Aussie astronomers find ‘lost city’ of stars

Like archaeologists unearthing a lost city, an Australian-led team of astronomers using the 8-m Gemini South telescope have revealed the faint ancient outer parts of the galaxy NGC 300, showing that the galaxy is at least twice as big as previously thought.

The finding implies that our own Galaxy too is probably much bigger than textbooks say.

And ideas on how galaxies form will have to be rethought, to explain how NGC 300 could have stars so far out from its centre.

The research is published today [10 August] in the Astrophysical Journal.

NGC300 is a spiral galaxy 6.1 million light-years away. It looks rather like our own Galaxy, with most of its stars lying in a thin disk like a pancake.

Using the Gemini Multi-Object Spectrograph instrument on the Gemini South telescope in Chile, the observers were able to see stars in the disk up to 47,000 light-years [14.4 kpc] from the galaxy’s centre—double the previously known radius of the disk.

These were extremely sensitive measurements capturing light levels more than ten times fainter than any previous images of this galaxy.

A few billion years ago the outskirts of NGC 300 were brightly lit suburbs that would have shown up as clearly as its inner metropolis. But the suburbs have dimmed with time, and are now inhabited only by faint, old stars—stars that need large telescopes such as Gemini South to detect them.

The finding has profound implications for our own Galaxy. Most current estimates put its size at 100,000 light-years across, about the same as the new estimate for NGC 300. “However, our galaxy is much more massive and brighter than NGC 300. So on this basis, our Galaxy is also probably much larger than we previously thought—perhaps as much as 200,000 light-years across,” said the paper’s lead author, Professor Joss Bland-Hawthorn of the Anglo-Australian Observatory.

Galaxy keeps on keeping on

The observers found no evidence that the outer part of NGC 300 was falling abruptly in brightness, or truncating, as many galaxies do.

“We now realize that there are distinctly different types of galaxy disks,” said team member Professor Ken Freeman of the Research School of Astronomy and Astrophysics at the Australian National University. “Probably most truncate—the density of stars in the disk drops off sharply. But NGC 300 just seems to go on forever. The density of stars in the disk falls off very smoothly and gradually.”

The observers traced NGC 300’s disk out to the point where the surface density of stars was equivalent to a one-thousandth of a Sun per square light-year.

“This is the most extended and diffuse population of stars ever seen,” said Bland-Hawthorn.

How did the stars get there? Were they formed in place or did they come from somewhere else?

A previous study has shown that the disk contains cold hydrogen gas that extends well out past even the new-found stars. Some stars could have formed from this. But it’s hard to see how that hydrogen gas could have come into the galaxy—either accreting onto the galaxy as gas per se, or dumped in by mergers with other small galaxies—and still left the disk density so smooth.

Mixing processes could have scattered stars out from the inner parts of the disk. “But it’s hard to see how that could produce such an extensive stellar disk that falls off so smoothly in density,” said Freeman.

The researchers have been granted more time on Gemini South to determine exactly what kind of stars they are seeing in the outskirts of NGC 300, and to make similar studies of other galaxies.

NGC 300 is the nearest member of the Sculptor group of galaxies, and is the first galaxy outside of our Local Group of galaxies to be studied to great depth. There have only been two other galaxies studied to such faint levels, the Andromeda galaxy and its neighbour M33, both in our Local Group. “We still have a lot to learn about how galaxies like ours formed,” said Bland-Hawthorn. “The next Gemini observations later in the year should provide more important clues.”  

The Gemini Observatory

The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna Kea, Hawai'i (Gemini North) and the other telescope at Cerro Pachón in central Chile

(Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Particle Physics and Astronomy Research Council (PPARC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

Bruce Draine was partly supported by NSF grant AST-9988126.


Joss Bland-Hawthorn
Anglo-Australian Observatory, Sydney, Australia
+61-2-9960-6553 (Home)
+61-2-9372-4851 (Office)
+61-404-858-054 (Mobile)

Ken Freeman
Australian National University, Canberra, Australia
+61-2-6125-0264 (Office)
+61 402 134 289 (Mobile)

Bruce Draine
Princeton University
Princeton, NJ
+1-609-258-3810 (Office)


Joss Hawthorn
Hear Joss Bland-Hawthorn talk about this research.
Why the team chose to study NGC 300. (MP3, 1:4 6)
The approach they took: counting stars. (MP 3, 1:24)
Observing with Gemini. The results were surpr ising. (MP3, 2:23)
What do we know about how disks form? (M P3, 2:21)
What the finding suggests (MP3, 2:09)
Joss Bland-Hawthorn

Helen Sim

Anglo-Australian Observatory

+61-2-9372-4251 (Office)

+61-419-635-905 (Mob.)


Researcher contacts at end


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