Distant ‘cannibal twin’ shows how galaxies grow

The Umbrella Galaxy, NGC 4651

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 Galaxy

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

Twenty years ago, astronomers using AAT identified a new ‘dwarf’ galaxy, the Sagittarius dwarf, 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 other galaxies.

New follow-up study

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

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 José 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 Subaru and 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).

Publication

C. Foster, H. Lux, A.J. Romanowsky, D. Martínez-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”. MNRAS 442, 3544 (2014). Download refereed preprint from http://arxiv.org/abs/1406.5511

Image

(High-resolution: ngc4651_gabany.jpg)

Caption: 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, that has been shredded apart by gravity.  The image is a combination of data from the 0.5-metre BlackBird Remote Observatory Telescope and Suprime-Cam on the 8-meter Subaru Telescope.

Image copyright © 2014 R. J. GaBany (Blackbird Obs.)

Animation

File: Nbody_progenitor_300.gif

Caption: Animated 3D computer model of the Umbrella galaxy and its infalling prey. The trajectory of the cannibalised dwarf is shown in green, with shredded stars in white. The disc of NGC 4651 is illustrated with blue circles.

Credit: N. Singh/UCSC.