Abstract
Synchrotron emission is one of few observable tracers of galactic magnetic fields (B) and cosmic rays (CRs). Much of our understanding of B in galaxies comes from utilizing synchrotron observations in conjunction with several simplifying assumptions of equipartition models, however, it remains unclear how well these assumptions hold, and what B these estimates physically represent. Using Feedback in Realistic Environments project simulations which self-consistently evolve CR proton, electron, and positron spectra from MeV to TeV energies, we present the first synthetic synchrotron emission predictions from simulated L∗ galaxies with 'live' spectrally resolved CR-magnetohydrodynamic. We find that synchrotron emission can be dominated by relatively cool and dense gas, resulting in equipartition estimates of B with fiducial assumptions underestimating the 'true' B in the gas that contributes the most emission by factors of 2-3 due to small volume-filling factors. Motivated by our results, we present an analytical framework that expands upon equipartition models for estimating B in a multiphase medium. Comparing our spectrally resolved synchrotron predictions to simpler spectral assumptions used in galaxy simulations with CRs, we find that spectral evolution can be crucial for accurate synchrotron calculations towards galactic centres, where loss terms are large.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 11707-11718 |
| Number of pages | 12 |
| Journal | Monthly Notices of the Royal Astronomical Society |
| Volume | 527 |
| Issue number | 4 |
| DOIs | |
| State | Published - Feb 1 2024 |
Funding
We wish to recognize and acknowledge the past and present Gabrielino–Tongva people and their Indigenous lands upon which this research was conducted. Additionally, we thank the staff at our institutes, without whose endless efforts this work would not be possible during the ongoing pandemic. We thank the anonymous referee for their helpful comments which improved the quality of this manuscript. We thank Marco Padovani for providing lookup tables for the synchrotron emissivity calculation. SP thanks Dr Viviana Casasola for providing gas surface density data for the galaxies compared to in this study, and thanks Dr Aritra Basu for providing fits files of radio continuum observations for the same galaxies. Support for SBP and PFH was provided by NSF Research Grants 1911233, 20009234, 2108318, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, HST-AR-15800. 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. CH was supported by NSF grant AAG-1911233 and NASA grants HST-AR-15800, HST-AR-16633, and HST-GO-16703. Numerical calculations were run on the Caltech compute cluster ‘Wheeler,’ allocation AST21010 supported by the NSF and TACC, and NASA HEC SMD-16-7592. The Flatiron Institute is supported by the Simons Foundation. CAFG was supported by NSF through grants AST-2108230 and CAREER award AST-1652522; by NASA through grants 17-ATP17-0067 and 21-ATP21-0036; by STScI through grant HST-GO-16730.016-A; by CXO through grant TM2-23005X; and by the Research Corporation for Science Advancement through a Cottrell Scholar Award. ISB was supported by the DuBridge Postdoctoral Fellowship at Caltech. DK was supported by NSF grant AST2108314. KS acknowledges support from the Black Hole Initiative at Harvard University, which is funded by grants from the John Templeton Foundation and the Gordon and Betty Moore Foundation. This work was supported by NSF grant AST-2109127.
Keywords
- ISM: magnetic fields
- galaxies: formation
- methods: Analytical-methods: numerical-cosmic rays
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science