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First predicted cosmic ray spectra, primary-to-secondary ratios, and ionization rates from MHD galaxy formation simulations

Hopkins, Philip F. and Butsky, Iryna S. and Panopoulou, Georgia V. and Ji, Suoqing and Quataert, Eliot and Faucher-Giguère, Claude-André and Kereš, Dušan (2022) First predicted cosmic ray spectra, primary-to-secondary ratios, and ionization rates from MHD galaxy formation simulations. Monthly Notices of the Royal Astronomical Society, 516 (3). pp. 3470-3514. ISSN 0035-8711. doi:10.1093/mnras/stac1791.

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We present the first simulations evolving resolved spectra of cosmic rays (CRs) from MeV–TeV energies (including electrons, positrons, (anti)protons, and heavier nuclei), in live kinetic-magnetohydrodynamics galaxy simulations with star formation and feedback. We utilize new numerical methods including terms often neglected in historical models, comparing Milky Way analogues with phenomenological scattering coefficients ν to Solar-neighbourhood [Local interstellar medium (LISM)] observations (spectra, B/C, e⁺/e⁻, p̅/ρ, ¹⁰Be/⁹Be, ionization, and γ-rays). We show it is possible to reproduce observations with simple single-power-law injection and scattering coefficients (scaling with rigidity R), similar to previous (non-dynamical) calculations. We also find: (1) The circumgalactic medium in realistic galaxies necessarily imposes an ~10 kpc CR scattering halo, influencing the required ν(R). (2) Increasing the normalization of ν(R) re-normalizes CR secondary spectra but also changes primary spectral slopes, owing to source distribution and loss effects. (3) Diffusive/turbulent reacceleration is unimportant and generally sub-dominant to gyroresonant/streaming losses, which are sub-dominant to adiabatic/convective terms dominated by ~ 1.0 - 1 turbulent/fountain motions. (4) CR spectra vary considerably across galaxies; certain features can arise from local structure rather than transport physics. (5) Systematic variation in CR ionization rates between LISM and molecular clouds (or Galactic position) arises naturally without invoking alternative sources. (6) Abundances of CNO nuclei require most CR acceleration occurs around when reverse shocks form in SNe, not in OB wind bubbles or later Sedov–Taylor stages of SNe remnants.

Item Type:Article
Related URLs:
URLURL TypeDescription ItemDiscussion Paper
Hopkins, Philip F.0000-0003-3729-1684
Butsky, Iryna S.0000-0003-1257-5007
Panopoulou, Georgia V.0000-0001-7482-5759
Ji, Suoqing0000-0001-9658-0588
Quataert, Eliot0000-0001-9185-5044
Faucher-Giguère, Claude-André0000-0002-4900-6628
Kereš, Dušan0000-0002-1666-7067
Additional Information:We thank Jonathan Squire for a number of insightful conversations, and Michael Korsmeier for useful comments, as well as our referee for a number of suggestions. Support for PFH was provided by NSF Research Grants 1911233 and 20009234, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, HST-AR-15800.001-A. Numerical calculations were run on the Caltech compute cluster ‘Wheeler,’ allocations AST21010 and AST20016 supported by the NSF and TACC, and NASA HEC SMD-16-7592. CAFG was supported by NSF through grants AST-1715216 and CAREER award AST-1652522; by NASA through grant 17-ATP17-0067; by STScI through grant HST-AR-16124.001-A; and by the Research Corporation for Science Advancement through a Cottrell Scholar Award and a Scialog Award. EQ’s research was supported in part by NSF grants AST-1715070 and AST-2107872 and a Simons Investigator award from the Simons Foundation. GVP acknowledges support by NASA through the NASA Hubble Fellowship grant #HST-HF2-51444.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
NASAHST AR-16124.001-A
Cottrell Scholar of Research CorporationUNSPECIFIED
Simons FoundationUNSPECIFIED
NASA Hubble FellowshipHST-HF2-51444.001-A
Issue or Number:3
Record Number:CaltechAUTHORS:20221003-756400000.16
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117209
Deposited By: Melissa Ray
Deposited On:06 Oct 2022 22:17
Last Modified:06 Oct 2022 22:17

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