The giant that turned out to be a dwarf

March 7, 2007

New data obtained on the apparent celestial couple, NGC 5011 B and C, taken with the 3.6-m ESO telescope, reveal that the two galaxies are not at the same distance, as was believed for the past 23 years. The observations show that NGC 5011C is not a giant but a dwarf galaxy, an overlooked member of a group of galaxies in the vicinity of the Milky Way.

The galaxy NGC 5011C is located towards the Centaurus constellation, in the direction of the Centaurus A group of galaxies and the Centaurus cluster of galaxies. The former is about 13 million light-years from our Milky Way, while the latter is about 12 times farther away.

The appearance of NGC 5011C, with its low density of stars and absence of distinctive features, would normally lead astronomers to classify it as a nearby dwarf elliptical galaxy. On the other hand, the galaxy's distance - as reported in the scientific literature - makes it a member of the more distant Centaurus cluster. As such, it would belong to the same cluster as its brighter, redder companion on the sky, the lenticular galaxy NGC 5011B, which is seen almost edge-on.

"This is however a problem," says Ivo Saviane, from ESO, who together with colleague Helmut Jerjen (Mt Stromlo Observatory, Australia) studied this peculiar system, "as despite the small distance between the two galaxies this would imply from their projection on the sky if they were indeed at the same distance - only 45,000 light-years, half the size of our Milky Way - there is no obvious sign of interaction between the two."

Moreover, if the two galaxies were at the same distance, then NGC 5011C would be larger than NGC 5011B in real size, making it a kind of galaxy never seen before.

Saviane and Jerjen therefore used the ESO 3.6-m telescope at La Silla to take images and spectra of the galaxies. The astronomers then found that contrary to what is published, the two galaxies have very different redshifts, with NGC 5011C moving away from us five times slower than its companion on the sky. "This indicates they are at different distances and not at all associated", says Jerjen. "Clearly, NGC 5011C belongs to the close group of galaxies centred around Centaurus A, while NGC 5011B is part of the much farther Centaurus cluster."

The astronomers also established that the two galaxies have very different intrinsic properties. NGC 5011B contains for example more heavy chemical elements than NGC 5011C, and the latter seems to contain only about 10 million times the mass of the Sun in stars and is therefore a true dwarf galaxy. For comparison, our Milky Way contains thousands of times more stars.

"Our new observations with the 3.6-m ESO telescope thus confirm a new member of the nearby Centaurus A group whose true identity remained hidden because of coordinate confusion and wrong distance estimates in the literature for the last 23 years," says Saviane.

With this new distance determination, the astronomers also established that NGC 5011C lies 500,000 light-years away from the dominant galaxy in its group, Centaurus A. Centaurus A (NGC 5128) is the nearest giant elliptical galaxy, at a distance of about 13 million light-years. It is currently merging with a spiral companion galaxy. It possesses a very massive black hole at its centre and is a source of strong radio and X-ray emission.

The research discussed here is presented in an article in press in the Astronomical Journal, vol. 133, p. 1892: "NGC 5011C: An overlooked dwarf galaxy in the Centaurus A group", by Ivo Saviane and Helmut Jerjen. It is also available at .

Source: European Southern Observatory

Explore further: Hubble spots a peculiar compact Blue Dwarf Galaxy

Related Stories

A deeper look at Centaurus A

May 16, 2012

( -- The strange galaxy Centaurus A is pictured in a new image from the European Southern Observatory. With a total exposure time of more than 50 hours this is probably the deepest view of this peculiar and spectacular ...

Watching a Cannibal Galaxy Dine

November 20, 2009

( -- A new technique using near-infrared images, obtained with ESO’s 3.58-metre New Technology Telescope (NTT), allows astronomers to see through the opaque dust lanes of the giant cannibal galaxy Centaurus ...

Black hole outflows from Centaurus A detected with APEX

January 28, 2009

( -- Astronomers have a new insight into the active galaxy Centaurus A (NGC 5128), as the jets and lobes emanating from the central black hole have been imaged at submillimetre wavelengths for the first time. ...

Recommended for you

Hubble captures a galactic waltz

November 26, 2015

This curious galaxy—only known by the seemingly random jumble of letters and numbers 2MASX J16270254+4328340—has been captured by the NASA/ESA Hubble Space Telescope dancing the crazed dance of a galactic merger. The ...

Earth might have hairy dark matter

November 23, 2015

The solar system might be a lot hairier than we thought. A new study publishing this week in the Astrophysical Journal by Gary Prézeau of NASA's Jet Propulsion Laboratory, Pasadena, California, proposes the existence of ...

Scientists detect stellar streams around Magellanic Clouds

November 23, 2015

(—Astronomers from the University of Cambridge, U.K., have detected a number of narrow streams and diffuse debris clouds around two nearby irregular dwarf galaxies called the Magellanic Clouds. The research also ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.