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Register a new userCharacterising the signatures of star-forming galaxies in the extra-galactic $gamma$-ray background
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Galaxies experiencing intense star-formation episodes are expected to be rich in energetic cosmic rays (CRs). These CRs undergo hadronic interactions with the interstellar gases of their host to drive $gamma$-ray emission, which has already been detected from several nearby starbursts. Unresolved $gamma$-ray emission from more distant star-forming galaxies (SFGs) is expected to contribute to the extra-galactic $gamma$-ray background (EGB). However, despite the wealth of high-quality all-sky data from the Fermi-LAT $gamma$-ray space telescope collected over more than a decade of operation, the exact contribution of such SFGs to the EGB remains unsettled. We investigate the high-energy $gamma$-ray emission from SFGs up to redshift $z=3$ above a GeV, and assess the contribution they can make to the EGB. We show the $gamma$-ray emission spectrum from a SFG population can be determined from just a small number of key parameters, from which we model a range of possible EGB realisations. We demonstrate that populations of SFGs leave anisotropic signatures in the EGB, and that these can be accessed using the spatial power spectrum. Moreover, we show that such signatures will be accessible with ongoing operation of current $gamma$-ray instruments, and detection prospects will be greatly improved by the next generation of $gamma$-ray observatories, in particular the Cherenkov Telescope Array.
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In recent years, $gamma$-ray emission has been detected from star-forming galaxies (SFGs) in the local universe, including M82, NGC 253, Arp 220 and M33. The bulk of this emission is thought to be of hadronic origin, arising from the interactions of cosmic rays (CRs) with the interstellar medium of their host galaxy. Distant SFGs are presumably also bright in $gamma$-rays. Although they would not be resolvable as point sources, distant unresolved SFG populations contribute $gamma$-rays to the extra-galactic $gamma$-ray background (EGB). Despite the wealth of high-quality all-sky EGB data collected over more than a decade of operation with the textit{Fermi}-LAT $gamma$-ray space telescope, the exact contribution of SFGs to the EGB remains unsettled. In this study, we model the $gamma$-ray emission from SFG populations and demonstrate that such emission can be characterized by just a small number of physically-motivated parameters. We further show that source populations would leave anisotropic signatures in the EGB, which could be used to yield information about the underlying properties, dynamics and evolution of CR-rich SFGs.
The Fermi Gamma-ray Space Telescope has revealed a diffuse $gamma$-ray background at energies from 0.1 GeV to 1 TeV, which can be separated into Galactic emission and an isotropic, extragalactic component. Previous efforts to understand the latter have been hampered by the lack of physical models capable of predicting the $gamma$-ray emission produced by the many candidate sources, primarily active galactic nuclei and star-forming galaxies, leaving their contributions poorly constrained. Here we present a calculation of the contribution of star-forming galaxies to the $gamma$-ray background that does not rely on empirical scalings, and is instead based on a physical model for the $gamma$-ray emission produced when cosmic rays accelerated in supernova remnants interact with the interstellar medium. After validating the model against local observations, we apply it to the observed cosmological star-forming galaxy population and recover an excellent match to both the total intensity and the spectral slope of the $gamma$-ray background, demonstrating that star-forming galaxies alone can explain the full diffuse, isotropic $gamma$-ray background.
We derive the contribution to the extragalactic gamma-ray background (EGB) from AGN winds and star-forming galaxies by including a physical model for the gamma-ray emission produced by relativistic protons accelerated by AGN-driven and supernova-driven shocks into a state-of-the-art semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers of AGN accretion and starburst activity and on expanding blast wave as the mechanism to communicate outwards the energy injected into the interstellar medium by the active nucleus. We compare the model predictions with the latest measurement of the EGB spectrum performed by the Fermi-LAT in the range between 100 MeV and 820 GeV. We find that AGN winds can provide ~35$pm$15% of the observed EGB in the energy interval E_{gamma}=0.1-1 GeV, for ~73$pm$15% at E_{gamma}=1-10 GeV, and for ~60$pm$20% at E_{gamma}>10 GeV. The AGN wind contribution to the EGB is predicted to be larger by a factor of 3-5 than that provided by star-forming galaxies (quiescent plus starburst) in the hierarchical clustering scenario. The cumulative gamma-ray emission from AGN winds and blazars can account for the amplitude and spectral shape of the EGB, assuming the standard acceleration theory, and AGN wind parameters that agree with observations. We also compare the model prediction for the cumulative neutrino background from AGN winds with the most recent IceCube data. We find that for AGN winds with accelerated proton spectral index p=2.2-2.3, and taking into account internal absorption of gamma-rays, the Fermi-LAT and IceCube data could be reproduced simultaneously.
A majority of the $gamma$-ray emission from star-forming galaxies is generated by the interaction of high-energy cosmic rays with the interstellar gas and radiation fields. Star-forming galaxies are expected to contribute to both the extragalactic $gamma$-ray background and the IceCube astrophysical neutrino flux. Using roughly 10,years of $gamma$-ray data taken by the {it Fermi} Large Area Telescope, in this study we constrain the $gamma$-ray properties of star-forming galaxies. We report the detection of 11 bona-fide $gamma$-ray emitting galaxies and 2 candidates. Moreover, we show that the cumulative $gamma$-ray emission of below-threshold galaxies is also significantly detected at $sim$5,$sigma$ confidence. The $gamma$-ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8-1000,$mu$m) infrared luminosity as previously determined. Above 1,GeV, the spectral energy distribution of resolved and unresolved galaxies is found to be compatible with a power law with a photon index of $approx2.2-2.3$. Finally, we find that star-forming galaxies account for roughly 5,% and 3,% of the extragalactic $gamma$-ray background and the IceCube neutrino flux, respectively.
Cosmic ray transport on galactic scales depends on the detailed properties of the magnetized, multiphase interstellar medium (ISM). In this work, we post-process a high-resolution TIGRESS magnetohydrodynamic simulation modeling a local galactic disk patch with a two-moment fluid algorithm for cosmic ray transport. We consider a variety of prescriptions for the cosmic rays, from a simple purely diffusive formalism with constant scattering coefficient, to a physically-motivated model in which the scattering coefficient is set by critical balance between streaming-driven Alfven wave excitation and damping mediated by local gas properties. We separately focus on cosmic rays with kinetic energies of $sim 1$ GeV (high-energy) and $sim 30$~MeV (low-energy), respectively important for ISM dynamics and chemistry. We find that simultaneously accounting for advection, streaming, and diffusion of cosmic rays is crucial for properly modeling their transport. Advection dominates in the high-velocity, low-density, hot phase, while diffusion and streaming are more important in higher density, cooler phases. Our physically-motivated model shows that there is no single diffusivity for cosmic-ray transport: the scattering coefficient varies by four or more orders of magnitude, maximal at density $n_mathrm{H} sim 0.01, mathrm{cm}^{-3}$. Ion-neutral damping of Alfven waves results in strong diffusion and nearly uniform cosmic ray pressure within most of the mass of the ISM. However, cosmic rays are trapped near the disk midplane by the higher scattering rate in the surrounding lower-density, higher-ionization gas. The transport of high-energy cosmic rays differs from that of low-energy cosmic rays, with less effective diffusion and greater energy losses for the latter.
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