A Gamma-ray Emitting Collisional Ring Galaxy System in our Galactic Neighborhood

The astrophysical $\gamma$-ray photons carry the signatures of the violent phenomena happening on various astronomical scales in our Universe. This includes supernova remnants, pulsars, and pulsar wind nebulae in the Galactic environment and extragalactic relativistic jets associated with active galactic nuclei (AGN). However, $\sim$30\% of the \gm-ray sources detected with the Fermi Large Area Telescope lack multiwavelength counterpart association, precluding us from characterizing their origin. Here we report, for the first time, the association of a collisional ring galaxy system in our Galactic neighborhood (distance $\sim$10 Mpc), formed as a consequence of a smaller `bullet' galaxy piercing through a larger galaxy, as the multi-frequency counterpart of an unassociated $\gamma$-ray source 4FGL~J1647.5$-$5724. The system, also known as "Kathryn's Wheel", contains two dwarf irregular galaxies and an edge-on, late-type, spiral galaxy surrounded by a ring of star-forming knots. We utilized observations taken from the Neil Gehrels Swift observatory, Rapid ASKAP Continuum Survey, SuperCOSMOS H$\alpha$ Survey, Dark Energy Survey, and Visible MultiObject Spectrograph at Very Large Telescope to ascertain the association with 4FGL~J1647.5$-$5724 and to explore the connection between the star-forming activities and the observed $\gamma$-ray emission. We found that star-formation alone cannot explain the observed $\gamma$-ray emission, and additional contribution likely from the pulsars/supernova remnants or buried AGN is required. We conclude that arcsecond/sub-arcsecond-scale observations of this extraordinary $\gamma$-ray emitting galaxy collision will be needed to resolve the environment and explore the origin of cosmic rays.
Comment: ApJ Letters, in press