Study provides clues about the formation and evolution of structures in the universe.
A Texas Tech University researcher and his team have used images from the Hubble Space Telescope to confirm three supernovae exploded in the emptiness of space after being flung from their galaxies millions or even billions of years earlier.
David J. Sand, an assistant professor in the Texas Tech Department of Physics; Melissa Graham, University of California-Berkeley; Dennis Zaritsky, University of Arizona-Tucson; and Chris J. Pritchet, University of Victoria in British Columbia, Canada, will present their analysis of the three supernovae at a conference at North Carolina State University in Raleigh. Their paper also has been accepted by the Astrophysical Journal.
"The universe is made of billions of galaxies, and there appears to be darkness in between," Sand said. "But it turns out to be more than that – there are stars between galaxies, they are just too faint to see individually. We undertook a big supernova survey to study these seemingly empty regions between galaxies because we want to know how many stars are there, too faint for us to see in typical observations. One way to light up those regions is to watch and look for a supernova. Then we compare that number in empty space to the number inside galaxies, and it tells you how many stars there are."
While most supernovae are found inside galaxies containing hundreds of billions of stars, these lonely supernovae were found between galaxies in three large clusters of several thousand galaxies each. Any planets around these intracluster stars, which exploded in what are called Type Ia supernovae, were no doubt obliterated by the explosions.
The study confirms the discovery between 2008 and 2010 of three apparently hostless supernovae by the Multi-Epoch Nearby Cluster Survey conducted by the Canada-France-Hawaii Telescope (CFHT). At that time, the CFHT was unable to rule out a faint galaxy hosting those supernovae, but the sensitivity and resolution of these images from the Hubble Space Telescope's Advanced Camera for Surveys clearly showed the stars exploded in empty space, far from any galaxy. They belong to a population of solitary stars that exist in most galaxy clusters.
"When we found these supernovae, we thought maybe there was a tiny galaxy there we couldn't see from the ground. But we got the Hubble Space Telescope and found there is literally nothing between these galaxies. There's not a little tiny galaxy, just maybe a couple stars here and there," Sand said. "It's the first time we've really confirmed that."
Stars and supernovae normally exist within galaxies, but sometimes, when galaxies are clustered together, extreme gravitational forces will wrench away about 15 percent of the stars. The cluster's mass keeps the stars bound within the sparsely populated intracluster regions.
Once dispersed, these lonely stars are too faint to be seen individually unless they explode as supernovae. Sand and his research teammates are studying bright supernovae in intracluster space as tracers to determine the population of unseen stars. Such information provides clues about the formation and evolution of large scale structures in the universe.
"Supernovae are great signposts for the universe," Sand said. "Certain types of supernovae are like 60-watt light bulbs, all with the same intrinsic brightness, so they let us measure distances. In this situation, we are using supernovae to light up some of the lowest density regions of the universe. Without them these stellar backwaters may never be understood."