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Title: Simulating Gamma-Ray Emission in Star-forming Galaxies

Abstract

Star-forming galaxies emit GeV and TeV gamma-rays that are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. To understand the emission, we have used the moving-mesh code Arepo to perform magnetohydrodynamical galaxy formation simulations with self-consistent CR physics. Our galaxy models exhibit a first burst of star formation that injects CRs at supernovae. Once CRs have sufficiently accumulated in our Milky Way–like galaxy, their buoyancy force overcomes the magnetic tension of the toroidal disk field. As field lines open up, they enable anisotropically diffusing CRs to escape into the halo and to accelerate a bubble-like, CR-dominated outflow. However, these bubbles are invisible in our simulated gamma-ray maps of hadronic pion-decay and secondary inverse-Compton emission because of low gas density in the outflows. By adopting a phenomenological relation between star formation rate (SFR) and far-infrared emission and assuming that gamma-rays mainly originate from decaying pions, our simulated galaxies can reproduce the observed tight relation between far-infrared and gamma-ray emission, independent of whether we account for anisotropic CR diffusion. This demonstrates that uncertainties in modeling active CR transport processes only play a minor role in predicting gamma-ray emission from galaxies. We find that in starbursts, mostmore » of the CR energy is “calorimetrically” lost to hadronic interactions. In contrast, the gamma-ray emission deviates from this calorimetric property at low SFRs due to adiabatic losses, which cannot be identified in traditional one-zone models.« less

Authors:
 [1]; ; ;  [2]
  1. Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam (Germany)
  2. Heidelberg Institute for Theoretical Studies, Schloss-Wolfsbrunnenweg 35, D-69118 Heidelberg (Germany)
Publication Date:
OSTI Identifier:
22654381
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 847; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ANISOTROPY; CALORIMETRY; COSMIC RADIATION; DENSITY; DIFFUSION; EMISSION; GALAXIES; GAMMA RADIATION; GEV RANGE; INTERACTIONS; LOSSES; MAGNETOHYDRODYNAMICS; MILKY WAY; NUCLEI; PIONS; SIMULATION; STAR EVOLUTION; SUPERNOVAE; TEV RANGE

Citation Formats

Pfrommer, Christoph, Pakmor, Rüdiger, Simpson, Christine M., and Springel, Volker, E-mail: cpfrommer@aip.de. Simulating Gamma-Ray Emission in Star-forming Galaxies. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA8BB1.
Pfrommer, Christoph, Pakmor, Rüdiger, Simpson, Christine M., & Springel, Volker, E-mail: cpfrommer@aip.de. Simulating Gamma-Ray Emission in Star-forming Galaxies. United States. doi:10.3847/2041-8213/AA8BB1.
Pfrommer, Christoph, Pakmor, Rüdiger, Simpson, Christine M., and Springel, Volker, E-mail: cpfrommer@aip.de. Sun . "Simulating Gamma-Ray Emission in Star-forming Galaxies". United States. doi:10.3847/2041-8213/AA8BB1.
@article{osti_22654381,
title = {Simulating Gamma-Ray Emission in Star-forming Galaxies},
author = {Pfrommer, Christoph and Pakmor, Rüdiger and Simpson, Christine M. and Springel, Volker, E-mail: cpfrommer@aip.de},
abstractNote = {Star-forming galaxies emit GeV and TeV gamma-rays that are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. To understand the emission, we have used the moving-mesh code Arepo to perform magnetohydrodynamical galaxy formation simulations with self-consistent CR physics. Our galaxy models exhibit a first burst of star formation that injects CRs at supernovae. Once CRs have sufficiently accumulated in our Milky Way–like galaxy, their buoyancy force overcomes the magnetic tension of the toroidal disk field. As field lines open up, they enable anisotropically diffusing CRs to escape into the halo and to accelerate a bubble-like, CR-dominated outflow. However, these bubbles are invisible in our simulated gamma-ray maps of hadronic pion-decay and secondary inverse-Compton emission because of low gas density in the outflows. By adopting a phenomenological relation between star formation rate (SFR) and far-infrared emission and assuming that gamma-rays mainly originate from decaying pions, our simulated galaxies can reproduce the observed tight relation between far-infrared and gamma-ray emission, independent of whether we account for anisotropic CR diffusion. This demonstrates that uncertainties in modeling active CR transport processes only play a minor role in predicting gamma-ray emission from galaxies. We find that in starbursts, most of the CR energy is “calorimetrically” lost to hadronic interactions. In contrast, the gamma-ray emission deviates from this calorimetric property at low SFRs due to adiabatic losses, which cannot be identified in traditional one-zone models.},
doi = {10.3847/2041-8213/AA8BB1},
journal = {Astrophysical Journal Letters},
number = 2,
volume = 847,
place = {United States},
year = {Sun Oct 01 00:00:00 EDT 2017},
month = {Sun Oct 01 00:00:00 EDT 2017}
}