A Radio Continuum Study of NGC 2082

We present radio continuum observations of NGC 2082 using ASKAP, ATCA and Parkes telescopes from 888 MHz to 9000 MHz. Some 20 arcsec from the centre of this nearby spiral galaxy, we discovered a bright and compact radio source, J054149.24-641813.7, of unknown origin. To constrain the nature of J054149.24-641813.7, we searched for transient events with the Ultra-Wideband Low Parkes receiver, and compare its luminosity and spectral index to various nearby supernova remnants (SNRs), and fast radio burst (FRB) local environments. Its radio spectral index is flat (${\alpha} = 0.02 \pm 0.09$) which is unlikely to be either an SNR or pulsar. No transient events were detected with the Parkes telescope over three days of observations, and our calculations show J054149.24-641813.7 is two orders of magnitude less luminous than the persistent radio sources associated with FRB 121102&190520B. We find that the probability of finding such a source behind NGC 2082 is P = 1.2%, and conclude that the most likely origin for J054149.24-641813.7 is a background quasar or radio galaxy.


Introduction
In the absence of an active galactic nucleus (AGN), a spiral galaxy's radio emission primarily derives from non-thermal synchrotron radiation from supernova remnants (SNRs), and thermal bremsstrahlung from Hii regions (Condon 1992;Filipović and Tothill 2021b,a). Thus, deep and wide radio surveys from the new generation of radio telescopes such as the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT can shed important light on the processes by which star formation shapes the interstellar medium (ISM).
NGC 2082 is G-type spiral galaxy (of SB(r)b morphology) in the Dorado constellation. It has an absolute B-band magnitude of M B = 12.79 (Lauberts and Valentijn 1989), a diameter of 10.16 kpc, and is located at a distance of 18.5 Mpc (Olivares E. et al. 2010) and redshift z = 0.00395. Unlike some other galaxies in the Dorado constellation (e.g. NGC 1566), NGC 2082 remains poorly studied, with its most notable feature being a type II supernova, SN1992ba (Evans and Phillips 1992).
Here, we study the radio properties of NGC 2082 using ASKAP, Australia Telescope Compact Array (ATCA) and Parkes radio telescope observations. We will also draw on Hubble Space Telescope (HST) observations. Section 2 presents our observations and data analysis of NGC 2082. Section 3 gives our results and discussion, and conclusions are presented in Section 4.

Observations & Data
NGC 2082 has been observed in the ASKAP-EMU 888 MHz radio continuum survey of the Large Magellanic Cloud (LMC; Pennock et al. 2021;Filipović et al. 2022), as well as in the ATCA 20 cm mosaic survey arXiv:2205.11144v2 [astro-ph.GA] 2 Jul 2022 . We have also made new observations from Parkes radio telescope, and obtained new and archival data from ATCA (pre-CABB) and the HST.

CABB
We observed NGC 2082 on 2019 November 30 th using the Australian Telescope Compact Array (ATCA) (project code C3275 with 1.5C array configuration).
The miriad 1 (Sault et al. 1995) and karma 2 (Gooch 1995) software packages were used for reduction and analysis. Imaging was completed using the multifrequency synthesis invert task with natural Briggs weighting (robust=0 for all images), and beam size of 4.5 × 4.1 arcsec, 1.9 × 1.8 arcsec, and 1.3 × 1.0 arcsec for 2100, 5500, and 9000 MHz images, respectively. The mfclean and restor algorithms were used to deconvolve the images, with primary beam correction applied using the linmos task. We follow the same process with Stokes Q and U parameters to produce polarisation maps, except with beam size of 5 × 5 arcsec (see Section 3.2 below).

Australian Square Kilometre Array Pathfinder
NGC 2082 was observed serendipitously in the ASKAP-EMU radio continuum survey of the Large Magellanic Cloud , at the edge of the 120 deg 2 field. This survey was performed at 888 MHz with a 288 MHz bandwidth and 13.9 × 12.1 arcsec beam size.

