Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst M82: Implications for Galactic Wind Physics (1908.09824v1)

Abstract: Cosmic rays (CRs) and magnetic fields may be dynamically important in driving large-scale galactic outflows from rapidly star-forming galaxies. We construct two-dimensional axisymmetric models of the local starburst and super-wind galaxy M82 using the CR propagation code GALPROP. Using prescribed gas density and magnetic field distributions, wind profiles, CR injection rates, and stellar radiation fields, we simultaneously fit both the integrated gamma-ray emission and the spatially-resolved multi-frequency radio emission extended along M82's minor axis. We explore the resulting constraints on the gas density, magnetic field strength, CR energy density, and the assumed CR advection profile. In accord with earlier one-zone studies, we generically find low central CR pressures, strong secondary electron/positron production, and an important role for relativistic bremsstrahlung losses in shaping the synchrotron spectrum. We find that the relatively low central CR density produces CR pressure gradients that are weak compared to gravity, strongly limiting the role of CRs in driving M82's fast and mass-loaded galactic outflow. Our models require strong magnetic fields and advection speeds of order ~1000 km/s on kpc scales along the minor axis in order to reproduce the extended radio emission. Degeneracies between the controlling physical parameters of the model and caveats to these findings are discussed.

• The paper constrains CR propagation by matching simulated gamma-ray and radio emissions with observations, revealing a degeneracy between core magnetic field strength and gas density.
• The study shows that M82 is not a perfect CR proton calorimeter, with strong hadronic cooling limiting CR pressure relative to gravity.
• The paper highlights that radio halos are shaped by secondary electron production and ~1000 km/s wind advection, indicating magnetic fields significantly influence wind dynamics.

Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst M82: Implications for Galactic Wind Physics

The paper "Cosmic Rays and Magnetic Fields in the Core and Halo of the Starburst M82: Implications for Galactic Wind Physics" by Buckman et al. explores the role of cosmic rays (CRs) and magnetic fields in the dynamics of galactic winds, particularly focusing on the starburst galaxy M82. The researchers adopt a rigorous approach using two-dimensional axisymmetric models facilitated by the CR propagation code GALPROP. This investigative framework enables them to analyze both the gamma-ray and radio emissions from M82, aiming to understand the underlying physical parameters such as gas density, magnetic field strength, and CR propagation dynamics.

Key Findings

1. CR and Magnetic Field Models: The paper provides constraints on CR propagation by comparing simulated gamma-ray and radio emissions with observational data. The integration of data reveals a degeneracy between core magnetic field strength and gas density that influences the spectral index of the radio emissions. This indicates that both components critically affect the non-thermal emission resulting from CR interactions.
2. Spectral Analysis and Energetic Constraints: The analysis highlights that M82 is not a perfect CR proton calorimeter—indicating that a notable fraction of CRp energies escape rather than being entirely consumed in hadronic interactions. The core of M82 exhibits strong hadronic cooling due to dense gas, leading to a relatively low CR pressure that is dynamically weak compared to gravity.
3. Extended Radio Halo: The observed radio halo is substantially shaped by secondary electron/positron production, with bremsstrahlung, ionization losses, and wind advection playing significant roles in determining the spatial extension of radio emission along the minor axis. The models suggest an alignment between advection velocities (~1000 km/s) and magnetic field strengths that effectively aid in describing the observed radio morphology.
4. Implications for Galactic Winds: Given the limited contribution of CR pressure compared to gravitational forces, the paper posits that CRs are not the dominant drivers of M82's galactic outflow. Instead, the magnetic field is identified as potentially significant in influencing wind dynamics, provided it remains substantial across larger spatial scales than previously assumed.

Implications and Future Directions

The paper advances the understanding of galactic winds in starburst galaxies by illustrating how CRs and magnetic fields interact in high-energy environments. It underscores the necessity to examine these dynamics beyond simple one-zone models—advocating for considering 2D approximations that capture spatial variations in CR-driven processes.

Future research could explore:

• The dynamic interplay between CR streaming and diffusion with implications for the energy transport within galactic halos.
• The evolution and impact of magnetic field topology on gas outflows and wind acceleration in star-forming galaxies.
• Extended observational campaigns using higher resolution telescopes to refine existing models and address discrepancies noted in extended radio emissions.

Buckman et al.'s contribution lays significant ground for integrated theoretical and observational frameworks that could effectively unravel the nuances of galactic wind dynamics, providing insights crucial for models of galaxy evolution, particularly in systems undergoing intense star formation and feedback episodes.