Astronomers from the European Southern Observatory (ESO) on Monday published a colorful image of a small galaxy in the constellation of Sagittarius.
In this week’s issue of Science, Oliver Müller (University of Basel, Switzerland) and colleagues report on the discovery of surprisingly organized motion among the satellite galaxies that swarm around Centaurus A (NGC 5128), a huge elliptical galaxy some 12 million light-years away.
Astronomers took the snapshot with the VIsible Multi-Object Spectrograph (VIMOS) instrument on ESO’s Very Large Telescope in Chile. The image shows the faint and fuzzy blue group of stars, which is just 3,000 light-years in diameter.
The object is officially known as the Sagittarius Dwarf Irregular Galaxy, or ‘SagDIG’ for short.
The French astronomer Diego Cesarsky together with his colleagues discovered the dwarf galaxy on a photographic plate obtained on June 13, 1977, with the 1-m Schmidt telescope at ESO’s La Silla observatory.
The Milky Way, together with SagDIG are both members of the local group of galaxies. However, the latter is the most distant member, thus, lying about 3 million light-years away from us.
The Weird Galaxies
Most of the dwarf galaxies around Centaurus A orbit the giant galaxy along a single plane. However, this is not the same case with the latest discovery.
Out of the 16 Cen A satellites with known radial velocities, 14 of them move in an orderly pattern. They orbit on a broad plane that’s more or less perpendicular to the large galaxy’s famous dust band, all 14 moving in the same circular direction.
However, unlike normal galaxies, dwarf galaxies are typically smaller and host a relatively small number of stars.
Gravitational tugs from nearby galaxies can often distort the spherical and disc-like shapes of these fragile objects. This very process may be responsible for SagDIG’s rectangular shape.
The likelihood of finding just one example of the coordinated motion of dwarf satellites in ΛCDM simulations is smaller than 0.5%, according to Müller and his co-authors. “Finding three such systems in the nearby universe seems extremely unlikely,” they write in Science.
“It’s an interesting result,” says Eline Tolstoy (University of Groningen, The Netherlands), an expert on dwarf galaxy evolution. “But it’s still small number statistics. Velocity measurements are only available for the very brightest satellites of Centaurus A — there may be an unknown bias there.”
“But still,” she adds, “it’s very intriguing.”
SagDIG has relatively few elements more massive than helium. So, the lack of heavy elements might mean that SagDIG is very young. Thus, the component stars had little time to create and disperse massive elements.
The small size could also indicate that it formed in the early Universe.
Thumbnail image: Centaurus A (NGC 5128) as seen through the MPG/ESO 2.2-meter telescope at the La Silla Observatory in Chile. Credit: ESO