Characterizing the host galaxies and delay times of Ca-rich gap transients vs 91bg-like SNe and normal Type Ia SNe

TL;DR

This study systematically compares the host environments of Ca-rich gap transients, 91bg-like SNe, normal SNe Ia, and Type II SNe using ZTF-CLU data and two parallel host-property pipelines (CLU catalog and Prospector SED fitting with a continuity SFH). It finds that Ca-rich gap transients and 91bg-like SNe preferentially reside in massive, quiescent hosts, with their delay-time distributions peaking at long delays around $t_{ m peak} \,\approx\, 10^4$ Myr, much longer than SNe Ia ($\sim 10^3$ Myr) and II ($\sim 10$ Myr). The results support a shared old progenitor channel for Ca-rich gap transients and 91bg-like SNe and underscore the value of homogeneous host analyses, while acknowledging systematic mass and SFR discrepancies between Prospector and CLU. The work also pioneers DTDs for Ca-rich gap transients and motivates further hydrodynamical and population studies to connect progenitor scenarios with host demographics. $

Abstract

Calcium-rich gap transients are a faint, fast-evolving class of supernovae that show strong nebular Ca emission lines. Their progenitor systems are uncertain, but they are often associated with old and quiescent host galaxies. In this work, we compare the properties of the hosts of hydrogen-poor Ca-rich gap transients to the hosts of 3 other classes of supernova (SNe): normal Type Ia, 91bg-like, and Type II. We use data from the Zwicky Transient Facility (ZTF) Census of the Local Universe (CLU) experiment to build up our 4 SNe samples and identify the host galaxies. A combination of precomputed host properties from the CLU catalog and those derived from SED fitting are used to characterize each host's stellar mass, star formation rate, and specific star formation rate (sSFR). We find that the hosts of Ca-rich gap transients and 91bg-like SNe occupy a similar parameter space of mass and sSFR, and are more massive and quiescent compared to the hosts of Type Ia and Type II SNe. Additionally, we construct delay time distributions (DTDs) for our 4 samples, finding that Ca-rich gap transients and 91bg-like SNe have the longest peak delay times $\sim 10^4$ Myr, compared to the peak delay times of Type Ia SNe ($\sim 10^3$ Myr) and Type II SNe ($\sim 10$ Myr). The similarity of host environment and DTDs for Ca-rich gap transients and 91bg-like SNe motivates further analysis of the relationship of these two transient classes.

Figures (6)

• Figure 1: The discrepancy between stellar mass as found in prospector and the CLU catalog, plotted versus sSFR as found in prospector, for 91bg-like SNe and SNe Ia . Additional 91bg-like SNe detected later than 2020 and not in our main sample, but which we modeled in prospector, are also included. There appears to be a trend where the discrepancy decreases for star-forming galaxies with sSFR $> 10^{-11} \ \rm{yr}^{-1}$ (the boundary marked by the vertical dashed line).
• Figure 2: The stellar masses and specific star formation rates (sSFR) of the galaxy hosts of 4 classes of transients (Ca-rich gap transients, 91bg-like SNe, normal SNe Ia, Type II SNe). The horizontal line designates the fiducial boundary between star-forming and quiescent galaxies. Top: Masses and SFRs are retrieved directly from the CLU catalog for all hosts. Bottom: For the hosts of Type II SNe and SNe Ia, same as top panel. For the hosts of Ca-rich gap transients and 91bg-like SNe, masses and SFRs are estimated using prospector modeling.
• Figure 3: Similar to the bottom panel of Fig. \ref{['fig:SNe_comp']}, but with the addition of the Silver Sample, Ca-rich transient hosts identified in dong_physical_2022 that were modeled in prospector. Those from dong_physical_2022 labeled as "Other Ca-rich" have strong calcium emission, but are not identified as Ca-rich gap transients. The two points with downward arrows indicate upper limits.
• Figure 4: Posterior delay time distributions (DTDs) for Ca-rich gap transients, based on the stellar masses and star-forming/quiescent status of their host galaxies. Host properties are determined in prospector. The DTDs are assumed to be a power law, and the light gray lines show individual DTDs. The black line is the median event rate from the DTD posterior.
• Figure 5: Similar to Fig. \ref{['fig:DTD_Ca']}, but based on the host galaxies of 91bg-like SNe. Host properties are determined in prospector.