Parkes Radio Telescope
We observed NGC 2082 on 2021 June 14 th , 15 th and 18 th with the Parkes radio telescope (project code PX075) using the Ultra-Wideband-Low (UWL) receiver (Hobbs et al. 2020), which delivers radio frequency coverage from 704 MHz to 4032 MHz. All observations were pointed at J054149.24-641813.7 and executed using the transient search mode where data are recorded with 2-bit sampling every 64 µs in each of the 0.125 MHz wide frequency channels (26624 channels across the whole band).
The full UWL band was split into multiple 512 MHz subbands for the search of bursts. The search was performed using the pulsar searching software package PRESTO (Ransom 2001). Radio-frequency interference (RFI) were identified and marked using the PRESTO routine RFIFIND with a 1 s integration time. To determine the optimal dispersion measure (DM) steps of the search, we used the DDPLAN.PY routine of PRESTO for a DM range of 200 to 3000 cm −3 pc. Data were then dedispersed at each of the trial DMs using the PREPDATA routine with RFI removal based on the mask file produced by RFIFIND.
Single pulse candidates with a signal-to-noise ratio larger than seven were identified using the SIN-GLE PULSE SEARCH.PY routine for each dedispersed time series and for different boxcar filtering parameters (from 1 to 300 samples). Burst candidates were manually examined.

Hubble Space Telescope
NGC 2082 was first imaged by the Hubble Space Telescope in 1997, following the type II supernova SN1992ba (Evans and Phillips 1992), revealing a bright, face-on spiral galaxy (Fig. 1). Fig. 1 is a 3-colour image created with APLpy (Robitaille and Bressert 2012), using archival HST data 4 (Carollo et al. 2002), where the 4 Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). red channel uses I-band data (F814W filter), the blue channel uses B-band data (F435W filter), and the green channel is pseudo-green which has been constructed by stacking the red and blue channels (B+I-band). No detection of SN1992ba is reported; it cannot be seen in any of these images.

Results and Discussion
A striking feature in all our radio images of NGC 2082 is strong point radio source (J054149.24-641813.7) positioned 20 arcsec from the galaxy centre, as seen in the bottom-left image of Fig. 1 by the ATCA contours. We also note no detection of SN1992ba in any of our images. The 9000 MHz ATCA observations, with our highest resolution, shows an unresolved point radio source, regardless of the parameters used in the data reduction. The top-right subplot in Fig. 1 provides a better look at an 888 MHz emission peak flux density of 0.0013 Jy beam −1 opposing J054149.24-641813.7, and also away from the centre of galaxy. It is unlikely that the two radio sources are related. Finally, we note HST observations with F435W and F814W filters show no optical counterparts to either source, and there are no counterparts at other wavelengths.

Spectral index
In Table 1 we show the flux densities of J054149.24-641813.7, measured using CARTA 5 and treated as a point source. We assume that the flux density errors are <10 per cent. We estimate a flat radio spectral index of α = +0.02 ± 0.09 suggesting that the emission is predominantly of thermal origin if the source is located in NGC 2082 (Fig. 2). Such a flat spectral index would be very unusual among SNRs and radio pulsar sources (Urošević 2014;Bates et al. 2013;Dai et al. 2015) unless this source is an unresolved pulsar wind nebulae (PWN). However, a background galaxy (quasar) could explain this radio spectrum (see Section 3.5).
To measure the flux densities of NGC 2082's entire extended emission, we use the method described in  and , which includes careful region selection that also excludes J054149.24-641813.7. We measure reliable NGC 2082 flux densities at two frequencies (888 and 2100 MHz; Table 1), which allows us to estimate the spectral index of α = −0.15 ± 0.23. This flat radio spectral index is unusual for spiral galaxies Gioia et al. (1982), but consistent with thermal emission from Hii 5 https://cartavis.org/ regions across NGC 2082. As the nucleus of NGC 2082 does not show any radio compact source or emission above 0.1 mJy beam −1 , we suggest this might account for the unusually flat radio spectral index.

