Po-Sheng Ou (ASIAA/NTU); You-Hua Chu (ASIAA); Chuan-Jui Li (ASIAA)
Balmer-dominated optical spectrum is a characteristic of young Type Ia supernova remnants (SNRs). To date, only 10 Balmer-dominated SNRs are known in the Galaxy and the Large Magellanic Cloud (LMC). We intend to search for Balmer-dominated SNRs in M33, the next sizeable galaxy beyond the LMC, to enlarge the sample in order to investigate properties of progenitors and environments of Type Ia SNe. The sample of young Type Ia SNRs in the Galaxy appears to show two distinct types of X-ray properties: the X-ray-bright thermal X-ray sources, such as Kepler and Tycho, and the X-ray-faint nonthermal sources, such as G1.9+0.3 and SN1006. The Balmer-dominated young Type Ia SNRs in the LMC are all X-ray-bright with X-ray luminosities greater than 5x10^35 erg/s. In our first search for Balmer-dominated Type Ia SNRs in M33, we used the criterion L_X > 5x10^35 erg/s, but did not find any convincing candidates in M33 (Lin et al. 2020, ApJ, in press). While this result is puzzling, it also indicates that different galaxies may have different populations of Type Ia SNe. In this work, we have examined all 662 X-ray sources in M33 identified by the ChASeM33 team (Tullmann et al. 2011), searched for their optical counterparts, and compared their H-alpha, [SII] and [OIII] images. We present the optical images of some promising candidates of X-ray-faint Balmer-dominated SNRs, and compare them with the Galactic Type Ia SNRs. If some of the candidates are confirmed in further observations, they will help us understand the populations of young Type Ia SNRs and their relationship with the types of their host galaxies.