In a quiet back alley of a galaxy, where stars are rare and old, a light suddenly flares up, brighter than an entire galaxy. This is a Type Ia supernova: a thermonuclear explosion of a white dwarf that astronomers use as a standard candle to measure cosmic distances. But among these candles, there are rare "defective" ones: the 2003fg subtype, which shines too brightly and can throw off the cosmic ruler. Exactly where they are born has long remained a mystery.
New work has, for the first time, systematically studied the host galaxies of twenty such supernovae. The scientists used a method that allows obtaining spectra for every point of a galaxy, like taking a map of its chemical composition. It turned out that 2003fg-like supernovae prefer poor outskirts: their median normalized distance from the galaxy center is 0.82, whereas for normal Type Ia supernovae it is 0.63. It is as if rare guests always showed up on the back streets rather than downtown.
The most striking difference is in oxygen content. At the explosion sites of 2003fg-like supernovae, the metallicity averages only 0.68 solar, and they occupy the most oxygen-poor environments among all Type Ia supernovae. This is no coincidence: if such explosions came from a single rapidly rotating white dwarf with super-Chandrasekhar mass, they would require a young stellar population rich in metals. But observations show the opposite: metal-poor, old outskirts. So the single-star scenario is ruled out, leaving the merger of two white dwarfs or a degenerate core.
Why does this matter for cosmology? Type Ia supernovae serve as "standard candles": knowing their true brightness, one can calculate distance. But 2003fg-like ones, after standardization, turn out to be systematically brighter, and if they end up in distant samples, they distort dark energy measurements. Now astronomers know that such supernovae are linked to metal-poor environments and can account for this correction. Rare guests on poor outskirts have ceased to be mere noise: they have become a key to the precision of the cosmic ruler.
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.