New ASKAP radio-continuum surveys of the Small Magellanic Cloud

TL;DR

This study delivers deep, high-precision ASKAP radio-continuum catalogues of the Small Magellanic Cloud at 944 and 1367 MHz, achieving sub-arcsecond astrometric accuracy and robust flux calibration through cross-matches with MeerKAT data. By applying a rigorous, multi-step source-detection and validation pipeline and deriving spectral indices across 14 frequencies via Orthogonal Distance Regression, the authors produce 36,571 and 15,227 reliable sources for the two bands and demonstrate strong consistency with external catalogues. The resulting mean spectral index of α ≈ −0.62 (with a peak near −0.8) reflects a dominant synchrotron population, while the spatially-resolved spectral-index mapping provides a valuable tool for distinguishing thermal and non-thermal emission in extended SMC structures. Public availability of the catalogues enables future multi-wavelength studies of SMC sources and informs our understanding of radio-source populations in dwarf galaxies and the capabilities of modern radio facilities like ASKAP.

Abstract

We present two new radio continuum images from the ASKAP POSSUM survey in the direction of the Small Magellanic Cloud. The two new source lists contain 36,571 radio continuum sources detected at 944 MHz and 15,227 sources at 1367 MHz, with beam sizes of approximately 14.5 by 12.2 arcsec and 8.7 by 8.2 arcsec, respectively. We used the Aegean software package to generate these point source catalogues, and together with the previously published MeerKAT catalogue, we estimated spectral indices for the full set of matched radio point sources. By cross-matching our ASKAP catalogues with the MeerKAT data, we identified 21,442 and 12,654 common point sources at 944 MHz and 1367 MHz, respectively, using a 2 arcsec matching radius. These new catalogues improve our understanding of the Small Magellanic Cloud and demonstrate the capability of current radio telescopes such as ASKAP to investigate diverse galactic source populations

Figures (12)

• Figure 1: ASKAP image of the SMC at 944 MHz. The beam size is $14\hbox{$.\!\!^{\prime\prime}$}5\times12\hbox{$.\!\!^{\prime\prime}$}2$ and the linearly scaled colour bar represents the image scale intensity range. The background noise level in the image is 26 $\mu$Jy beam$^{-1}$. Coordinates are in the epoch J2000.
• Figure 2: ASKAP image of the SMC at 1367 MHz. The beam size is 8$.\!\!^{\prime\prime}$7$\times8\hbox{$.\!\!^{\prime\prime}$}2$ and the linearly scaled colour bar represents the image scale intensity range. The background noise level in the image is 28 $\mu$Jy beam$^{-1}$. Coordinates are in the epoch J2000.
• Figure 3: Distribution of SIMBAD object types from cross-matched sources. G = galaxy; Rad = radio source; AG? = possible active galaxy; AGN = active galactic nucleus; rG = radio galaxy; RB? = possible radio burst source; X = X-ray source; PN = planetary nebula; Y*? = possible young stellar object; Psr = pulsar; Others = rare types.
• Figure 4: Overview of ASKAP 944 MHz analysis results. Top Left: Peak flux as a function of RMS for ASKAP 944 MHz on a logarithmic scale. The grey line indicates the threshold at 65 $\mu$Jy beam$^{-1}$. Retained sources are in blue and discarded sources in green. Top Right: Major Axis Ratio distribution for ASKAP 944 MHz sources. Retained sources are in blue, and discarded sources are in green. Bottom Left: Minor Axis Ratio distribution for ASKAP 944 MHz sources. Retained sources are in blue, and discarded sources are in green. Bottom Right: ASKAP 944 MHz image with green circles representing the masked sources, predominantly located at the edges of the image and in regions with BCE sources.
• Figure 5: Positional differences between ASKAP and external catalogues, based on cross-matching at 2$^{\prime\prime}$ radius. Each panel displays local density-colour-coded hexbin maps and marginal histograms. The black dashed lines in the red histograms represent the median values of the respective distributions. The points are colour-coded to indicate local density, from yellow for high density to purple for low density.