Abstract:Starburst galaxies are efficient γ-ray producers, because their high supernova rates generate copious cosmic ray (CR) protons, and their high gas densities act as thick targets off which these protons can produce neutral pions and thence γ-rays. In this paper we present a first-principles calculation of the mechanisms by which CRs propagate through such environments, combining astrochemical models with analysis of turbulence in weakly ionised plasma. We show that CRs cannot scatter off the strong large-scale t… Show more
“…However, Ref. [11] recently introduced a model for f cal (E), based on the idea that the rate at which CRs diffuse through the magnetised ISM, and thus the fraction that escape before colliding, is determined by the balance between driving of turbulence by the CR streaming instability and ion-neutral damping. This model successfully reproduces the γ-ray spectra of NGC 253, M82, and Arp 220, and we therefore adopt it for this work.…”
Section: Galactic Emission Modelmentioning
confidence: 99%
“…We employ the model of Ref. [11] for CR transport and calorimetry in SFGs. The basic premise of the model is that, in the neutral phase that dominates the mass of the ISM and thus the set of available targets for γ-ray production, CR transport is primarily by streaming along magnetic field lines.…”
Section: Model For Calorimetrymentioning
confidence: 99%
“…the ISM is molecule-dominated, ii) the main ionised species is C + , and iii) the magnetic field is set by a turbulent dynamo, we adopt the fiducial parameters of Ref. [11] appropriate for such galaxies. Specifically, we take χ = 10 −4 , µ i = 12, M A = 2, V Ai = u LA /χ 1/2 M A , and u LA = σ g / √ 2, where σ g is the gas velocity dispersion of the galaxy.…”
The Fermi Gamma Ray Space Telescope has revealed a diffuse, isotropic γ-ray background at energies ranging from 0.1 GeV to 1 TeV [1] whose astrophysical sources remain uncertain. Previous efforts to understand the origin of this background have been hampered by the lack of physical models capable of predicting the γ-ray emission produced by the many candidate sources, which include star-forming galaxies (SFGs) [2–6], active galactic nuclei (particularly blazars [7–9]), millisecond pulsars[7], and dark matter annihilation [10]. In the absence of predictive models, estimates of the contribution from potential sources have relied on a highly-uncertain process of empirically scaling the emission from a small sample of local, resolved sources by their estimated cosmological abundances. Here we present the first calculation of the contribution of SFGs to the γ-ray background that is based on a physical model for the γ-ray emission produced when cosmic ray ions accelerated in supernova remnants interact with the interstellar medium [11]. We validate the model by showing that it reproduces the γ-ray spectra, source count distribution and far infrared-γ-ray correlation observed for nearby, resolved SFGs. When we apply the model to the observed cosmological SFG population, we recover an excellent match to the γ-ray background from 1 GeV to 1 TeV. Our result shows that SFGs alone can explain the full diffuse γ-ray background over this energy range, and strongly suggests that emission in excess of our model at energies <1 GeV originates from cosmic ray electrons produced in the same galaxies.
