi-LAT data, eight mergers are
detected individually, and the remaining sample yields a high-significance
stacked γ-ray detection with a hard spectrum, establishing mergers as a
new γ-ray
source class. Finally, I discuss late-stage mergers via SMBHB candidates:
a stacking analysis of 693 systems reports positive correlations with
IceCube events at up to ~3σ significance, motivating joint neutrino–GW
tests in the pulsar-timing era. Overall, these results suggest a coherent
pathway: mergers are efficient γ-ray emitters, their neutrino output is
tightly constrained, and SMBHB candidates may be promising multi-messenger
targets for the next generation of coordinated searches.