"The surface of the Earth is the shore of the cosmic ocean... Recently, we've managed to wade a little way out, and the water seems inviting." - Carl Sagan
Merging galaxies illuminate the cosmic food chain
Scientists studying a ‘twin’ of the Milky Way have used the W. M. Keck Observatory and Subaru Observatory to accurately model how it is swallowing another, smaller galaxy.
Their findings have opened the way to a better understanding of how structure forms in the universe and are being published in the Monthly Notices of the Royal Astronomical Society this week.
The work, led by Caroline Foster of the Australian Astronomical Observatory, has used the Umbrella (NGC 4651) galaxy to reveal insights in galactic behavior.
The Umbrella lies 62 million light-years away, in the northern constellation of Coma Berenices. Its faint parasol is composed of a stellar stream, thought to be the remnants of a smaller galaxy being pulled apart by the large galaxy’s intense gravitational field. The Umbrella will eventually absorb this small galaxy completely.
The merging of small galaxies into larger ones is common throughout the universe, but because the shredded galaxies are so faint it has been hard to extract details in three-dimensions about how such mergers proceed. Using the most powerful optical facilities in the world, the twin, 10-meter Keck Observatory and the 8-meter Subaru Telescope, near the summit of Mauna Kea, Foster and her collaborators have determined enough about the character of the merger to provide a detailed model of how and when it occurred.
After taking panoramic images of the Umbrella with Suprime-Cam on Subaru, the scientists used the DEIMOS instrument, installed on the Keck II telescope, to map out the motions of the stream and hence determine how the galaxy is being shredded.
The stars in the stream are incredibly faint, so it was necessary to use a proxy technique to measure the speeds of brighter tracer objects moving along with the stream stars. These bright tracers include globular star clusters, planetary nebulae (dying stars that glow like neon lights), and patches of glowing hydrogen gas.
"This is important because our whole concept about what galaxies are and how they grow has not been fully verified," said co-author Aaron Romanowsky, an astronomer at both San Jose State University and University of California Observatories. "We think they are constantly consuming smaller galaxies as part of a cosmic food chain, all pulled together by a mysterious form of invisible ‘dark matter.’ When a galaxy is torn apart, we sometimes get a glimpse of the hidden vista because the stripping process lights it up. That’s what occurred here."