ASKAP-EMU radio continuum detection of planetary nebula NGC 5189: the "Infinity" nebula

Abstract

We report the radio continuum detection of well known Galactic Planetary Nebula (PN) NGC 5189, observed at 943 MHz during the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey. Two detections of NGC 5189 have been made during the survey, of better resolution than previous radio surveys. Both measurements of the integrated flux density are consistent with each other, at $S_{943\mathrm{MHz}}=0.33\pm 0.03$ Jy, and the spectral luminosity is $L_{943\mathrm{MHz}}$ = 8.89 $\times$ 10${}^{13}$ W m${}^{-2}$ Hz${}^{-1}$. Using available flux density measurements for radio-detections of NGC 5189, we calculate a radio surface brightness at 1 GHz and measure ${\Sigma }_{1\mathrm{GHz}}$ = 6.0 $\times$ 10${}^{-21}$ W m${}^{-2}$ Hz${}^{-1}$ sr${}^{-1}$, which is in the expected range for Galactic PNe. We measure an apparent size of 3.4${}^{\prime }$ $\times$ 2.2${}^{\prime }$ corresponding to physical diameters of 1.48 pc $\times$ 0.96 pc, and combine available radio observations of NGC 5189 to estimate a spectral index of $\alpha$ = 0.12 $\pm$ 0.05. Hence, we agree with previous findings that NGC 5189 is a thermal (free-free) emitting nebula. Additional measurements of the optical depth ($\tau =0.00246$) and electron density ($N_e=138c{m}^{-3}$) support our findings that NGC 5189 is optically thin at 943 MHz. Furthermore, the radio contours from the ASKAP-EMU image have been overlaid onto a Hubble Space Telescope (HST) Wide Field Camera 3 image, demonstrating that the radio morphology closely traces the optical. Notably, the contour alignment for the innermost region highlights the two envelopes of gas previously reported to be low-ionisation structures, which is considered a defining feature of post common-envelope PNe that surround a central Wolf-Rayet star.

9 pages, 4 figures. Accepted for publication in PASA

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