On the size distribution of supernova remnants in the Magellanic Clouds
  The physical sizes of supernova remnants (SNRs) in a number of nearby
galaxies follow an approximately linear cumulative distribution, contrary to
what is expected for decelerating shock fronts. This has been attributed to
selection effects, or to a majority of SNRs propagating in "free expansion", at
constant velocity, into a tenuous ambient medium. We compile a list of 77 known
SNRs in the Magellanic Clouds (MCs), and argue that they are a fairly complete
record of the SNe that have exploded over the last ~20kyr, with most now in the
adiabatic, Sedov phase of their expansions. The roughly linear cumulative size
distribution (uniform in a differential distribution) can result from the
combination of a deceleration during this phase, a transition to a
radiation-loss-dominated phase at a radius that depends on the local gas
density, and a distribution of ambient densities varying roughly as rho^{-1}.
This explanation is supported by the observed -1 power-law distributions of
three independent tracers of density: HI column density, Halpha surface
brightness, and star formation rate from resolved stellar populations. In this
picture, the observed cutoff at r~30 pc in the SNR size distribution is due to
a minimum in the mean ambient gas density in the regions where supernovae (SNe)
explode. We show that M33 has a SNR size distribution similar to that of the
MCs, suggesting these features, and their explanation, may be universal. In a
companion paper (Maoz & Badenes 2010), we use our sample of SNRs as an
effective "SN survey" to calculate the SN rate and delay time distribution in
the MCs. The hypothesis that most SNRs are in free expansion, rather than in
the Sedov phase of their evolution, would result in SN rates that are in strong
conflict with independent measurements, and with basic stellar evolution
theory.
