Using these supernovae, which act as natural “cosmic lenses,” astronomers can capture a magnified view of the remote universe.
Two teams of astronomers working independently, including Or Graur, an assistant research scientist at Johns Hopkins University and a research associate in the Museum’s Department of Astrophysics, have found three such exploding stars, called supernovae, far behind massive clusters of galaxies.
Their light was amplified and brightened by the immense gravity of the foreground clusters in a phenomenon called gravitational lensing. This effect is similar to a glass lens bending light to form an image. Astronomers use the gravitational-lensing technique to search for distant objects that might otherwise be too faint to see, even with today’s largest telescopes.
Astronomers from the Supernova Cosmology Project and the Cluster Lensing And Supernova survey with Hubble (CLASH) are using these supernovae in a new method to check the predicted magnification, or prescription, of the gravitational lenses.
Luckily, two and possibly all three of the supernovae appear to be a special type of exploding star called Type Ia supernovae, prized by astronomers because they provide a consistent level of peak brightness that makes them reliable for making distance estimates.
“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 in Piscataway, N.J., a member of the CLASH team. “Because we can estimate the intrinsic brightness of the Type Ia supernovae, we can independently measure the magnification of the lens, unlike for other background sources.”
The CLASH team’s results appear in the May 1 issue of The Astrophysical Journal and the Supernova Cosmology Project’s findings in the May 1 edition of the Monthly Notices of the Royal Astronomical Society.
Having a precise prescription for a gravitational lens will help astronomers probe objects in the early universe and better understand a galaxy cluster’s structure and its distribution of dark matter, say researchers. Dark matter cannot be seen directly but is thought to make up most of the universe’s matter.
“Type Ia supernovae first became famous when they were used to measure the accelerating expansion of the universe and bring about the realization that most of the universe is made of dark energy,” Graur said. “It’s exciting to discover that we can also use these distant explosions to study dark matter, another great mystery of modern astronomy.”
How much a gravitationally lensed object is magnified depends on the amount of matter in a cluster, including dark matter, which is the source of most of a cluster’s gravity. Astronomers develop maps that estimate the location and amount of dark matter in a cluster. The maps are the lens prescriptions that predict how distant objects behind the cluster will be magnified when their light passes through it.
The three supernovae in the Hubble study, two of which were found in 2012 and the other in 2010, were each gravitationally lensed by a different cluster. The teams measured the brightness of the lensed supernovae and compared them to the explosions’ intrinsic brightness to calculate how much they were 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 exploded between 7 billion and 9 billion years ago, when the universe was slightly more than half its current age of 13.8 billion years old.
To perform their analyses, both teams of astronomers used observations in visible light from Hubble’s Advanced Camera for Surveys and in infrared light from the Wide Field Camera 3. The research teams also obtained spectra from both space and ground-based telescopes that provided independent estimates of the distances to these exploding stars. Each team then compared its results with independent theoretical models of the clusters’ dark-matter content, concluding that the predictions fit the models.
Now that the researchers have proven the effectiveness of this method, the goal is to find more Type Ia supernovae behind behemoth lensing galaxy clusters. The astronomers estimate that they need about 20 supernovae spread out behind a cluster so they can map the entire cluster field and ensure that the lens model is correct.