A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. I. Long-Term Evolution of M83

Abstract

We present analyses of deep radio observations of M83 taken with the Very Large Array spanning 15 yr, including never-before-published observations from 1990 and 1998. We report on the evolution of 55 individual point sources, which include four of the six known historical supernovae in this galaxy. A total of 10 sources have X-ray counterparts from a Chandra survey. Each of these sources show nonthermal spectral indices, and most appear to be X-ray supernova remnants. Comparing the radio source list to surveys in the optical and X-ray, we identify three optical/ X-ray supernova remnants. Nearly half of the detected radio sources in these observations are coincident with known H ii regions lying in the spiral arm structures of the galaxy. We also report on changes in emission from the complex nuclear region, which has shown variability at 20 cm wavelengths. We confirm that the peak radio emission from the nucleus is not coincident with the known optical center. One lesser nuclear peak is consistent with the optical/IR nucleus. Previous dynamical studies of a ''dark'' nuclear mass indicate a possible match to other radio nuclear emission regions in M83.

A C-Band Study of the Historical Supernovae in M83 with the Karl G. Jansky Very Large Array

“…The deep continuum VLA observations span a period of approximately 15 years, from 1982 to 1998. In Maddox et al (2006), we presented the data over the fifteen years of VLA monitoring with a consistent data analysis and imaging scheme to ensure the best possible comparative study of more than 50 sources and extensive diffuse emission regions across the galaxy. Some of our key findings include:…”

“…From the initial VLA maps, Maddox et al (2006) found 17 candidate radio SNRs, about half of which are also detected at optical and/or X-ray wavelengths. With this project, we will use previously detected VLA sources like SN1950B and other marginally radio-identified SNRs to explore their spectral index across both bands and apply what we learn to newly discovered sources to more clearly identify as SNRs or not with the VLA.…”

“…As part of a long-term study to detect radio emission from known (i.e., historical) supernovae (SNe), the Karl G. Jansky Very Large Array (VLA) 3 has been used to observe a number of galaxies (Cowan & Branch 1985;Cowan, Goss & Sramek 1991;Cowan, Roberts & Branch 1994;Eck et al 1998;Eck, Cowan & Branch 2000;Stockdale et al 2001;Stockdale et al 2006;Maddox et al 2006). The primary goal of these observations has been to learn more about the evolution of SNe into supernova remnants (SNRs), how radio emission is produced in these events, and the environment in which they occur.…”

“…Figure 1 illustrates an initial analysis of the new Australia Telescope Compact Array radio data in the C band and 160 ks of recent Chandra (160 Ks) X-ray observations of M83 and what we can begin to expect from deeper imaging of the galaxy with a total of 750ks of Chandra new observations. Previous studies of M83 by Maddox et al (2006) identified a number of radio point sources with C and L bands pseudo-continuum measurements that had radio spectral indices (α, S ν ∼ ν α ) that were neither clearly thermal nor non-thermal. This was likely caused by spatially coincident HII regions and SNRs with blended emission, causing the flux from thermal HII regions to dominate the higher frequency emission and the SNR flux to dominate at lower frequencies.…”

“…This made it difficult to definitively identify a possibly significant portion of the SNRs that are in the galaxy. In 2011, we proposed a scaled-array experiment to measure the spectral index evolution of a sample roughly double the size of the ∼ 50 sources that were detected by Maddox et al (2006), as they vary across the EVLA L and C Bands. This project gives comparable resolution at both C and L band, and allows us to image each sub-band separately, to contrast the full spectral profiles of each source detected in the two EVLA bands.…”