Distant “Hungry Twin” Shows How Galaxies Grow

galaxy

The Umbrella Galaxy takes its name from a mysterious feature seen on the left here, that is now found to be debris from a tiny galaxy, only a 50th its size, shredded apart by gravity. The image is a combination of data from the 0.5-meter BlackBird Remote Observatory Telescope and Suprime-Cam on the 8-meter Subaru Telescope. The inset shows a small cluster of stars embedded in the stream, which marks the center of the disrupted galaxy (image by R. Jay Gabany).

MAUNA KEA, HAWAII – 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, according to newly published research co-authored by San Jose State University assistant professor of physics and astronomy Aaron Romanowsky.

This is important because our whole concept about what galaxies are and how they grow has not been fully verified,” Romanowsky said. “We think they are constantly swallowing up smaller galaxies as part of a cosmic food chain, all pulled together by a mysterious form of invisible dark matter. We sometimes get a glimpse of the hidden vista being lit up when a galaxy is torn apart. That’s what occurred here.”

The work, led by Caroline Foster of the Australian Astronomical Observatory, has used the Umbrella (NGC 4651) galaxy to reveal insights in galactic behavior.

An Umbrella Galaxy

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.

“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 present work is a follow-up to a 2010 study, led by Dr. David Martínez-Delgado (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 the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and world-leading laser guide star adaptive optics systems.

Speedy Tracers

DEIMOS (the DEep Imaging and Multi-Object Spectrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the Keck instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.

Keck Observatory is a private 501(c)3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

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.