Effects of Different Cosmic Ray Transport Models on Galaxy Formation. (arXiv:2004.02897v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hopkins_P/0/1/0/all/0/1">Philip F. Hopkins</a> (Caltech), <a href="http://arxiv.org/find/astro-ph/1/au:+Chan_T/0/1/0/all/0/1">T. K. Chan</a> (ICC Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Squire_J/0/1/0/all/0/1">Jonathan Squire</a> (Otago), <a href="http://arxiv.org/find/astro-ph/1/au:+Quataert_E/0/1/0/all/0/1">Eliot Quataert</a> (Berkeley), <a href="http://arxiv.org/find/astro-ph/1/au:+Ji_S/0/1/0/all/0/1">Suoqing Ji</a> (Caltech), <a href="http://arxiv.org/find/astro-ph/1/au:+Keres_D/0/1/0/all/0/1">Dusan Keres</a> (UCSD), <a href="http://arxiv.org/find/astro-ph/1/au:+Faucher_Giguere_C/0/1/0/all/0/1">Claude-Andre Faucher-Giguere</a> (Northwestern)
Cosmic rays (CRs) with ~GeV energies can contribute significantly to the
energy and pressure budget in the interstellar, circumgalactic, and
intergalactic medium (ISM, CGM, IGM). Recent cosmological simulations have
begun to explore these effects, but almost all studies have been restricted to
simplified models with constant CR diffusivity and/or streaming speeds.
Physical models of CR propagation/scattering via extrinsic turbulence and
self-excited waves predict transport coefficients which are complicated
functions of local plasma properties. In a companion paper, we consider a wide
range of observational constraints to identify proposed physically-motivated
cosmic-ray propagation scalings which satisfy both detailed Milky Way (MW) and
extra-galactic $gamma$-ray constraints. Here, we compare the effects of these
models relative to simpler ‘diffusion+streaming’ models on galaxy and CGM
properties at dwarf through MW mass scales. The physical models predict large
local variations in CR diffusivity, with median diffusivity increasing with
galacto-centric radii and decreasing with galaxy mass and redshift. These
effects lead to a more rapid dropoff of CR energy density in the CGM (compared
to simpler models), in turn producing weaker effects of CRs on galaxy star
formation rates (SFRs), CGM absorption profiles and galactic outflows. The
predictions of the more physical CR models tend to lie ‘in between’ models
which ignore CRs entirely and models which treat CRs with constant diffusivity.
Cosmic rays (CRs) with ~GeV energies can contribute significantly to the
energy and pressure budget in the interstellar, circumgalactic, and
intergalactic medium (ISM, CGM, IGM). Recent cosmological simulations have
begun to explore these effects, but almost all studies have been restricted to
simplified models with constant CR diffusivity and/or streaming speeds.
Physical models of CR propagation/scattering via extrinsic turbulence and
self-excited waves predict transport coefficients which are complicated
functions of local plasma properties. In a companion paper, we consider a wide
range of observational constraints to identify proposed physically-motivated
cosmic-ray propagation scalings which satisfy both detailed Milky Way (MW) and
extra-galactic $gamma$-ray constraints. Here, we compare the effects of these
models relative to simpler ‘diffusion+streaming’ models on galaxy and CGM
properties at dwarf through MW mass scales. The physical models predict large
local variations in CR diffusivity, with median diffusivity increasing with
galacto-centric radii and decreasing with galaxy mass and redshift. These
effects lead to a more rapid dropoff of CR energy density in the CGM (compared
to simpler models), in turn producing weaker effects of CRs on galaxy star
formation rates (SFRs), CGM absorption profiles and galactic outflows. The
predictions of the more physical CR models tend to lie ‘in between’ models
which ignore CRs entirely and models which treat CRs with constant diffusivity.
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