Two Yale astronomers have discovered that, contrary to previous beliefs, the area around the Milky Way contains the remains of small galaxies that were torn apart by the Milky Way’s gravity.
"It was once thought that the Milky Way's halo had a smooth distribution of stars," said Anna Katherina Vivas, an astronomy graduate student at Yale who presented the results today at the American Astronomical Society meeting in San Diego, CA. "Our study shows that this is clearly incorrect. The clumps we observed provide strong evidence in favor of the view that the halo contains debris from these small, ancient galaxies."
The study, titled, "The Quest RR Lyrae Survey: Searching for Debris of Ancient Galaxies in the Milky Way," looked at RR Lyrae variables—stars that are roughly half the mass of the sun and are more than 10 billion years old. Vivas said these stars have evolved to roughly 100 times the sun’s luminosity.
"Because the luminosities of RR Lyrae variables are well-determined, their distances from the sun can be accurately measured from their brightness," said Vivas, who conducted the research with her advisor, Robert J. Zinn, professor of astronomy at Yale.
RR Lyrae variables are found in all galaxies, including the low-mass dwarf galaxies that orbit the Milky Way. Vivas said these stars are easily discovered because they pulsate every six to 24 hours, which increases their brightness by two to three times.
The RR Lyrae variables were detected by observing the same region of the sky roughly 20 times over the course of one year with the QUEST camera. This yielded thousands of variable stars, but only about 150 that have the proper periods and light variations to be RR Lyrae variables.
The QUEST CCD camera was built under the direction of Charles Baltay, Eugene Higgins Professor and chair of physics, and professor of astronomy at Yale. QUEST, the quasar equatorial survey team, is a multi-national collaboration of astronomers and physicists, headed by Baltay. While QUEST's primary scientific goal is to search for new quasars and gravitational lenses, its powerful instrumentation is being used for a wide variety of astrophysical problems, including this RR Lyrae search. The QUEST camera, which consists of 16 2048x2048 pixel CCDs, is mounted on 1m Schmidt telescope at the Llano del Hato Observatory in Venezuela.
About 40 percent of the area of the sky that was surveyed by QUEST had been previously surveyed by the Sloan Digital Sky Survey (SDSS). SDSS had detected a clump of stars, both RR Lyrae variables and A type stars, that appeared to be the remains of a satellite galaxy that had been accreted by the Milky Way. Zinn said the QUEST survey has confirmed that the stars suggested to be RR Lyrae variables by the SDSS are indeed of that type.
"In addition," Zinn said, "QUEST has also detected approximately 2.5 times more RR Lyrae variables in the region of overlap with the SDSS, which greatly strengthens the evidence that this clump is a real feature. It has also shown that the clump extends far outside the region surveyed by SDSS."
This clump of stars lies approximately 150,000 light-years from the center of the Milky Way. It is roughly 30,000 light-years across and at least 60,000 light-years long.