Washington - In 2010 astronomers were baffled when they found a supernova 30 times brighter than it should have been.
Named PS1-10afx it was identical to similar types of supernova saves for its intense brightness, leaving scientists wondering if their models were wrong. But new observations suggest it may not have been a ‘super-supernova’ at all, but rather its brightness was magnified by a hidden lens galaxy that has now recently discovered.
Type 1a supernovas, caused when a large star transfers material to a white dwarf until it explodes, are useful tools for understanding the universe. This is because they have similar luminosities regardless of where they occur in the universe, meaning they can be used as ‘distance markers’. So when PS1-10afx was found to break those previous theories, it had astronomers worried they had been wrong about the supernovas. This particular supernova occurred nine billion light years away, and shone with an intensity of 100 billion suns. In 2013, scientists at Harvard concluded it must be a type of new, significantly brighter class of supernova. But a new study in Science suggests it is nothing more than an average supernova, amplified by an intervening galaxy.
The study was carried out by a team of researchers led by Robert Quimby at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU). On analysis of the supernova, they weren’t so quick to agree with their compatriots at Harvard that it was a super-supernova.
‘PS1-10afx looked a lot like a Type 1a supernova,’ says Quimby, ‘but it was just too bright.’ Generally, when a supernova is found to be brighter than a regular Type 1a supernova it is either hotter, appearing blue, or involves a much larger initial size.
‘New physics would thus be required to explain PS1-10afx as an intrinsically luminous supernova,’ continues Quimby. Instead, the scientists believed that it was nothing more than an illusion and that a hidden galaxy was altering what we were seeing.
So in September 2013 they spent seven hours collecting light at the location of PS1-10afx using the Low-Resolution Imaging Spectrograph on the 10-metre Keck-1 telescope in Hawaii. In the glare of the galaxy containing PS1-10afx they found another galaxy, much dimmer, but of the right size to cause the lensing effect.
‘This second galaxy was faint enough to have previously gone unnoticed,’ says Anupreeta More, Astronomer at the Kavli IMPU. ‘But our analysis showed that it was still the right size to explain the gravitational lensing of PS1-10afx.’ Gravitational lensing occurs when a large, massive object like a galaxy bends the light of a distant source. The light from the distant source is gathered by the intervening galaxy and, when we finally see it on Earth, it appears amplified. The effect was first predicted by Einstein’s Theory of General Relativity – now, this discovery serves as another validation of the theory.
Such lensing galaxies for Type 1a supernovas had been predicted before, but none had been found. However, this discovery could lead to similar lensing galaxies being found in future. ‘We had existing predictions of what a gravitationally lensed Type Ia supernova would look like,’ says Masamune Oguri from the Department of Physics at the University of Tokyo.
‘But the small size of this lens galaxy and the large magnification it produced was not exactly what we were expecting for the first discovery. ‘However, this system may very well prove typical of discoveries to come. ‘Because more distant supernovae are more likely to be gravitationally lensed, lensed supernovae are typically highly magnified and located in the distant universe.
‘Our new approach allows us to find unresolved strong lensing events produced by such low-mass galaxies. ‘Thus, the expected number of gravitationally lensed Type Ia supernovae to be found in future surveys increases by an order of magnitude.’