Searching for Balmer-dominated Type Ia Supernova Remnants in M33
  We have searched for Balmer-dominated Type Ia SNRs in M33 by selecting
thermal X-ray sources with $L_{\rm X} \ge 5\times10^{35}$ ergs s$^{-1}$,
identifying associated H$\alpha$ emission features, and checking their [S II]
and [O III] emission properties. Our search did not find any Balmer-dominated
Type Ia SNRs in M33. This result is puzzling because M33 is 2-3 times more
massive than the LMC, yet the LMC hosts five Balmer-dominated Type Ia SNRs and
M33 has none. We have considered observational biases, interstellar densities
and ionization conditions, Type Ia SN rate expected from the star formation
history and Type Ia SN delay time distribution function, and metallicity
effect. None of these can explain the absence of X-ray-bright Balmer-dominated
Type Ia SNRs in M33. It is intriguing that the Galaxy has X-ray-bright and
thermal Type Ia SNRs (Kepler and Tycho) as well as X-ray-faint and nonthermal
Type Ia SNRs (G1.9+0.3, SN1006, and RCW86), while the LMC does not have the
X-ray-faint and nonthermal ones and M33 does not have the X-ray-bright and
thermal ones.
