The $\mathsf{\Sigma}$ – Drelation for supernova remnants in nearby galaxies

Abstract

Abstract. This paper examines relations between the radio surface brightness Σ and the diameter D (also known as Σ − D relations) for a sample of extragalactic supernova remnants (SNRs) as constructed from a combination of published data and data from our own surveys. Our sample of extragalactic SNRs is the largest ever devised for the purpose of analyzing Σ − D relations. The main results of this paper may be summarized as follows: (i) the empirical relations for SNRs in 10 of the 11 nearby galaxies studied have the approximately trivial Σ ∝ D −2 form, therefore limiting their interpretation as physically meaningful relations. In addition, these relations are subject to selection effects rendering them even less useful. Further Monte Carlo simulations suggest that the effect of survey sensitivity has the opposite effect of volume selection (e.g. Malmquist bias, a volume selection effect that shapes the Galactic sample) by tending to flatten the slopes toward a trivial relation. In this case, the true slopes may be steeper than the observed slopes; (ii) compact M 82 SNRs appear to follow a uniquely different Σ− D relation in comparison to the larger, older SNRs in the other 10 galaxies. Monte Carlo simulations suggest that the probability of this difference arising by chance is ≈1% to 10%, depending on what is assumed regarding the underlying SNR population; (iii) three candidate hypernova remnants were identified in our sample of 11 nearby galaxies.

The $\Sigma-{\sf \textit D}$ relation for planetary nebulae

“…For Galactic samples, Malmquist bias 4 may be the most severe effect; this is similar to the situation for Galactic SNR samples (Urošević et al 2005).…”

“…1. Malmquist bias tends to increase the slope of the Σ − D relation for SNRs (Urošević et al 2005). Similar implications should be valid for PNe as well.…”

A statistical approach to radio emission from shell-type SNRs

“…For Galactic SNRs, Green (2005) highlighted the selection effects: (1) in the surface brightness, with a completeness limit of about 10 −20 W m −2 Hz −1 sr −1 , while SNRs below this limit are predominantly in regions where the Galactic background is low; (2) in the angular size, so that young but distant SNRs may be missed. For SNRs in other galaxies, Urošević et al (2005) also showed that there may be a selection effect in the integrated flux (valid for unresolved or mildly resolved SNRs), with the effect of leading to an observed slope shallower than the intrinsic one.…”

“…Various authors (e.g., Clark & Caswell 1976;Milne 1979;Caswell & Lerche 1979;Case & Bhattacharya 1998;Urošević et al 2005) have investigated this correlation. Originally applied as a tool for estimating SNR distances, it has been found to be unreliable for this purpose.…”

“…Originally applied as a tool for estimating SNR distances, it has been found to be unreliable for this purpose. Some authors (e.g., Green 2005;Urošević et al 2005) have also argued that this correlation may be affected by selection effects. For Galactic SNRs, Green (2005) highlighted the selection effects: (1) in the surface brightness, with a completeness limit of about 10 −20 W m −2 Hz −1 sr −1 , while SNRs below this limit are predominantly in regions where the Galactic background is low; (2) in the angular size, so that young but distant SNRs may be missed.…”