ON THE GeV AND TeV DETECTIONS OF THE STARBURST GALAXIES M82 AND NGC 253

Abstract

The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and VERITAS constrain the physics of cosmic rays (CRs) in these dense starbursts. We argue that the γ -rays are predominantly hadronic in origin, as expected by previous studies. The measured fluxes imply that pionic losses are efficient for CR protons in both galaxies: we show that a fraction F cal ≈ 0.2-0.4 of the energy injected in high-energy primary CR protons is lost to inelastic proton-proton collisions (pion production) before escape, producing γ -rays, neutrinos, and secondary electrons and positrons. We discuss the factor of ∼2 uncertainties in this estimate, including supernova rate and leptonic contributions to the GeV-TeV emission. We argue that γ -ray data on ULIRGs like Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we present upper limits on Arp 220 from the Fermi data. We show that the observed ratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron cooling processes are important for cooling the CR electron/positron population. We briefly reconsider previous predictions in light of the γ -ray detections, including the starburst contribution to the γ -ray background and CR energy densities. Finally, as a guide for future studies, we list the brightest star-forming galaxies on the sky and present updated predictions for their γ -ray and neutrino fluxes.

M 82 – A radio continuum and polarisation study

“…This might be a collision of the hot superwind with a preexisting neutral cloud (Lehnert et al 1999) and a remnant of a previously stronger star-bursting activity that was blowing material even further into the halo. The detection of hard γ-ray emission by the VERITAS Collaboration et al (2009) further verifies a closed environment scenario since it favours the idea of M 82 being a proton calorimeter (Lacki et al 2011). Additionally, the high particle and energy densities in the core region lead to a scenario where even charged particles accelerated by supernova explosions would not be able to escape this region and could not build up a synchrotron halo, assuming the recent activity of star formation and particle densities.…”

SEARCH FOR TIME-INDEPENDENT NEUTRINO EMISSION FROM ASTROPHYSICAL SOURCES WITH 3 yr OF IceCube DATA

“…The high number of sources does not penalize the stacking search because sources are weighted according to their FIR flux at 60 μm, and therefore, dim sources will contribute less to the likelihood. The highest-weighted starburst galaxy in the catalog is M82 (Lacki et al 2011), followed by NGC 253 (Romero & Torre 2003). Stacking search for five nearby clusters of galaxies.…”

Nonthermal particles and photons in starburst regions and superbubbles

“…Depending on the particular mechanism of emission of the accelerated particles the measured gamma-ray luminosity may constrain the energy content in the non-thermal components, i.e., relativistic particles and magnetic fields (see, e.g., Ohm 2012, Torres and Reimer 2013, Lacki 2013, Martin 2014, Yoast-Hull et al 2014. For instance, Lacki et al (2011) argued that if the gammarays observed from M82 and NGC 253 are of hadronic origin, i.e., they come from pion decays following inelastic collisions of relativistic nuclei with the interstellar matter, then 20-40% of the energy injected in the high-energy primary CR protons is lost in the inelastic collisions before escape from the starburst region. On the other hand, Mannheim et al (2012) argued that apart from the hadronic emission from CRs the gamma-ray pulsar wind nebulae left behind by the supernovae may substantially contribute to the observed TeV luminosity of starburst galaxies.…”