Diving for pearls with the Hubble Space Telescope: Astronomers help find star 'necklace' connecting elliptical galaxies

Scientists from Rochester Institute of Technology helped analyze data from the Hubble Space Telescope showing elliptical galaxies coalescing at the core of a dense galaxy cluster. The study is part of a program sponsored by the Hubble Space Telescope -- an international cooperation between NASA and the European Space Agency -- to look inside 23 massive clusters first catalogued in the Sloan Digital Sky Survey.

Findings of the study, "A thirty-kiloparsec chain of 'beads-on-a-string' star formation between two merging early type galaxies in the core of a strong-lensing galaxy cluster," will be published in an upcoming issue of The Astrophysical Journal Letters.

"These data were originally taken for a completely different purpose -- to study the bluish arcs on larger scales in the cluster," said Chris O'Dea, professor in RIT's School of Physics and Astronomy and a co-author on the paper. "We were not expecting to catch these two elliptical galaxies in this spectacular burst of star formation."

O'Dea and co-author Stefi Baum, professor and director of RIT's Chester F. Carlson Center for Imaging Science, were thesis advisers and mentors of the paper's lead author, Grant Tremblay, a post-doctoral fellow at the European Southern Observatory in Garching, Germany, and an inaugural alumnus of RIT's astrophysical sciences and technology Ph.D. program. Tremblay will join Yale University as a NASA Einstein Fellow in September.

The 100,000-light-year-long structure identified in the Hubble data is dotted with 19 young, blue star clusters like pearls on a string, evenly spaced and separated by 3,000 light-years. The star necklace will lose its shape in about 10 million years as each of the 19 stellar superclusters follows a different orbit, Tremblay said.

Earlier observations of star clusters forming in evenly distributed clumps in spiral galaxies could explain Tremblay's "serendipitous discovery" in the Hubble data.

"This phenomenon has never been seen before in merging elliptical galaxies," Tremblay said. "We have two big monsters and they're playing tug-of-war with this necklace."

Tremblay and his team suggest three possible scenarios that could have created the string-of-pearl stars between two elliptical galaxies:

• Merger -- Coalescing galaxies triggered a reservoir of cold gas into star formation
• Cooling flow of gas -- Hot gas from the X-ray atmosphere around the galaxies cooled into puddles of cold molecular gas and started to form stars
• Collision -- A galactic collision created an X-ray shock catalyzing the star formation by compressing the gas and cooling the plasma.

"Compared to a galaxy's lifetime of billions of years, star formation processes -- which take millions of years -- are quite brief," said Kevin Cooke, graduate student in RIT's astrophysical sciences and technology program. "To find such an event in early type galaxies where star formation is rare is an incredibly fortunate find. Research into star formation in galaxies helps address many fundamental questions about the universe, and this rare star formation event will help propel this field of knowledge."