ASKAP EMU detection of an Odd Radio Circle (ORC) candidate: J094412-751016 (Anglerfish)

Abstract

We report diffuse extended radio-continuum emission spatially coinciding with the IR source WISEA J094409.17-751012.8, and a semi-variable star, V687 Carinae. We use 944 MHz radio data from the large-scale Evolutionary Map of the Universe (EMU) survey to analyse this diffuse emission (EMU J094412-751016), which we nickname "Anglerfish". We investigate if the spatially correlated infrared (IR) source, WISEA J094409.17-751012.8, is physically related to Anglerfish. The IR colours of WISEA J094409.17-751012.8 are indicative of an elliptical galaxy, raising the possibility that Anglerfish may belong to the newly-discovered class of extragalactic radio sources known as Odd Radio Circles (ORCs) with WISEA J094409.17-751012.8 as the host galaxy. We also investigate the possibility that Anglerfish is physically related to the star, V687 Carinae, and whether it may be a remnant from a previous epoch of stellar mass-loss. We determine that a physical association between the radio emission and the star is unlikely due to the emission's non-thermal nature and the star's weak stellar winds compared to the theoretical expansion velocity of the 'shell'. It is possible that Anglerfish may be a Galactic high-latitude supernova remnant (SNR); however, we find that the observed size and luminosity are not consistent with this scenario. We also investigate the ORC scenario, which we deem the most likely scenario based on the Anglerfish's observed properties such as size, brightness, lack of other frequency detections, and spectral index. We therefore propose Anglerfish as an ORC candidate, but note that additional radio and optical observations are vital to further constrain the properties and confirm this classification.

Figures (5)

• Figure 1: Four-panel image of Anglerfish radio-continuum emission. Top left: 944 MHz ASKAP radio-continuum image (linearly scaled) with a measured RMS noise level of $\sim$25-30$\mu$Jy beam$^{-1}$, and a 15$^{\prime\prime}$$\times$15$^{\prime\prime}$ convolved beam size shown in the bottom left corner. Contours are from the same image at levels of 60, 100, and 150 $\mu$Jy beam$^{-1}$. Top right: DSS2 IR image. The variable star V687 Car and the elliptical galaxy WISEA J094409.17$-$751012.8 are annotated in the image with the solid blue arrows. The dashed orange arrow shows the direction of proper motion of V687 Car. The image is linearly scaled and the contours are from the radio-continuum image at the same levels as the top left panel. Bottom left: Polarised intensity (PI) image with an RMS noise level of 10 $\mu$Jy beam$^{-1}$. The image is linearly scaled, and the contours are from the radio-continuum image at the same levels as the top left panel. The image is convolved to a beam size of 18$^{\prime\prime}$$\times$18$^{\prime\prime}$, shown in the bottom left corner. There are two point sources in PI at levels of $\sim$8$\sigma$ and $\sim$6$\sigma$, indicated by the blue arrows. Bottom right: RGBY image using radio, optical, and IR data. Red is EMU 944 MHz, green is DSS2 Red, blue is DSS2 blue, and yellow is DSS2 IR. All images are linearly scaled.
• Figure 2: Faraday Depth Spectrum of the brightest peak in the PI map displayed in Figure \ref{['Figure:RGBY']}, where it is indicated by the left blue arrow. In the top right inset, we show an inset zooming in on the peak, indicating a non-zero RM.
• Figure 3: Spectral index graph of the Anglerfish emission, using the flux density measurement from the EMU data, and the upper limits from the RACS and GLEAM data to generate shallow (dotted line) and steep (dashed line) limits for the spectral index. The uncertainty ranges for each of these spectral index fits is shown as a shaded blue region around each line. The hashed area in between represents the possible spectral index range.
• Figure 4: ASKAP EMU 944 MHz image of the Anglerfish radio emission with superimposed measurements used in section \ref{['stellar mass loss']}. The image is linearly scaled with the beam size shown in the bottom left corner. The black circle denotes the elliptical region defined in Section \ref{['results and discussion']}. The white dashed line represents V687 Car's tangential movement, with two arrows indicating its direction. The green square shows the location of V687 Car, the white 'X' shows the geometric centre of the emission (discussed in section \ref{['results and discussion']}, and the red circle denotes V687 Car's closest approach to the geometric centre.
• Figure A1: Histogram of the pixel values for RACS image (left) and MWA image (right). All pixel values are shown in grey, and the negative pixel values are shown in blue, both normalised to unit area. The different heights of the grey and blue histograms result from their different sample sizes when normalised to probability density. The blue histogram represents only the negative pixels (a narrower range) while the grey inclues all pixels (spanning a wider range including positive sources). The fitted Gaussian (the thick red line) was fit to only the negative values to estimate the background noise level, and the dashed red vertical lines on the left panel show the $\pm$3$\sigma$ limits.