Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma ray emission. (arXiv:1812.10496v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Chan_T/0/1/0/all/0/1">T.K. Chan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Keres_D/0/1/0/all/0/1">D. Keres</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hopkins_P/0/1/0/all/0/1">P.F. Hopkins</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Quataert_E/0/1/0/all/0/1">E. Quataert</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Su_K/0/1/0/all/0/1">K.-Y. Su</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hayward_C/0/1/0/all/0/1">C.C. Hayward</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Faucher_Giguere_C/0/1/0/all/0/1">C.-A. Faucher-Giguere</a>
We present the first results of cosmic ray (CR) feedback implemented in the
Feedback In Realistic Environments (FIRE) simulations. We investigate CR
feedback in non-cosmological simulations of dwarf, sub-$Lstar$ starburst, and
$Lstar$ galaxies with different propagation models, including advection,
isotropic and anisotropic diffusion, and streaming along field lines with
different transport coefficients. We simulate CR diffusion and streaming
simultaneously in galaxies with high resolution, using a newly-developed two
moment method. We forward-model and compare to observations of $gamma$-ray
emission from nearby and starburst galaxies. We reproduce the $gamma$-ray
observations with constant isotropic diffusion coefficients $kappa sim
3times 10^{28-29},{rm cm^{2},s^{-1}}$. Advection-only and streaming-only
models (even allowing for super-Alfvenic streaming speeds) produce
order-of-magnitude too large $gamma$-ray luminosities in dwarf and $Lstar$
galaxies. We show that in models that match the $gamma$-ray observations, most
CRs escape low-gas-density galaxies (e.g. dwarfs) before significant
collisional losses, while starburst galaxies are CR proton calorimeters. While
adiabatic losses can be significant, they occur only after CRs escape galaxies,
so they are only of secondary importance for $gamma$-ray emissivities. Models
where CRs are “trapped” in the star-forming disk have lower star formation
efficiency, but these models are ruled out by $gamma$-ray observations. For
models with constant $kappa$ that match the $gamma$-ray observations, CRs
form extended halos with scale heights of several kpc to several tens of kpc.
We present the first results of cosmic ray (CR) feedback implemented in the
Feedback In Realistic Environments (FIRE) simulations. We investigate CR
feedback in non-cosmological simulations of dwarf, sub-$Lstar$ starburst, and
$Lstar$ galaxies with different propagation models, including advection,
isotropic and anisotropic diffusion, and streaming along field lines with
different transport coefficients. We simulate CR diffusion and streaming
simultaneously in galaxies with high resolution, using a newly-developed two
moment method. We forward-model and compare to observations of $gamma$-ray
emission from nearby and starburst galaxies. We reproduce the $gamma$-ray
observations with constant isotropic diffusion coefficients $kappa sim
3times 10^{28-29},{rm cm^{2},s^{-1}}$. Advection-only and streaming-only
models (even allowing for super-Alfvenic streaming speeds) produce
order-of-magnitude too large $gamma$-ray luminosities in dwarf and $Lstar$
galaxies. We show that in models that match the $gamma$-ray observations, most
CRs escape low-gas-density galaxies (e.g. dwarfs) before significant
collisional losses, while starburst galaxies are CR proton calorimeters. While
adiabatic losses can be significant, they occur only after CRs escape galaxies,
so they are only of secondary importance for $gamma$-ray emissivities. Models
where CRs are “trapped” in the star-forming disk have lower star formation
efficiency, but these models are ruled out by $gamma$-ray observations. For
models with constant $kappa$ that match the $gamma$-ray observations, CRs
form extended halos with scale heights of several kpc to several tens of kpc.
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