If you need to check whether the prescription for your eye glasses or contact lenses is still accurate, you visit an ophthalmologist for an eye exam. The doctor will ask you to read an eye chart, which tests your visual acuity. Your score helps the doctor determine whether to change your prescription.
Astronomers don't have a giant eye chart to check the prescription for natural cosmic lenses, created by galaxy clusters. The gravity of these cosmic lenses warps space around them, magnifying and brightening the light from distant objects behind them. Without these lenses, background objects would be too dim to be detected by even NASA's Hubble Space Telescope. But how do astronomers know whether the prescription for these zoom lenses, which tells them how much an object will be magnified, is accurate? Astronomers using the Hubble telescope have discovered the next best thing to a giant cosmic eye chart: the light from distant exploding stars behind galaxy clusters.
What could be more exciting than watching the fireworks of cataclysmic stellar explosions outshining entire galaxies of stars? How about watching them through the funhouse lens of a massive cluster of galaxies whose powerful gravity warps space around it?
In fact, distant exploding stars observed by NASA's Hubble Space Telescope are providing astronomers with a powerful tool to check the prescription of these natural "cosmic lenses," which are used to provide a magnified view of the remote universe.
Two teams of astronomers working independently 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. First predicted by Albert Einstein, 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 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."
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 believed to make up most of the universe's matter.
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 based on theoretical models and on the observed amplification and bending of light from sources behind the cluster. The maps are the lens prescriptions that predict how distant objects behind the cluster are magnified when their light passes through it.
"Building on our understanding of these lensing models also has implications for a wide range of key cosmological studies," explained Supernova Cosmology Project leader Saul Perlmutter of the E.O. Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley. "These lens prescriptions yield measurements of the cluster masses, allowing us to probe the cosmic competition between gravity and dark energy as matter in the universe gets pulled into galaxy clusters." Dark energy is a mysterious, invisible energy that is accelerating the universe's expansion.
The three supernovae in the Hubble study were each gravitationally lensed by a different cluster. The teams measured the brightnesses of the lensed supernovae and compared them to the explosions' intrinsic brightnesses 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 supernovae were discovered in the CLASH survey, a Hubble census that probed the distribution of dark matter in 25 galaxy clusters. Two of the supernovae were found in 2012, the other in 2010. The three supernovae exploded between 7 billion and 9 billion years ago, when the universe was slightly less 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. In some cases the spectra 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 astronomers also used different colored filters on Hubble to help establish the supernova type.
Each team then compared its results with independent theoretical models of the clusters' dark-matter content, concluding that the predictions fit the models.
"It is encouraging that the two independent studies reach quite similar conclusions," explained Supernova Cosmology Project team member Jakob Nordin of Berkeley Lab and the University of California, Berkeley. "These pilot studies provide very good guidelines for making future observations of lensed supernovae even more accurate." Nordin also is the lead author on the team's science paper describing the findings.
Now that the researchers have proven the effectiveness of this method, they need to find more Type Ia supernovae behind behemoth lensing galaxy clusters. In fact, the astronomers estimate 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.
They are optimistic that Hubble and future telescopes, including NASA's James Webb Space Telescope, an infrared observatory, will nab more of these unique exploding stars.
"Hubble is already hunting for them in the Frontier Fields, a three-year Hubble survey of the distant universe using massive galaxy clusters as gravitational lenses," said CLASH team member Brandon Patel of Rutgers University, the lead author on the science paper announcing the CLASH team's results. Steven Rodney of Johns Hopkins University, and co-leader of the CLASH supernova team, will direct the search for Type Ia supernovae in the Frontier Fields data.
The CLASH team's results will 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.
The CLASH survey is led by Marc Postman of the Space Telescope Science Institute in Baltimore, Md. The CLASH supernova project is co-led by Rodney and Adam Riess of the Space Telescope Science Institute and Johns Hopkins University. Aiding with the analysis on the Hubble study are Curtis McCully of Rutgers University, Or Graur of the American Museum of Natural History in New York City, and Julian Merten and Adi Zitrin of the California Institute of Technology in Pasadena.
Other members of the Supernova Cosmology Project who worked on the supernova analysis are David Rubin of Florida State University in Tallahassee and Greg Aldering of Berkeley Lab. The project's galaxy cluster models were created by Johan Richard of the University of Lyon in France and Jean-Paul Kneib of École Polytechnique Fédérale de Lausanne in Switzerland.