Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies

The luminosity of a star-forming galaxy like the Milky Way (MW) is dominated by the relatively narrow frequency range of the spectral energy distribution (SED) from the ultraviolet (UV) to far infrared (FIR), which is due to stellar emission and dust reprocessing in the interstellar medium (ISM). Related to the birth and death of massive stars, cosmic rays (CRs) are pervasive throughout the ISM (see, e.g., Strong et al. 2007 for a recent review). The diffuse emissions arising from various interactions between CRs and the ISM, interstellar radiation field (ISRF -the UV-FIR component of the galactic SED), and magnetic field span radio frequencies to high-energy γ-rays (>100 MeV), but at a lower level of intensity compared to the stellar and dust component. These broadband emissions are currently the best way to trace CR intensities and spectra throughout the MW and other galaxies. Gamma rays are particularly useful in this respect because this energy range gives access to the dominant hadronic component in CRs via the observation of π 0 -decay radiation produced by CR nuclei inelastically colliding with the interstellar gas. Understanding the global energy budget of processes related to the injection and propagation of CRs, and how the energy is distributed across the electromagnetic spectrum, is essential to interpret the radio/far-infrared relation (Helou et al. 1985, Murphy et al. 2006), galactic calorimetry (e.g., Völk 1989), and predictions of extragalactic backgrounds (e.g., Thompson et al. 2007, Murphy et al. 2008), and for many other studies.

Troy A. Porter

The MW is the best studied non-AGN dominated star-forming galaxy, and the only galaxy that direct measurements of CR intensities and spectra are available. However, because of our position inside, the derivation of global properties is not straightforward and requires detailed models of the spatial distribution of the emission. In recent work by Strong et al. (2010) using the GALPROP (e.g., Strong et al. 2000, Moskalenko et al. 2002 ; also see http://galprop.stanford.edu ) and FRaNKIE (Fast Radiative Numerical Kode for Interstellar Emission - Porter et al. 2008) codes, we have calculated the broadband SED of the MW for the first time. Emission by stars and dust is included, together with the diffuse emissions for different CR propagation models consistent with local CR data. Figure 1 (left) shows the broadband luminosity spectrum of the Galaxy, including the input luminosity for CRs for a 4 kpc halo using a diffusive-reacceleration CR propagation model. Strong et al. (2010) considered a range of halo sizes (2-10 kpc). For this range, the relative decrease in the injected CR proton and helium luminosities is ∼ 10%. For smaller halo sizes, the CRs escape quicker requiring more injected power to maintain the local CR spectrum. In addition, for larger halo sizes CR sources located at further distances can contribute to the local spectrum, which is the normalisation condition, hence less power is required. Contrasting with the CR nuclei, the injected primary CR electron luminosity increases with z h , which is required to counter the increased inverse-Compton (IC) energy losses in the halo from the longer escape time. The energy channelled into γ-ray and other secondary production from the CR nuclei component is only a very small fraction of the total power injected in these particles. For the model shown, the total of synchrotron, IC, and bremsstrahlung luminosities for the primary electrons accounts for over half of the total luminosity injected in Cosmic-Ray Induced Diffuse Emissions from Local Group Galaxies 3 these particles. Note that it is the IC emission at γ-ray energies that is responsible for the majority of the energy losses, not synchrotron radiation as usually assumed. Including the contribution by secondary electron/positron production, approximately half of the total output in γ-rays is provided by these particles, even though they count for only ∼ 2% of the injected power. Non-AGN dominated galaxies like the MW turn out to be very good lepton calorimeters if all the energy-loss processes are taken into account.

Until recently, the MW was the only galaxy that was resolved in high-energy γ-rays. However, observations by the Fermi Large Area Telescope (LAT) have extended the sample of resolved star-forming galaxies to include the Magellanic Clouds (Abdo et al. 2010, Abdo et al. 2010), with M31 also being detected (Abdo et al. 2010).

Figure 2 (left) shows the background subtracted counts map for a 20 • × 20 • region of interest (ROI) surrounding the LMC. The remaining feature is extended emission that is spatially confined to within the LMC boundaries, which are traced by the iso column density contour N H = 1×10 21 H cm -2 of neutral hydrogen in the LMC (Kim et al. 2005). The extended γ-ray emission from the LMC can be resolved into several components. The brightest emission feature is located near (α J2000 , δ J2000 ) ≈ (05 h 40 m , -

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