Free Astronomy Magazine May-June 2015

SPACE CHRONICLES the astronomers fine-tune the mod- els that map out the cluster's mass. "We will measure the time delays, and we'll go back to the models and com- pare them to the model predictions of the light paths," Kelly said. "The lens modelers, such as Adi Zitrin (Califor- nia Institute of Tech- nology) from our team, will then be able to adjust their models to more ac- curately recreate the landscape of dark matter, which dictates the light travel time." While making a rou- tine search of the GLASS team's data, Kelly spotted the four images of the exploding star on Nov. 11, 2014. The FrontierSN and GLASS teams have been searching for such highly magni- fied explosions since 2013, and this object is their most spec- tacular discovery. The supernova ap- pears about 20 times brighter than its natural brightness, due to the com- bined effects of two overlapping lenses. The dominant lensing effect is from the massive galaxy cluster, which focuses the supernova light along at least three separate paths. A secondary lensing effect occurs when one of those light paths hap- pens to be precisely aligned with a specific elliptical galaxy within the cluster. "The dark matter of that in- dividual galaxy then bends and refo- cuses the light into four more paths," Rodney explained, "generating the rare Einstein Cross pattern we are currently observing." The two teams spent a week analyzing the object's light, confirming it was the signature of a supernova. They then turned to the W.M. Keck Observa- tory on Mauna Kea, in Hawaii, to measure the distance to the supernova's host ga- laxy. The astronomers nicknamed the super- nova Refsdal in hon- or of Norwegian as- tronomer Sjur Ref- sdal, who, in 1964, first proposed using time-delayed images from a lensed super- nova to study the ex- pansion of the uni- verse. "Astronomers have been looking to find one ever since," said Tommaso Treu of the University of Cali- fornia, Los Angeles, the GLASS project's principal investigator. "The long wait is over!" The Frontier Fields survey is a three-year program that uses Hubble and the grav- itational-lensing ef- fects of six massive galaxy clusters to probe not only what is inside the clusters but also what is be- yond them. The three-year Fron- tierSN program stud- ies supernovae that appear in and around the galaxy clusters of the Frontier Fields and GLASS surveys. The GLASS survey is using Hubble's spectroscopic capa- bilities to study remote galaxies through the cosmic telescopes of 10 massive galaxy clusters, including the six in the Frontier Fields. T his illustration shows how four different images of the same supernova were created when its light was distorted and magnified by the huge galaxy cluster MACS J1149 +2223 in front of it. The light has been magni- fied and distorted due to gravitational lensing and as a result the images are arranged around the elliptical galaxy in a formation known as an Ein- stein cross. The massive galaxy cluster focuses the supernova light along at least three separate paths, and then when one of those light paths happens to be precisely aligned with a single elliptical galaxy within the cluster, a secondary lensing effect occurs. The dark matter associated with the ellipti- cal galaxy bends and refocuses the light into four more paths, generating the rare Einstein cross pattern that the team observed. [NASA & ESA] n