Discovery of gamma-ray emission from the Sagittarius dwarf spheroidal galaxy

by Roland Crocker (Australian National University)

Thursday 2 Dec 2021, 16:00 → 17:00 Australia/Sydney

The Fermi Bubbles are giant, γ-ray emitting lobes emanating from the nucleus of the Milky Way discovered in∼1-100 GeV data collected by the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi Bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s super-massive black hole. I will show that much of the γ-ray emission associated to the brightest region of substructure – the so-called cocoon– is instead due to the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. This large Milky Way satellite is viewed through the Fermi Bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sgr dSph has no on-going star formation, but I will demonstrate that its γ-ray signal is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron-positron pairs injected by the dwarf’s millisecond pulsar (MSP) population, combined with these objects’ magnetospheric emission. This finding suggests that MSPs likely produce significant γ-ray emission amongst old stellar populations, potentially confounding indirect dark matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dwarf spheroidals.