CHILLING: Continuum Halos in LVHIS Local Irregular Nearby Galaxies - Radio continuum spectral behavior of dwarf galaxies

TL;DR

This study analyzes radio continuum emission in 15 dwarf galaxies (11 LVHIS irregulars and 4 blue compact dwarfs) using ATCA multi-band data (L/S-, C-, X-bands) to assess detectability, spectral shapes, and the RC–FIR relation. It finds RC emission in 11 galaxies, with detection strongly linked to star formation rate and stellar mass rather than HI content or rotation velocity, and reveals prevalent spectral curvature incompatible with a simple power-law. Through a 19-model spectral fitting framework implemented with Bayesian MCMC, the authors show many spectra require free–free absorption and/or energy-loss breaks, highlighting complex CRE transport and cooling in low-mass systems. The observed RC deficit relative to the Yun et al. RC–FIR relation, especially for extended-diffuse emitters, underscores CRE energy losses as a key factor in dwarf galaxies, with implications for feedback, magnetic-field evolution, and the interpretation of RC surveys of low-mass systems.

Abstract

Dwarf galaxies, due to their shallow gravitational potentials, provide critical environments for studying feedback mechanisms from star formation and its impacts on dwarf galaxy evolution. In particular, radio continuum (RC) observations offer valuable insights into cosmic ray dynamics, which play a significant role in shaping these processes. This study investigates the detectability and spectral characteristics of RC emission in a sample of 15 dwarf galaxies (11 gas-rich, star forming dwarfs and 4 blue compact dwarfs) spanning a broad range of stellar masses and star formation histories. Using multi-band RC data (L/S-, C-, and X-band) from the Australia Telescope Compact Array, we analyse the physical conditions responsible for RC emission and explore the dominant emission mechanisms within these systems. RC emission is detected in 11 out of the 15 galaxies. Our results indicate that RC emission correlates strongly with star formation rate, far-infrared, and stellar mass, while dynamic parameters such as HI and rotational velocity exhibit no significant correlation with RC detectability. Spectral analysis reveals that the RC spectral energy distribution in these galaxies frequently deviate from a simple power-law behavior, instead displaying curvature that suggests more complex underlying physical processes. Statistical model comparison confirms that a single power-law model is inadequate to capture the observed spectral shapes, emphasising the necessity of more sophisticated approaches. Additionally, the observed radio-far-infrared correlation indicates that cosmic ray electrons in lower-mass dwarf galaxies cool more rapidly than they can escape (e.g. via galactic winds), resulting in a measurable RC deficit.

Figures (13)

• Figure 1: Colour-composite images from the DESI Legacy Imaging Surveys of the RC detected CHILLING sample, showing more diffuse emission, with overlaid ATCA radio emission contours starting at $3\,\sigma$ and increasing by a factor of $\sqrt{2}$ at a central frequency of 2.1 GHz. For IC 4662, we use the H$\alpha$ map from Hunter_2004 as the declination is too low for the DESI Legacy Imaging Surveys. The scale in the top left corner is calculated using the distance to the source. The beam is shown in the bottom left corner. The noise level $\sigma_{\rm 2.1\,GHz}$ can be taken from Table \ref{['obs_params']}.
• Figure 2: Colour-composite images from the DESI Legacy Imaging Surveys of the RC non-detected CHILLING sample, showing more compact emission, with overlaid 2.1 GHz ATCA radio emission contours starting at $3\,\sigma$ and increasing by a factor of $\sqrt{2}$. The scale in the top left corner is calculated using the distance to the source. The beam is shown in the bottom left corner. The noise level $\sigma_{\rm 2.1\,GHz}$ can be taken from Table \ref{['obs_params']}.
• Figure 3: Colour–composite images from the DESI Legacy Imaging Surveys of the CHILLING sample with non-detected or marginally detected RC emission, overlaid 2.1 GHz ATCA radio emission contours starting at $3\,\sigma$ and increasing by a factor of $\sqrt{2}$. The scale in the top left corner is calculated using the distance to the source. The beam is shown in the bottom left corner. The noise level $\sigma_{\rm 2.1\,GHz}$ can be taken from Table \ref{['obs_params']}.
• Figure 4: WISE color–color diagram showing in the background the locations of interesting classes of objects Wright_2010, while showing the CHILLING galaxies in dependency of their detectability category of RC emission. Dwarf galaxies with diffuse RC emission are shown in green, those with compact RC emission are shown in blue, and non-detections or weak detections are shown in orange. The WISE W1, W2 and W3 bands are $3.4\,\upmu$m, $4.6\,\upmu$m and $12\,\upmu$m, respectively.
• Figure 5: Correlation analysis of several galaxy parameters with the dependence of RC emission detection. It represents different physical properties, including absolute magnitude in the B-band $M_{\rm B}$, star formation rate (SFR), H i mass M$_{\rm HI}$, maximum rotation velocity of the gas corrected for inclination ($\varv_{\rm rot}$), mid-infrared magnitude at $3.4\,\upmu$m ($L_{\rm 3.4\upmu m}$) from WISE and far-infrared (FIR) luminosity at $60\,\upmu$m ($L_{\rm 60\upmu m}$) from IRAS. Galaxies with diffuse RC emission are shown in green, those with compact emission in blue, and galaxies with no or very weak detection in orange. The Kolmogorov–Smirnov (KS) test results are visualised as circles on the diagonal of the plot, with sizes ranging from 0 to 1; larger circles indicate greater differences between the distributions. The exact value from the KS-test are shown inside the circles. The color represents statistical significance based on the $p$-value: pink circles indicate significant differences ($p < 0.05$), while gray circles indicate non-significant results.