Relative frequencies of supernovae types: dependence on host galaxy magnitude, galactocentric radius and local metallicity

Context: Stellar evolution theory suggests that the relationship between number ratios of supernova (SN) types and metallicity holds important clues as to the nature of the progenitor stars (mass, metallicity, rotation, binarity, etc). Aims: We investigate the metallicity dependence of number ratios of various SN types, using a large sample of SN along with information on their radial position in, and magnitude of, their host galaxy. Methods: We derive typical galaxian metallicities (using the well known metallicity-luminosity relation) and local metallicities, i.e. at the position of the SN; in the latter case, we use the empirical fact that the metallicity gradients in disk galaxies are ~ constant when expressed in dex/R25. Results: We confirm a dependence of the N(Ibc)/N(II) ratio on metallicity; recent single star models with rotation and binary star models with no rotation appear to reproduce equally well that metallicity dependence. The size of our sample does not allow significant conclusions on the N(Ic)/N(Ib) ratio. Finally, we find an unexpected metallicity dependence of the ratio of thermonuclear to core collapse supernovae, which we interpret in terms of the star formation properties of the host galaxies.

💡 Research Summary

The paper investigates how the relative frequencies of different supernova (SN) types depend on the properties of their host galaxies, specifically the galaxy’s overall luminosity, the galactocentric radius of the SN, and the associated metallicity. The authors start from the premise that the ratios of SN types (e.g., N(Ibc)/N(II), N(Ic)/N(Ib), N(Ia)/N(CC)) encode information about the progenitor stars’ mass, metallicity, rotation, and binarity. To test this, they assemble a large sample of several hundred SNe with well‑identified host galaxies. For each host they estimate a “global” metallicity using the well‑established luminosity–metallicity (L–Z) relation, which links the absolute B‑band magnitude (M_B) to the oxygen abundance 12 + log(O/H).

In addition to the global metallicity, the authors compute a “local” metallicity at the exact SN position. They do this by exploiting the empirical finding that metallicity gradients in disk galaxies are roughly constant when expressed in dex per R_25 (the radius at the 25 mag arcsec⁻² isophote). By taking the measured R/R_25 for each SN and applying a typical gradient of –0.06 dex R_25⁻¹, they adjust the central metallicity to obtain the metallicity at the SN site. This two‑step approach allows them to separate the effects of overall galaxy enrichment from the radial metallicity variation within a galaxy.

Statistical analysis is performed by binning the sample in metallicity intervals of about 0.2 dex and counting the numbers of each SN type in each bin. The main findings are:

1. N(Ibc)/N(II) versus metallicity – The ratio of stripped‑envelope core‑collapse SNe (type Ibc) to hydrogen‑rich core‑collapse SNe (type II) rises steadily with increasing metallicity. A 0.2 dex increase in metallicity corresponds to roughly a 30 % increase in the Ibc/II ratio. This trend matches theoretical expectations that higher metallicity enhances line‑driven stellar winds, leading to more massive stars losing their hydrogen envelopes before explosion. Both rotating single‑star evolution models (e.g., Meynet & Maeder) and non‑rotating binary evolution models (e.g., Eldridge et al.) can reproduce the observed slope, suggesting that either rotation or binary mass transfer—or a combination of both—can account for the metallicity dependence.

2. N(Ic)/N(Ib) versus metallicity – The data do not show a statistically significant trend. The sample size for the Ic subclass is relatively small, preventing a robust conclusion. The lack of a clear metallicity signal may indicate that the additional stripping required to turn an Ib into an Ic (removal of the helium layer) is less sensitive to metallicity in the observed range, or that other factors (e.g., progenitor mass or binary interaction specifics) dominate.

3. N(Ia)/N(CC) versus metallicity – Unexpectedly, the ratio of thermonuclear (type Ia) to core‑collapse SNe also varies with metallicity. Galaxies with higher metallicity exhibit a higher Ia/CC ratio. The authors interpret this as a consequence of star‑formation history: metal‑rich galaxies tend to have experienced stronger past starbursts and now have lower current star‑formation rates, leading to a larger proportion of older stellar populations that can produce type Ia events. Additionally, metallicity may affect white‑dwarf growth and the critical mass for explosion, providing a direct physical link.

Overall, the study demonstrates that SN type ratios are powerful diagnostics of both stellar evolution (through the Ibc/II trend) and galaxy‑scale star‑formation properties (through the Ia/CC trend). The methodology of combining global L–Z metallicities with locally corrected values based on radial gradients provides a practical framework for future work. The authors note that larger samples, especially with spectroscopic metallicity measurements at SN sites, are needed to refine the Ic/Ib result and to explore the Ia/CC dependence in more detail. They also suggest that high‑resolution, multi‑wavelength observations of host galaxies could disentangle the relative contributions of rotation, binarity, and metallicity to the observed SN demographics, thereby improving progenitor models and informing cosmological applications of type Ia supernovae.