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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 (2021) First Predicted Cosmic Ray Spectra, Primary-to-Secondary Ratios, and Ionization Rates from MHD Galaxy Formation Simulations. . (Unpublished)

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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-MHD 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-neighborhood (LISM) observations (spectra, B/C, e⁺/e⁻, p̅/p, ¹⁰Be/⁹Be, ionization). 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 circum-galactic medium in realistic galaxies necessarily imposes a ~10kpc 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 ~0.1-1 kpc 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:Report or Paper (Discussion Paper)
Related URLs:
URLURL TypeDescription Paper ItemGIZMO code
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. Support for PFH was provided by NSF Research Grants 1911233 & 20009234, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, HST-AR-15800.001-A. Numerical calculations were run on the Caltech compute cluster “Wheeler,” allocations FTA-Hopkins 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. DATA AVAILABILITY STATEMENT. The data supporting the plots within this article are available on reasonable request to the corresponding author. A public version of the GIZMO code is available at
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Cottrell Scholar of Research CorporationUNSPECIFIED
Heising-Simons FoundationScialog Award
Simons FoundationUNSPECIFIED
NASA Hubble FellowshipHST-HF2-51444.001-A
Subject Keywords:cosmic rays — plasmas — methods: numerical — MHD — galaxies: evolution — ISM: structure
Record Number:CaltechAUTHORS:20220228-183248631
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113644
Deposited By: George Porter
Deposited On:01 Mar 2022 00:16
Last Modified:01 Mar 2022 00:16

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