Why is the Radio Continuum Spectral Index of a Star-Forming Galaxy Approximately -0.7?
Abstract
For local star-forming galaxies, the observed radio continuum spectral index typically falls within +/- 0.1 of -0.7. This narrow distribution is often attributed to a high degree of uniformity in both the ratio of the constituent synchrotron and free-free emission components, as well as to the constancy of their spectral shapes. However, astrophysical models do not generally predict simple power law shapes for the constituent components, nor do they predict a ubiquitous ratio between their contributions to the total integrated spectrum. In this talk, I summarize the results of an investigation into the detailed spectral shapes of these constituent components, as well as their relative contributions to the total spectrum, using observations with high spatial and spectral resolution.I have analyzed over 30 high-resolution images of two starburst galaxies, M82 and NGC 2146, using observations with the Karl G. Jansky Very Large Array (VLA) at frequencies from 1 to 46 GHz. Substantial variations were detected in the radio spectrum across the face of each galaxy, which demonstrates that the integrated spectrum is a composite of diverse spectral shapes. I have separated the resolved radio emission into thermal and nonthermal components and will present a multi-zone description, with elements including a star-forming disk and galactic wind. I will present evidence for spectral curvature in the synchrotron emission from these objects, and compare these findings to models of energy-dependent cosmic ray cooling, as well as to the predictions from calorimeter models of the radio/far-IR relation. Additionally, I have analyzed faint filamentary emission detected in my very high resolution VLA image of the galaxy M82, and I will discuss this emission in the context of wind outflow channels and magnetic structures.
- Publication:
-
American Astronomical Society Meeting Abstracts #225
- Pub Date:
- January 2015
- Bibcode:
- 2015AAS...22541102M