High energy extragalactic multimessenger backgrounds from starburst and dead galaxies

Starburst galaxies are $\gamma$-ray sources. Canonically, their emission is driven by hadronic cosmic rays (CRs) interacting with interstellar gas, forming $\gamma$-rays via the decay of neutral pions. Charged pions are also formed in this process. They decay into secondary leptons, including electrons and neutrinos. Starburst galaxies are therefore also expected to be neutrino sources, and their high-energy $\gamma$-ray emission may include a secondary leptonic component. Leptonic $\gamma$-rays may also originate from electrons directly energized by shocks within the interstellar medium of galaxies, or from pulsars and their surrounding halos. In the Milky Way, pulsars/pulsar halos are the dominant $\gamma$-ray source class. They are associated with stellar remnants or old stellar populations, and are presumably abundant in old galaxies. In this work, we show that the collective high-energy emission from galaxies can account for only a fraction of extragalactic neutrinos, but can form a major component of the extragalactic $\gamma$-ray background. Contrary to the traditional view, a substantial fraction of this radiation may originate from leptonic processes, including from old, quiescent galaxies.