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Australian Astronomical Observatory

Distant 'cannibal twin' shows how galaxies grow

1 July 2014                                


A distant 'twin' of the Milky Way that is swallowing another galaxy has opened the way to a better understanding of how galaxies grow.

A team led by Dr Caroline Foster of the Australian Astronomical Observatory (AAO) has been studying the Umbrella galaxy, so called because of its 'parasol' of stars - the remains of a smaller galaxy it's consuming.

The Umbrella (NGC 4651) lies 62 million light-years away, in the northern constellation of Coma Berenices.

Twenty years ago, astronomers using the AAO's 4-m Anglo-Australian Telescope (AAT) identified a new 'dwarf' galaxy, the Sagittarius dwarf, which is being engulfed by our own Milky Way Galaxy.

This was the first sign that the Milky Way had fattened up - acquired stars - by 'snacking' on other, smaller, galaxies. Since then, astronomers have spotted stellar streams in more galaxies.

The present work is a follow-up to a 2010 study, led by David Martinez-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.

That study posited that the Umbrella galaxy's distinctive arc was the result of a single merger, rather than of several events over time - a result confirmed by the present work.

"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," Dr Foster said. "This allows us to reconstruct the history of the system, which we couldn't before."

Being able to study streams this far out means that many more galaxies can be put under the microscope, said co-author Dr Aaron Romanowsky (San Jose State University and University of California Observatories).

"In turn that means we can get a handle on how often these 'minor mergers' - an important way that galaxies grow - actually occur," he said.

For this work the astronomers used data from the 8-m Subaru and 10-m Keck telescopes in Hawai'i.

They determined the movement of the stars in the stream by using three sets of 'tracers': clusters of old stars (globular clusters); old, brightly glowing stars (planetary nebulae); and patches of glowing hydrogen gas (HII regions).

Helen Sim
+61 2 9372 4251 (office - Sydney, Australia: UTC+10)
+61 419 635 905 (mob.)

Dr Caroline Foster
+61 2 9372 4894 (office - Sydney, Australia: UTC+10)
+61 430 453 532 (mob.)

IMAGE (gabany_v4_high_contrast_n4651.jpg - 2.8 MB)
The Umbrella Galaxy, NGC 4651. Image copyright © 2014 R. J. GaBany

C. Foster, H. Lux, A.J. Romanowsky, D. Martinez-Delgado, S. Zibetti, J.A. Arnold, J.P. Brodie, R. Ciardullo, R.J. GaBany, M.R. Merrifield, N. Singh and J. Strader. "Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy". Accepted for publication in Monthly Notices of the Royal Astronomical Society. Online at .