A Spatially Resolved Radio Spectral Index Study of the Dwarf Irregular Galaxy NGC\,1569

TL;DR

This study maps the spatially resolved radio spectral energy distribution of dwarf galaxy NGC 1569 using VLA data across 1-34 GHz, revealing a high thermal fraction and young cosmic ray electrons, with implications for internal extinction.

Contribution

First detailed spatially resolved radio spectral analysis of NGC 1569 using Bayesian fitting and multi-frequency VLA data, highlighting thermal/non-thermal emission variations.

Findings

Higher thermal fraction at 1 GHz than spiral galaxies

Average non-thermal spectral index of -0.53

Estimated line-of-sight reddening E(B-V)=0.49

Abstract

We study the resolved radio-continuum spectral energy distribution of the dwarf irregular galaxy, NGC 1569, on a beam-by-beam basis to isolate and study its spatially resolved radio emission characteristics. Utilizing high quality NRAO Karl G. Jansky Very Large Array (VLA) observations that densely sample the 1--34\,GHz frequency range, we adopt a Bayesian fitting procedure, where we use H$\alpha$ emission that has not been corrected for extinction as a prior, to produce maps of how the separated thermal emission, non-thermal emission and non-thermal spectral index vary across NGC\,1569's main disk. We find a higher thermal fraction at 1\,GHz than is found in spiral galaxies ($26^{+2}_{-3}\%$) and find an average non-thermal spectral index $\alpha = -0.53\pm0.02$, suggesting that a young population of cosmic ray electrons is responsible for the observed non--thermal emission. By comparing our recovered map of the thermal radio emission with literature H$\alpha$ maps, we estimate the total reddening along the line of sight to NGC\,1569 to be $E(B-V) = 0.49 \pm 0.05$, which is in good agreement with other literature measurements. Spatial variations in the reddening indicate that a significant portion of the total reddening is due to internal extinction within NGC\,1569.