A spatially resolved radio spectral index study of the dwarf irregular galaxy NGC 1569

Jonathan Westcott,Robert Beswick,Volker Heesen,Elias Brinks,Luke Hindson

doi:10.1093/mnras/sty028

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.

Highlights

Radio continuum emission from normal galaxies is made up of two key emission processes that are both closely related to recent star–formation (Condon 1992)
Jansky Very Large Array (VLA) observations that densely sample the 1–34 GHz frequency range, we adopt a Bayesian fitting procedure, where we use Hα 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 α found in = −0.53 s±pi0ra.0l2g,asluaxgigeess(t2in6g+−23t%ha)taandyofiunndgapnoapvuelraatgioennoofn-ctohsemrmicarlasypeecltercatlroinnds eixs responsible for the observed non–thermal emission

Summary

Introduction

Radio continuum emission from normal galaxies is made up of two key emission processes that are both closely related to recent star–formation (Condon 1992). Thermal (free–free) radio emission, originating from HII regions that have been ionized by massive stars (≥ 8 M ), has been shown to be a direct tracer of instantaneous star–formation that holds across many different galaxy types (Murphy et al 2012; Tabatabaei et al 2017). Thermal radio emission is an ideal, extinction–free tracer of star–formation (Murphy et al 2011), it is intrinsically faint, limited by its brightness temperature of typically 104 K and is overwhelmed at frequencies < 30 GHz by non–thermal (synchrotron) radio emission, and at frequencies > 200 GHz by the thermal re–radiation of stellar light by dust. Reference αJ2000 δJ2000 Galaxy Type IBm Distance.

Methods

Results

Discussion

Conclusion