2026ApJ...998...13R

We present high-resolution profiles of the Hα, [N II], and [O III] lines of supernova remnants (SNRs) in M33. The 8 km s^–1 resolution allows us to separate emission from the photoionized shock precursors from the emission from shocked gas. We find that there is considerable scatter in the ratios of precursor intensity to shock intensity and that the precursors account for up to 26% of the total Hα emission in SNRs in our sample and 35% of the total [N II] and [O III] emission. This is generally similar to what is predicted by steady-state precursor models from MAPPINGS for the shock speeds in the 150-180 km s^–1 range, but it is unlikely that all the observed SNRs have shock speeds in this narrow interval. The ratios of precursor to shock emission in Hα indicate faster shocks (though selection effects may play a role). The discrepancies between observed and model ratios of precursor to shock emission may be because the precursors are not in steady state, because our fibers select only a portion of the SNRs, so that accidents of geometry can affect the ratio of precursor to shocked emission that we see, or because the plane-parallel MAPPINGS models do not account for geometrical dilution of the radiation field outside of the SNR. Regardless of the modeling issues, these observations point out that precursor emission needs to be taken into account when analyzing spectra obtained on SNRs, especially in galaxies more distant than the local group.