Astronomers Find Runaway Mini-Galaxies

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By exploiting the public records of the Sloan digital sky survey (SDSS) and the GALEX satellite, two astronomers including a researcher from the Institut de Recherche en Astrophysique et Planétologie (IRAP – CNRS / Université Paul Sabatier Toulouse 3) have successfully identified almost 200 new compact elliptical galaxies and 11 escaping mini-galaxies. These are the remnants of larger galaxies that have lost most of their stars and have been catapulted into the intergalactic void. Such objects are extremely rare since to date only one had been found! This research demonstrates the fundamental role that survey programs of celestial objects such as the SDSS and the Virtual Observatories in general play in astronomical research.

Schematic artist’s view of the phenomenon of expulsion of a mini galaxy as described by the 3-body interaction model of this research: an intruding spiral galaxy approaches a system composed of an elliptical mini galaxy orbiting a giant elliptical galaxy. During its passage, the intruder acts as a gravitational catapult that modifies the orbit of the mini galaxy. The latter is then propelled out of the system while the spiral galaxy is absorbed by the giant elliptical galaxy. ESA/Hubble; Andrey Zolotov (artwork)

We know of about two dozen runaway stars, and have even found one runaway star cluster escaping its galaxy forever. Now, astronomers have spotted 11 small, runaway galaxies that have been flung out of their homes to wander the void of intergalactic space.

“These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in,” said astronomer Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics (CfA). Chilingarian is the lead author of the study, which is appearing in the journalScience.

An object is a runaway if it’s moving faster than escape velocity, which means it will depart its home never to return. For a runaway galaxy, that speed is more than a several million miles per hour (2500 km/s).

Chilingarian and his co-author, Ivan Zolotukhin (IRAP, Toulouse, France), initially set out to identify new members of a class of galaxies called compact ellipticals. These tiny blobs of stars are bigger than star clusters but smaller than a typical galaxy, spanning only a few hundred light-years. In comparison, the Milky Way is 100,000 light-years across. Compact ellipticals also weigh 1000 times less than a galaxy like our Milky Way.

Prior to this study, only about 30 compact elliptical galaxies were known, all of them residing in galaxy clusters. To locate new examples Chilingarian and Zolotukhin sorted through public archives of data from the Sloan Digital Sky Survey and the GALEX satellite.

Their search identified almost 200 previously unknown compact ellipticals. Of those, 11 were completely isolated and found far from any large galaxy or galaxy cluster.

“The first compact ellipticals were all found in clusters because that’s where people were looking. We broadened our search, and found the unexpected,” said Zolotukhin.

These isolated mini-galaxies were unexpected because theorists thought they originated from larger galaxies that had been stripped of most of their stars through interactions with an even bigger galaxy. So, the mini-galaxies should all be found near big galaxies.

Not only were the newfound compact ellipticals isolated, but also they were moving faster than their brethren in clusters.

“We asked ourselves, what else could explain them? The answer was a classic three-body interaction,” stated Chilingarian.

A hypervelocity star can be created if a binary star system wanders close to the black hole at the center of our galaxy. One star gets captured while the other is thrown away at tremendous speed.

Similarly, a compact elliptical could be paired with the big galaxy that stripped it of its stars. Then a third galaxy blunders into the dance and flings the compact elliptical away. As punishment, the interloper gets accreted by the remaining big galaxy.

This discovery represents a prominent success of the Virtual Observatory – a project to make data from large astronomical surveys easily available to researchers. So-called data mining can result in finds never anticipated when the original data was collected.

“We recognized we could use the power of the archives to potentially unearth something interesting, and we did,” added Chilingarian.

Further Resources

  • 1 Smithsonian Astrophysical Observatory, 60 Garden Street MS09, Cambridge, MA 02138, USA.
  • 2 Sternberg Astronomical Institute, Moscow State University, 13 Universitetsky prospect, Moscow 119992, Russia.
  • 3 Institut de Recherche en Astrophysique et Planétologie, 9 Avenue du Colonel Roche BP 44346, 31028, Toulouse Cedex 4, France.

IRAP Contact

  • Ivan Zolotukhin (CNRS – IRAP) : ; Tel : 05 61 55 75 32



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