Hubble Unveils a Mystery 1.2 Billion Light Years Distant

While observing stars in our own Milky Way galaxy with the Hubble Space in an ongoing study that has spanned several years, astronomers stumbled upon a distant elliptical galaxy 1.2 billion light years away packed with clusters of stars too dim for most telescopes to see. Oddly, the light from some of these clusters are not young and blue as expected, but rather, older and redder -an observation Jason Kalirai of the University of California, Santa Cruz, and colleagues are wrestling to understand.

Globular clusters are tight-knit collections of stars that are among the oldest surviving structures in the universe. Our own Milky Way has at least 158 known clusters. While taking an image of one Kalirai accidentally captured a rare gem in the background: a distant elliptical galaxy brimming with its own collection of the clusters.

Kalirai and Harvey Richer of the University of British Columbia led an earlier study, which began as an investigation of a globular star cluster in the Milky Way galaxy known as NGC 6397. The researchers acquired one of the deepest optical images ever taken with the Hubble Space Telescope's Advanced Camera for Surveys, focusing on a small field within NGC 6397. This cluster, home to hundreds of thousands of stars, is 8,500 light-years away, making it one of the closest globular clusters to Earth.

Hidden in the background, however, were findings that may hold even greater promise for understanding the evolution of such clusters, Kalirai said. Within the population of stars and galaxies behind NGC 6397, the Hubble image revealed the large elliptical galaxy that contains several hundred globular clusters, which led to the new discovery and mystery of the apparently older, reddish stars.

Kalirai and Richer followed up the Hubble imaging observations with spectroscopic observations using the Gemini Multi-Object Spectrograph on the Gemini South Telescope on Cerro Pachon in Chile. They were able to determine the distance of the elliptical galaxy hosting the globular clusters by measuring its redshift (a measure of how the expansion of the universe shifts the wavelengths of light from a distant object). This showed that the globular clusters are the most distant ever studied.

Previous studies by other researchers of globular clusters in nearby galaxies, including the Milky Way, have shown that these systems play a very important role in understanding the formation and evolution of galaxies. With a sample of almost 200 clusters in this one distant galaxy, Kalirai's team will test whether the properties of these globulars are consistent with the idea that elliptical galaxies formed the bulk of their stars at early times. For the first time, the observations may also allow astronomers to test for evolution in the properties of globular clusters themselves, Kalirai told New Scientist.

Posted by Casey Kazan.

Sources:

New Scientist Link

http://www.universityofcalifornia.edu/news/article/8826
Journal reference: Astrophysical Journal Letters (vol 682, forthcoming)