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TYPE IA SUPERNOVAE LUMINOSITY DEPENDS ON THE METALLICITY OF THEIR HOST GALAXIES

Image: Combined HST and Chnadra imaging of the supernova remnant SNR 0509-67.5 in the LMC

To find distances in space, astronomers use objects called "standard candles", objects that give a certain, known amount of light. Because astronomers know how bright these objects truly are, they can measure their distance from us by analyzing how dim they appear.


Type Ia supernovae, observed in all kinds of galaxies, are produced by white dwarf stars (the condensed remnant of what used to be sun-like stars) in a binary system and they are standard candles. The companion star can be a giant star or even a smaller white dwarf. White dwarf stars are one of the densest forms of matter, second only to neutron stars and black holes.

If accretion of matter from a companion star or the merger with another white dwarf, push a white dwarf star over the Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf will rise, triggering explosive nuclear fusion reactions that release an enormous amount of energy (supernova explosion). The star explodes in about ten seconds, leaving no remnant. The resulting light is 5 billion times brighter than the Sun.

Image: Evolution of a Type Ia supernova. Credit: NASA/CXC/M.Weiss

Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance. In recent years Type Ia supernova have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy.

The metallicity of the progenitor system producing a type Ia supernova (SN Ia) could play a role in its maximum luminosity, as suggested by theoretical predictions. In a recent paper (Moreno-Raya et al. 2016) published in ApJL, the authors present an observational study to investigate if such a relationship there exists. Using the 4.2m William Herschel Telescope (WHT), at El Roque de Los Muchachos Observatory (La Palma, Spain), they have obtained intermediate-resolution spectroscopy data of a sample of 28 local galaxies hosting SNe Ia. From the emission lines observed in their optical spectrum, they derived the gas-phase oxygen abundance in the region where each SN Ia exploded.

Their data show a trend, with a 80% of chance not to be due to random fluctuation, between SNe Ia absolute magnitudes and the oxygen abundances of the host galaxies, in the sense that luminosities tend to be higher for galaxies with lower metallicities. This result seems like to be in agreement with both the theoretically expected behavior, and with other observational results.

The authors argue that the standard calibration tends to overestimate the maximum luminosities of SNe Ia located in metal-rich galaxies. This dependence might induce to systematic errors when is not considered in deriving SNe Ia luminosities and then using them to derive cosmological distances.


▪ Moreno-Raya et al., 2016 ApJL - On the dependence of the type Ia SNe luminosities on the metallicity of their host galaxies (arXiv)

 ▪ Type Ia supernova - HubbleSite - Chandra - Wikipedia
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