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Our Galaxy could be a lot bigger than we thought. That's the
conclusion of an Australian-led team that's found whole new 'suburbs'
of stars in another galaxy.
Like archaeologists unearthing a lost city, the team used the 8-m
Gemini South telescope in Chile to reveal the faint ancient outer
parts of the galaxy NGC 300, showing that that 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, going 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.
The observers found no evidence that the outer part of NGC 300 was
falling abruptly in brightness, or truncating, as happens in many
galaxies.
"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.
Contacts
Joss Bland-Hawthorn
Anglo-Australian Observatory, Sydney, Australia
(AT HOME AUGUST 10)
+61-2-9960-6553 (Home)
+61-2-9372-4851 (Office)
+61-404-858-054 (Mobile)
Ken Freeman
Australian National University, Canberra, Australia
(IN CALIFORNIA UNTIL AUGUST 16)
+61-2-6125-0264 (Office)
+61 402 134 289 (Mobile)
Click here for more detail and images.
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