Type Ia supernovae serve as standard candles for measuring cosmic distances, but rare subtypes, such as 2003fg-like ones, can distort this picture. Previously, only a few such objects were known, and their hosts were studied individually. This work joins a line of research linking supernova properties to their environment, but for the first time systematically, on a sample of twenty objects, shows that the key factor is low metallicity, not the youth of the stellar population.
Twenty host galaxies were observed using integral field spectroscopy on the 3.5-meter telescope at Calar Alto and the 8.2-meter VLT. For each galaxy, maps of the stellar population and gas distribution were constructed, and spectra were extracted both for the entire galaxy and for a region of 1 kiloparsec around the explosion site. From the hydrogen and oxygen emission lines, the star formation rate, stellar mass, and oxygen abundance were estimated using two independent calibrations.
Globally, the host galaxies of 2003fg-like supernovae have lower stellar mass (on average 9.36 dex), lower metallicity (8.52 dex, about 0.68 solar), and a somewhat higher specific star formation rate than hosts of normal Type Ia supernovae, but they are not as extreme as hosts of superluminous supernovae. Locally, at the explosion site, the surface densities of star formation rate and stellar mass are lower than for normal supernovae, consistent with a preference for galaxy outskirts (median normalized distance 0.82 versus 0.63 for normal ones). The most striking difference is metallicity: 2003fg-like supernovae occupy the most oxygen-poor environments among all Type Ia supernovae. A tentative positive correlation is also found between the width of the light curve and oxygen abundance: broader light curves occur in more metal-rich environments.
This work differs from neighboring studies in that, for the first time for a large sample, it measures the local properties of the environment at the explosion site, rather than relying on global galaxy parameters or extrapolating metallicity gradients. This allowed us to directly show that low metallicity is the determining factor, while the age of the stellar population plays no role. This approach rules out the scenario of a single rapidly rotating white dwarf with super-Chandrasekhar mass, since it requires a young population that is not observed.
The topic could develop toward detailed modeling of white dwarf mergers at low metallicity, to understand which specific configurations lead to 2003fg-like explosions. It is also important to obtain early spectra of such supernovae to directly measure the metallicity of the outer ejecta. Given upcoming surveys such as the Nancy Grace Roman Space Telescope, it is critical to assess how the evolution of the fraction of 2003fg-like supernovae with redshift will affect cosmological parameters.
The results will affect cosmology, since 2003fg-like supernovae, after brightness standardization, turn out to be systematically brighter and could shift estimates of dark energy parameters in distant samples. They are also important for the theory of binary star evolution and nucleosynthesis.
The next step is to collect a larger sample of 2003fg-like supernovae at high redshift and measure their fraction among all Type Ia supernovae at different epochs. In parallel, merger models need to be refined to predict how metallicity affects explosion properties.
In supernova astrophysics, the nature of the progenitors of Type Ia supernovae remains an unresolved question. This work directly links the rare 2003fg subtype to low metallicity, pointing to white dwarf mergers, but does not allow a definitive distinction between the double-degenerate and the degenerate core scenarios. This contributes to the general problem of the multiplicity of explosion channels for Type Ia supernovae.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This paper presents the largest analysis to date of the host galaxies of twenty Type Ia supernovae of the 2003fg subtype, using integral field spectroscopy. For each galaxy, we constructed stellar population and gas maps and measured global and local properties at the explosion site. We find that such supernovae prefer low-mass, oxygen-poor galaxies and explode in their outskirts with minimal metallicity, which contradicts the scenario of a single rapidly rotating white dwarf of super-Chandrasekhar mass but agrees with the merger of two white dwarfs.
Where its strength is
The strength of the work lies in the direct measurement of local conditions at the explosion site for a significant sample, which allowed the single super-Chandrasekhar white dwarf scenario to be ruled out and showed that the key factor is low metallicity, not the youth of the stellar population.
Where it could go next
Worth trying
Why this may matter
If 2003fg-like supernovae systematically arise in metal-poor environments, their contribution to cosmological measurements can be corrected, improving the precision of dark energy parameter determination.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.57, against a usual 0.64. It looks like few have drilled nearby.
2003fg-like supernovae make up only about 0.2% of all Type Ia supernovae, but because of their high luminosity, they may appear in distant cosmological samples more often than expected.