Resolution of the Compact Radio Continuum Sources in Arp220

TL;DR

This study uses high-resolution radio imaging to analyze compact sources in Arp220, confirming their nature as supernovae and remnants, and deriving their physical properties and magnetic fields.

Contribution

It provides detailed measurements of SNe and SNRs in Arp220, extending size-luminosity relations to very small sources and estimating magnetic fields within SNR shells.

Findings

Resolved 7 sources at both wavelengths.

SNRs have diameters ≥ 0.27 pc and follow size-luminosity relations.

Estimated magnetic fields in SNR shells are 15-50 mG.

Abstract

We present 2 cm and 3.6 cm wavelength very long baseline interferometry images of the compact radio continuum sources in the nearby ultra-luminous infrared galaxy Arp220. Based on their radio spectra and variability properties, we confirm these sources to be a mixture of supernovae (SNe) and supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both wavelengths. The SNe generally only have upper size limits. In contrast all the SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent with them having just entered their Sedov phase while embedded in an interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the diameter-luminosity correlation for SNRs (and so also on the diameter-surface brightness relation) and extend these correlations to very small sources. The data are consistent with the relation L {\propto} D^{-9/4}. Revised equipartition arguments adjusted to a magnetic field to relativistic particle energy density ratio of 1% combined with a reasonable synchrotron-emitting volume filling factor of 10% give estimated magnetic field strengths in the SNR shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from compression of ambient ISM fields and must instead be internally generated. We set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in relativistic particles, 2%-20% of the explosion kinetic energy, is consistent with estimates from models that fit the IR-radio correlation in compact starburst galaxies.