The sizes of the ~200 catalogued supernova remnants (SNRs) in M31 and M33 are log-normally distributed with a small variance, and Bayesian inference under the assumption that the Sedov-Taylor phase dominates SNR visibility implies a remarkably narrow distribution of ambient interstellar densities centered on warm-ionized-medium values. Elwood, Murphy and Díaz-Rodríguez (2019) showed this admits a three-way degeneracy: either core-collapse supernovae genuinely prefer the warm ionized medium, or SNR catalogs are biased toward the subset that explodes there (including the specific conjecture that the most massive progenitors explode inside H II regions and escape detection, which would reduce the total catalog by approximately 14 percent if all such progenitors were affected, a depletion the source states current data likely cannot pin down at the ~10 percent level), or the Sedov-dominance assumption itself fails because most remnants expand inside wind-blown cavities and/or enter the radiative phase early. The 2018-2026 literature leaves the degeneracy unresolved: the mass-dependent depletion is neither confirmed nor ruled out, the now-available thousand-object samples have not yet been used to test the size distribution beyond mean and variance, and no self-consistent visibility model with cavities and the radiative transition exists. The problem of determining which explanation (or quantified mixture) holds, and of confirming or bounding the high-mass depletion, remains open.

Background

A supernova remnant (SNR) is the nebula produced when a supernova's blast wave sweeps up surrounding interstellar gas; SNR catalogs in nearby galaxies are a primary tracer of recent supernova activity and are routinely used to infer supernova rates, progenitor masses, and interstellar-medium (ISM) structure. During the Sedov-Taylor phase - the adiabatic similarity solution in which the swept-up mass far exceeds the ejecta mass and the shock radius grows as R∝(E/n)1/5t2/5R\propto(E/n)^{1/5}t^{2/5} for explosion energy EE, ambient particle density nn, and age tt - a measured SNR size combined with EE and tt constrains nn. Elwood, Murphy and Díaz-Rodríguez (2019, MNRAS 483, 4551) compiled the diameters of about 200 optically selected SNRs in M31 and M33 (identified by elevated [S II]/Hα\alpha ratios, the standard optical discriminator between shock-excited SNR gas and photoionized gas) and, using Bayesian inference under the assumption that the Sedov phase dominates the visible lifetime, inferred the distribution of ambient densities: a mean μlog⁡n=−1.33\mu_{\log n}=-1.33 and variance σlog⁡n2=0.49\sigma^2_{\log n}=0.49 of log⁡10(n/cm−3)\log_{10}(n/\mathrm{cm}^{-3}). This is a strikingly narrow distribution centered near the densities of the warm ionized medium (WIM), the diffuse, roughly 10410^4 K ionized phase of the ISM. Monte Carlo simulations in the same paper show that SNR size distributions are statistically indistinguishable from log-normal for samples smaller than about 600 objects, so present catalogs encode only the mean and variance of the underlying distributions.

The narrow result forces a trilemma, stated explicitly in the source. First, core-collapse supernovae may genuinely explode preferentially in the WIM. Second, SNR catalogs may be biased toward the subset of supernovae that explode in the WIM; a specific, quantified sub-case is that the most massive progenitors (above roughly 35 M⊙35\,M_\odot) are born and die inside H II regions - the dense photoionized nebulae surrounding young massive stars - and their remnants escape [S II]/Hα\alpha-based identification, which would remove about 14 percent of expected SNRs if all such progenitors were affected, a depletion at the ~10 percent level that the source's own split of the sample by published progenitor masses could neither confirm nor exclude for lack of statistical power. Third, the Sedov-dominance assumption may fail: the sizes of cavities blown by progenitor stellar winds (of order 2-20 pc) coincide with the small end of the observed size distribution, and remnants that expand inside such low-density cavities or that leave the Sedov phase early (entering the radiative, momentum-conserving stage) would have sizes that no longer map simply onto ambient density, so a narrow inferred density distribution could be an artifact of incomplete evolutionary physics. Deciding among these explanations matters because biased SNR catalogs distort supernova-rate estimates, progenitor-mass distributions inferred from local star-formation histories, and the placement of supernova feedback in galaxy simulations.

Problem Statement

Determine which of the three explanations offered by Elwood et al. 2019 - a genuine preference of core-collapse supernovae for the warm ionized medium, a selection bias of SNR catalogs toward remnants exploding there (a specific quantified sub-case of which is that progenitors above roughly 35 M⊙35\,M_\odot explode inside H II regions and escape detection, depleting catalogs at the ~10 percent level), or a breakdown of the assumption that Sedov evolution dominates SNR visibility - accounts for the narrow ambient-density distribution inferred from supernova remnant size distributions, or what quantified mixture of the three holds. The common objective is to break this degeneracy by a quantitative determination of which of the three explanations dominates - or of the quantified mixture that holds.

References

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