“…However, Ref. [11] recently introduced a model for f cal (E), based on the idea that the rate at which CRs diffuse through the magnetised ISM, and thus the fraction that escape before colliding, is determined by the balance between driving of turbulence by the CR streaming instability and ion-neutral damping. This model successfully reproduces the γ-ray spectra of NGC 253, M82, and Arp 220, and we therefore adopt it for this work.…”
Section: Galactic Emission Modelmentioning
confidence: 99%
“…We employ the model of Ref. [11] for CR transport and calorimetry in SFGs. The basic premise of the model is that, in the neutral phase that dominates the mass of the ISM and thus the set of available targets for γ-ray production, CR transport is primarily by streaming along magnetic field lines.…”
Section: Model For Calorimetrymentioning
confidence: 99%
“…the ISM is molecule-dominated, ii) the main ionised species is C + , and iii) the magnetic field is set by a turbulent dynamo, we adopt the fiducial parameters of Ref. [11] appropriate for such galaxies. Specifically, we take χ = 10 −4 , µ i = 12, M A = 2, V Ai = u LA /χ 1/2 M A , and u LA = σ g / √ 2, where σ g is the gas velocity dispersion of the galaxy.…”
The Fermi Gamma Ray Space Telescope has revealed a diffuse, isotropic γ-ray background at energies ranging from 0.1 GeV to 1 TeV [1] whose astrophysical sources remain uncertain. Previous efforts to understand the origin of this background have been hampered by the lack of physical models capable of predicting the γ-ray emission produced by the many candidate sources, which include star-forming galaxies (SFGs) [2–6], active galactic nuclei (particularly blazars [7–9]), millisecond pulsars[7], and dark matter annihilation [10]. In the absence of predictive models, estimates of the contribution from potential sources have relied on a highly-uncertain process of empirically scaling the emission from a small sample of local, resolved sources by their estimated cosmological abundances. Here we present the first calculation of the contribution of SFGs to the γ-ray background that is based on a physical model for the γ-ray emission produced when cosmic ray ions accelerated in supernova remnants interact with the interstellar medium [11]. We validate the model by showing that it reproduces the γ-ray spectra, source count distribution and far infrared-γ-ray correlation observed for nearby, resolved SFGs. When we apply the model to the observed cosmological SFG population, we recover an excellent match to the γ-ray background from 1 GeV to 1 TeV. Our result shows that SFGs alone can explain the full diffuse γ-ray background over this energy range, and strongly suggests that emission in excess of our model at energies <1 GeV originates from cosmic ray electrons produced in the same galaxies.
“…In this Proceedings article we report the results of our three recent papers [2,3,7] that incorporate a realistic model of cosmic ray (CR) transport through partially-ionised, but largely neutral, star-forming gas in galaxies. Our model for transport incorporates insights developed by Xu, Lazarian and co-workers in their recent studies [8,[12][13][14] of CR transport in such partiallyionised media.…”
Section: Introductionmentioning
confidence: 99%
“…There is, thus, an absence of extrinsic turbulence at the gyro-radius scale. This is because of the phenomenon of ion-neutral damping which, for ionisation fractions ∼ 10 −2 − 10 −4 (typical of star-formation-hosting gas: [7]) leads to dissipation of the turbulence at a scale significantly larger than the ∼ GeV gyro-radius scale.…”
Star formation proceeds inefficiently in galaxies for reasons that remain under debate. In the local ISM it is known that the cosmic rays (CRs) provide a significant fraction of total ISM pressure and therefore contribute to hydrostatic balance. We set out a model for the dynamical effect of CRs, directly accelerated as a result of star formation itself, on the ISM gas column. On the basis of this model, we will explain how CR feed-back sets an ultimate limit to the star formation efficiency of 'ordinary' galaxies (normal spirals and dwarfs). Interestingly, most such galaxies -including the Milky Way -have star formation efficiencies approaching the maximum allowed by cosmic ray feed-back, suggesting they exist in a state of delicate, dynamically-determined equilibrium. However, at the higher surface densities pertinent to star burst systems pionic losses imply that CRs are dynamically unimportant on global scales while, at the same time, guaranteeing that such galaxies are luminous gamma-ray sources. In passing, we explain how our model leads to new insights about the observed GeV to TeV spectra of local starbursts.
Star-forming and starburst galaxies (SBGs), which are well-known cosmic-ray (CR) reservoirs, are expected to emit gamma rays and neutrinos predominantly via hadronic collisions. In this work we analyze the 10-year Fermi-Low Energy Technique (LAT) spectral energy distributions of 13 nearby galaxies by means of a physical model that accounts for high-energy proton transport in starburst nuclei and includes the contribution of primary and secondary electrons. In particular, we test the hypothesis that the observed gamma-ray fluxes are mostly due to star-forming activity, which is in agreement with the available star formation rates coming from infrared (IR) and ultraviolet (UV) observations. Through this observation-based approach, we determine the most-likely neutrino counterparts from star-forming and SBGs and quantitatively assess the ability of current and upcoming neutrino telescopes to detect them as point-like sources.We also generate mock gamma-ray data to simulate the Cherenkov Telescope Array (CTA) performance in detecting these sources. Moreover, we propose a test to discriminate between the two different CR transport models for the starburst nuclei by looking at the different gamma-ray expectations. We point out that current data already gives a slight preference to CR models, which are dominated by advection.
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