Polarisation
We also investigate if any polarisation from J054149.24-641813.7 or NGC 2082 can be detected in our ATCA images. The fractional linear polarisation (P ) of NGC 2082 was calculated using the equation: where P is the mean fractional linear polarisation, S Q , S U , and S I are integrated intensities for the Q, U , and I Stokes parameters, respectively. We calculate P 5500 MHz = 6±2 % (see Fig. 3a) and P 9000 MHz = 8±4 % (see Fig. 3c). Their associated polarisation intensity maps are seen in Figures 3b and 3d respectively. This weak polarisation associated with J054149.24-641813.7 is most likely explained if the source is of background origin (see Section 3.5).   In addition to the very different radio spectral indexes of J054149.24-641813.7 and SNR 1987A, which indicate different emission origins, J054149.24-641813.7 is probably too bright to be an SNR originating from NGC 2082.
Fast radio bursts (FRBs) are extremely bright transient events of unknown origin (Lorimer et al. 2007).
Our Parkes observations (see Section 2.3) over 3 days detected no transient events. Despite this, considering both FRB 121102 and 190520B show sporadic outbursts (Rajwade et al. 2020;Dai et al. 2022), it is plausible J054149.24-641813.7 could host a repeating FRB and we observed during a quiescent period.
If intrinsic sources associated with J054149.24-641813.7 are implausible, the most likely remaining possibility is an extragalactic background source, such as a quasar, radio galaxy or AGN. If so, we may expect to see some Hi absorption; however, there is currently no high resolution Hi data for NGC 2082. The flat spectral index together with somewhat weak polarisation at 5500 and 9000 MHz images argue in favour of J054149.24-641813.7 background origin.
Our observations (Tab. 1) show that the flux density at 5500 MHz is ∼4.0 mJy. From Wall (1994), we find that for observations at 5500 MHz there are ∼15 sources/deg 2 at ≥4.0 mJy. The probability of finding a source of such brightness behind NGC 2082 is then, where A is NGC 2082s area on the sky in deg 2 . We calculate P = 1.2%, given the radius of NGC 2082 is r = 0.016 deg (de Vaucouleurs et al. 1991).

Conclusions
Nearby spiral galaxy NGC 2082 was found to contain an bright, compact radio source, J054149.24-641813.7 (Fig. 1), which is most likely of background origin. The flux densities reveal J054149.24-641813.7 has a flat spectral index, indicating its source may be of thermal origin. We compare the luminosity of J054149. 24-641813.7 to SNR 1987A, QRS 121102, and QRS 190520B, finding that J054149.24-641813.7 is likely too bright and flat to be a supernova, and is probably not bright enough to be a persistent radio source with an embedded FRB progenitor.

Acknowledgements
The Australia Telescope Compact Array (ATCA) and Australian SKA Pathfinder (ASKAP) are part of the Australia Telescope National Facility which is managed by CSIRO. Operation of the ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. The ASKAP uses the resources of the Pawsey Supercomputing Centre. Establishment of the ASKAP, the Murchison Radio-astronomy Observatory, and the Pawsey Supercomputing Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund. We acknowledge the Wajarri Yamatji people as the traditional owners of the Observatory site. SD is the recipient of an Australian Research Council Discovery Early Career Award (DE210101738) funded by the Australian Government. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research made use of APLpy, an open-source plotting package for Python (Robitaille and Bressert 2012). This research is based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). We thank the anonymous referee for a constructive report and useful comments.
Data Availability All data are publicly available: • Parkes (project code PX075): https://data.csiro.au/ • ATCA CABB (project code C3275) and Pre-CABB (project code C466): https://atoa.atnf.csiro.au/query.jsp • ASKAP (project code AS101): https://data.csiro.au/ • HST (proposal ID 9395): https://hla.stsci.edu/hlaview.html Author Contribution Miroslav Filipović and Shi Dai contributed to the original discovery, conception and design of this study. Parkes data collection and analysis were performed by Joel Balzan and Shi Dai. The first draft of this manuscript was written by Joel Balzan and all authors commented on previous versions of the manuscript. Rami Alsaberi observed and reduced ATCA data. All authors read and approved the final manuscript.
Funding No funding was acquired for this research.

Declarations
Conflict of interest All authors declare that they have no conflicts of interest.