TY - JOUR
T1 - Radio emission from supernova remnants
T2 - Implications for post-shock magnetic field amplification and the magnetic fields of galaxies
AU - Thompson, Todd A.
AU - Quataert, Eliot
AU - Murray, Norman
N1 - Copyright:
Copyright 2009 Elsevier B.V., All rights reserved.
PY - 2009/8
Y1 - 2009/8
N2 - Using observational data from the literature, we show that the non-thermal radio luminosity (L) of supernova remnants (SNRs) is a strong function of the average gas surface density (Σg) of the galaxy in which the remnants reside, from normal spirals to dense luminous starbursts. Our result supports the interpretation of the radio sources in M82 and Arp 220 as normal SNRs, and not 'radio' supernovae (SNe). We combine a simple theory for electron cooling in SNRs with their observed radio luminosities to estimate the remnant magnetic field strength (BSNR): the correlation between L and Σg implies that BSNR also increases with Σg. We explore two interpretations of this correlation: (1) BSNR is generated by post-shock magnetic field amplification, with B2SNR ∝ Σg and (2) BSNR results from shock compression of the ambient interstellar medium (ISM) magnetic field (BISM), with BISM being larger in denser galaxies. We find that shock compression is, on average, sufficient to produce the observed radio emission from SNRs in the densest starburst galaxies; amplification of post-shock magnetic fields is not required. By contrast, in normal spirals modest post-shock field amplification in some remnants (a factor of ∼ few - 10) is consistent with the data; we find tentative evidence that both the Alfvén speed within SNRs and the ratio of B2 SNR/8π to the post-shock pressure ('εB') are constant in SNRs from galaxy to galaxy. We discuss observational tests that can be used to more definitively distinguish between these two interpretations of the radio luminosities of SNRs. Regardless of which is correct, the radio emission from SNRs provides an upper limit to BISM that is independent of the minimum energy assumption. For the densest starbursts, the magnetic energy density in the ISM is below the total ISM pressure required for hydrostatic equilibrium; thus magnetic fields are not dynamically important on the largest scales in starbursts, in contrast with spiral galaxies like our own. This dichotomy may have implications for galactic dynamo theory.
AB - Using observational data from the literature, we show that the non-thermal radio luminosity (L) of supernova remnants (SNRs) is a strong function of the average gas surface density (Σg) of the galaxy in which the remnants reside, from normal spirals to dense luminous starbursts. Our result supports the interpretation of the radio sources in M82 and Arp 220 as normal SNRs, and not 'radio' supernovae (SNe). We combine a simple theory for electron cooling in SNRs with their observed radio luminosities to estimate the remnant magnetic field strength (BSNR): the correlation between L and Σg implies that BSNR also increases with Σg. We explore two interpretations of this correlation: (1) BSNR is generated by post-shock magnetic field amplification, with B2SNR ∝ Σg and (2) BSNR results from shock compression of the ambient interstellar medium (ISM) magnetic field (BISM), with BISM being larger in denser galaxies. We find that shock compression is, on average, sufficient to produce the observed radio emission from SNRs in the densest starburst galaxies; amplification of post-shock magnetic fields is not required. By contrast, in normal spirals modest post-shock field amplification in some remnants (a factor of ∼ few - 10) is consistent with the data; we find tentative evidence that both the Alfvén speed within SNRs and the ratio of B2 SNR/8π to the post-shock pressure ('εB') are constant in SNRs from galaxy to galaxy. We discuss observational tests that can be used to more definitively distinguish between these two interpretations of the radio luminosities of SNRs. Regardless of which is correct, the radio emission from SNRs provides an upper limit to BISM that is independent of the minimum energy assumption. For the densest starbursts, the magnetic energy density in the ISM is below the total ISM pressure required for hydrostatic equilibrium; thus magnetic fields are not dynamically important on the largest scales in starbursts, in contrast with spiral galaxies like our own. This dichotomy may have implications for galactic dynamo theory.
KW - Galaxies: magnetic fields
KW - Galaxies: starburst
KW - Radio continuum: galaxies
KW - Supernova remnants
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U2 - 10.1111/j.1365-2966.2009.14889.x
DO - 10.1111/j.1365-2966.2009.14889.x
M3 - Article
AN - SCOPUS:68149161098
VL - 397
SP - 1410
EP - 1419
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 3
ER -