Scientists using the W. M. Keck and Subaru observatories atop Mauna Kea have accurately modeled a “twin” galaxy of the Milky Way swallowing another, smaller galaxy.
The scientists’ 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, according to W. M. Keck Observatory.
The work, led by Caroline Foster of the Australian Astronomical Observatory, 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 gravitational field. The Umbrella will eventually absorb this small galaxy.
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 the mergers.
Using the twin, 10-meter Keck Observatory and the 8-meter Subaru Telescope atop Mauna Kea, Foster and her collaborators ascertained 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 the Suprime-Cam on Subaru, the scientists used the Deep Extragalactic Imaging Multi-Object Spectrograph instrument, which is installed on the Keck II telescope, to map the motions of the stream and determine how the galaxy is being shredded.
Because the stars in the stream are very faint, a proxy technique was used 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 Jos 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.”
“Through new techniques we have been able to measure the movements of the stars in the very distant, very faint, stellar stream in the Umbrella,” Foster said. “This allows us, for the first time, to reconstruct the history of the system.”
“Being able to study streams this far away means that we can reconstruct the assembly histories of many more galaxies,” Romanowsky said. “In turn that means we can get a handle on how often these ‘minor mergers’ — thought to be an important way that galaxies grow — actually occur. We can also map out the orbits of the stellar streams to test the pull of gravity for exotic effects, much like the moon going around the Earth but without having to wait 300 million years for the orbit to complete.”
The current work is a followup to a 2010 study, led by David Martinez-Delgado of the University of Heidelberg, which used small robotic telescopes to image eight isolated spiral galaxies, and found the signs of mergers — shells, clouds and arcs of tidal debris — in six of them.
The W. M. Keck Observatory operates two, 10-meter optical/infrared telescopes on the summit of Mauna Kea that feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and laser guide star adaptive optics systems.
