A long time ago, in a galaxy far, far away… a massive star exploded. That cataclysm created an object called a supernova (similar to the one we call the Crab Nebula). At the time this ancient star died, own galaxy, the Milky Way, was just starting to form. The Sun didn't even exist yet. Nor did the planets. The birth of our solar system still more than five billion years in the future.
Light Echoes and Gravitational Influences
The light from that long-ago explosion sped across space, carrying information about the star and its catastrophic death. Now, about 9 billion years later, astronomers have a remarkable view of the event. It shows up in four images of the supernova created by a gravitational lens created by a galaxy cluster. The cluster itself consists of a giant foreground elliptical galaxy collected together with other galaxies. All of them are embedded in a clump of dark matter. The combined gravitational pull of the galaxies plus the gravity of dark matter distorts light from more distant objects as it passes through. It actually shifts the direction of the light's travel slightly, and smears the "image" we get of those distant objects.
In this case, the light from the supernova traveled by four different paths through the cluster. The resulting images we see here from Earth form a cross-shaped pattern called an Einstein Cross (named after physicist Albert Einstein). The scene was imaged by the Hubble Space Telescope. The light of each image arrived at the telescope at a slightly different time - within days or weeks of each other. This is a clear indication that each image is the result of a different path the light took through the galaxy cluster and its dark matter shell. Astronomers study that light to learn more about the action of the distant supernova and the characteristics of the galaxy in which it existed.
How Does this Work?
The light streaming from the supernova and the paths it takes are analogous to several trains that leave a station at the same time, all traveling at the same speed and bound for the same final destination. However, imagine each train goes on a different route, and the distance for each one is not the same. Some trains travel over hills. Others go through valleys, and still others make their way around mountains. Because the trains travel over different track lengths across different terrain, they do not arrive at their destination at the same time. Similarly, the supernova images do not appear at the same time because some of the light is delayed by traveling around bends created by the gravity of dense dark matter in the intervening galaxy cluster.
The time delays between the arrival of each image's light tell astronomers something about the arrangement of the dark matter around the galaxies in the cluster. So, in a sense, the light from the supernova is acting like a candle in the dark. It helps astronomers map the amount and distribution of dark matter in the galaxy cluster. The cluster itself lies some 5 billion light-years from us, and the supernova is another 4 billion light-years beyond that. By studying the delays between the times that the different images reach Earth, astronomers can glean clues about the type of warped-space terrain the supernova's light had to travel through. Is it clumpy? How clumpy? How much is there?
Answers to these questions aren't quite ready yet. In particular, the appearance of the supernova images could change over the next few years. That's because light from the supernova continues to stream through the cluster and encounter other parts of the dark matter cloud surrounding the galaxies.
In addition to the Hubble Space Telescope's observations of this unique lensed supernova, astronomers also used the W.M. Keck telescope in Hawai'i to do further observations and measurements of the supernova host galaxy distance. That information will give further clues into conditions in the galaxy as it existed in the early universe.