First ever gravitationally lensed Type Ia supernovae found

Massive clusters of galaxies act as “gravitational lenses” because their powerful gravity bends light passing through them [1]. This lensing phenomenon makes faraway objects behind the clusters bigger and brighter — objects that might otherwise be too faint to see, even with the largest telescopes.

The new findings are the first steps towards the most precise prescription — or map — ever made for such a lens. How much a gravitationally lensed object is magnified depends on the amount of matter in a cluster — including dark matter, which we cannot see directly [2]. Astronomers develop maps that estimate the location and amount of dark matter lurking in a cluster. These maps are the lens prescriptions of a galaxy cluster and predict how distant objects behind a cluster will be magnified when their light passes through it. But how do astronomers know this prescription is accurate?

Now, two independent teams of astronomers from the Supernova Cosmology Project and the Cluster Lensing And Supernova survey with Hubble (CLASH) have found a new method to check the prescription of a gravitational lens. The three supernovae they analysed were each lensed by a different massive galaxy cluster. Luckily, one and possibly all three of them appeared to be a special type of exploding star that can be used as a standard candle. An astronomical “standard candle” is any type of luminous object whose intrinsic power is so accurately determined that it can be used to make distance measurements based on the rate the light dims over astronomical distances.

“Here, for the first time, we have found Type Ia supernovae that can be used like an eye chart for each lensing cluster,” explained Saurabh Jha of Rutgers University, USA, a member of the CLASH team. “Although other supernovae have been spotted behind galaxy clusters before, they have not been Type Ia. We cannot independently measure the magnification of the lens using other types of supernovae because we only know the intrinsic brightness of the Type Ia supernovae.”

The teams measured the brightnesses of the lensed supernovae and compared them to the explosion’s intrinsic brightness to calculate how much the exploding star was magnified due to gravitational lensing. One supernova in particular stood out, appearing to be about twice as bright as would have been expected if not for the cluster’s magnification power.

The three supernovae were discovered in the CLASH survey, which used Hubble to probe the distribution of dark matter in 25 galaxy clusters. Two of the supernovae were found in 2012; the other in 2010 to 2011.

To perform their analyses, both teams used Hubble observations alongside observations from both space and ground-based telescopes to provide independent estimates of the distances to these exploding stars [3].

In some cases the observations allowed direct confirmation of a Type Ia pedigree. In other cases the supernova spectrum was weak or overwhelmed by the light of its parent galaxy. In those cases the brightening and fading behaviour of the supernovae in different colours was used to help establish the supernova type.

“Our team believes that all three of the supernovae are likely to be Type Ia supernovae, although the classification of one remains somewhat ambiguous,” explained Supernova Cosmology Project team member Jakob Nordin of the E.O. Lawrence Berkeley National Lab and the University of California, both in Berkeley, USA. Nordin also is the lead author on the team’s science paper describing the findings.

Each team then compared its results with independent theoretical models of the clusters’ dark matter content. They each came to the same conclusions: that the predictions fit the models.

The Supernova Cosmology Project’s galaxy cluster models were created by team members Johan Richard of the University of Lyon in France, and Jean-Paul Kneib of Ecole Polytechnique Federale de Lausanne in Switzerland. “It’s really great to see that these supernovae are behaving in the way we expected,” says Kneib. “The more confirmation we get that our complex cluster models are correct, the more we can rely on them, and use them to probe the early Universe.”

However, astronomers still have more work ahead of them. “If you want to check the prescription of your lens, you really want to check it in more than one arbitrary place,” said Saul Perlmutter, of the E. O. Lawrence Berkeley National Lab and the University of California,and leader of the Supernova Cosmology Project. “If you could find about 20 supernovae behind that one cluster, then you would be able to map out the whole field and check that your whole understanding of the lens model is correct. But if you have only one, it’s still a good suggestive start that this one agrees with what you would have expected.”

The astronomers are optimistic that Hubble surveys such as Frontier Fields and future telescopes, including the infrared James Webb Space Telescope, will find more of these unique exploding stars.

Notes:
[1] Albert Einstein predicted this effect in his theory of general relativity.
[2] Dark matter is believed to make up the bulk of the Universe’s matter, and is therefore the source of most of a cluster’s gravity.
[3] The astronomers obtained observations in visible light from Hubble’s Advanced Camera for Surveys and in infrared light from the Wide Field Camera 3.