On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253

arXiv:1003.3257v3 [astro-ph.HE] 29 Apr 2011 DRAFT VERSION OCTOBER 30, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 ON THE GEV & TEV DETECTIONS OF THE STARBURST GALAXIES M82 & NGC 253 BRIAN C. LACKI1,2, TODD A. THOMPSON1,2,3, ELIOT QUATAERT4, ABRAHAM LOEB5, & ELI WAXMAN6 Draft version October 30, 2018 ABSTRACT The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and VERITAS constrains 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 Fcal ≈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 ∼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. Subject headings: galaxies: individual (M82, NGC 253), starburst – cosmic rays – gamma rays: theory, obser- vations – radio continuum: galaxies 1. INTRODUCTION M82 and NGC 253 are nearby (D ≈2.5 −4.0 Mpc), proto- typical starburst galaxies, each having an intense star-forming region of about 200 pc radius in the center of a more quies- cent disk galaxy. The starbursts are expected to have high supernova (SN) rates of about 0.03 −0.3 yr−1. SN rem- nants are believed to accelerate primary cosmic ray (CRs) protons and electrons. The high SN rates in starbursts im- ply high CR emissivities. The presence of CR electrons and positrons in these starbursts is inferred from the nonthermal synchrotron radio emission they produce (e.g., Klein et al. 1988; Völk et al. 1989). However, most of the CR energy is believed to be in the form of CR protons. When high energy CR protons collide with interstellar medium (ISM) nucleons, they create pions, which decay into secondary electrons and positrons, γ-rays, and neu- trinos. With their dense ISMs (⟨n⟩≈100 −500 cm−3) and possible high CR energy densities (as evinced by the bright radio emission Völk et al. 1989; Akyuz et al. 1991; Persic & Rephaeli 2010), M82 and NGC 253 are predicted to be bright γ-ray sources (e.g., Akyuz et al. 1991; Sreekumar et al. 1994; Völk et al. 1996; Paglione et al. 1996; Romero & Torres 2003; Domingo-Santamaría & Torres 2005; Thompson, Quataert, & Waxman 2007 [TQW]; Persic et al. 2008; de Cea del Pozo et al. 2009a; Rephaeli et al. 2009; Lacki et al. 2010 [LTQ]). As pro- totypical starbursts, if M82 and NGC 253 are seen in γ-rays, 1 Department of Astronomy The Ohio State University, Columbus, Ohio 43210, USA 2 Center for Cosmology & Astro-Particle Physics, The Ohio State Uni- versity, Columbus, Ohio 43210, USA 3 Alfred P. Sloan Fellow 4 Astronomy Department & Theoretical Astrophysics Center, 601 Campbell Hall, The University of California, Berkeley, CA 94720, USA 5 Astronomy Department, Harvard University, 60 Garden Street, Cam- bridge, MA 02138, USA 6 Physics Faculty, Weizmann Institute, Rehovot 7600, Israel starbursts in general may be sources of γ-rays (Pohl 1994; Torres et al. 2004a), with important implications for the dif- fuse γ-ray and neutrino backgrounds (e.g., Pavlidou & Fields 2002; Loeb & Waxman 2006 [LW06]; TQW). However, the γ-ray luminosity of starbursts depends not only on the injec- tion rate of CRs, but also on the efficiency of converting CR proton energy into pionic γ-rays, neutrinos, and secondary electrons and positrons. In turn, this efficiency depends on the ratio of the timescale for pion production to the escape timescale. The hypothesis that CR protons in starbursts lose all of their energy to pionic collisions before escaping is called “proton calorimetry” (c.f. Pohl 1994).7 If proton calorimetry is strongly violated, then M82 and NGC 253 and, by extension, other starbursts could in fact be weak γ-ray sources. Although γ-ray emission from M82 and NGC 253 has been sought for several years with no success (at GeV, Cillis et al. 2005; and at TeV, Aharonian et al. 2005; Itoh et al. 2007), the launch of Fermi and the advent of powerful VHE γ-ray telescopes has led to recent detections of both starbursts at GeV energies (with Fermi; Abdo e

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