Searching for Balmer-dominated Type Ia Supernova Remnants in M33

TL;DR

This study searched for specific Balmer-dominated Type Ia supernova remnants in M33 using X-ray and emission line data but found none, raising questions about their absence despite M33's larger mass compared to the LMC.

Contribution

The paper provides the first systematic search for Balmer-dominated Type Ia SNRs in M33 and discusses potential reasons for their absence, highlighting differences among galaxies.

Findings

No Balmer-dominated Type Ia SNRs found in M33.

M33's lack of these SNRs is puzzling given its mass and comparison with LMC.

The absence cannot be explained by observational biases or environmental factors.

Abstract

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.