[ExI] thick and thicker

[Damien Broderick]

Milky Way is much bigger than we thought

Wednesday, 20 February 2008
by John Pickrell
Comos Online
http://www.cosmosmagazine.com/node/1857


SYDNEY: By doing simple calculations on data 
freely available online, Australian 
astrophysicists have discovered that our Milky 
Way galaxy may be twice as thick as previously estimated.

Bryan Gaensler of the University of Sydney led 
the team who now estimate the disc of our galaxy 
to be 12,000 rather than 6,000 light years thick. 
Proving not all science requires big, expensive 
apparatus, the researchers downloaded data from 
the Internet and analysed it in a spreadsheet.

"We were tossing around ideas about the size of 
the Galaxy, and thought we had better check the 
standard numbers that everyone uses," said 
Gaensler. "It took us just a few hours to 
calculate this for ourselves. We thought we had 
to be wrong, so we checked and rechecked and couldn't find any mistakes."

Pulsating stars

Their results were presented at a meeting of the 
American Astronomical Society in Austin, Texas, 
last month, and are now slated for publication in 
an academic journal. The new estimate differs 
from previous calculations because Gaensler's 
team was more selective about the pulsars they used as a data source.

Short for 'pulsating stars', pulsars are rotating 
neutron stars - the remnants of massive suns that 
have collapsed into extremely dense objects. 
Rather like the beam of a lighthouse, they emit 
electromagnetic radiation in the form of radio waves as they rotate.

"As light from these pulsars travels to us, it 
interacts with electrons scattered between the 
stars (the Warm Ionised Medium, or WIM), which 
slows the light down," said Gaensler.

In particular, the longer (redder) wavelengths of 
these pulses slow down more than the shorter 
(bluer) wavelengths, so by seeing how far the red 
lags behind the blue we can calculate how much 
WIM the pulse has travelled through, he said. The 
concept is similar to counting the time between a 
crash of thunder and a flash of lightning to measure how far away it is.

By using the light from pulsars to measure where 
the WIM ends, we can estimate the edge of the 
galaxy, said Gaensler. "If you know the distance 
to the pulsar accurately, then you can work out 
how dense the WIM is and where it stops – in 
other words where the galaxy's edge is."

Less, but more

Of the thousands of pulsars known in and around 
our galaxy, only about 60 have well known 
distances. But to measure the thickness of the 
Milky Way, Gaensler's team focussed only on those 
sitting directly above or below us. Previous 
estimates measured the distance to pulsars 
diagonally above and below us in the direction of 
the upper and lower edge of the galaxy. But this 
yields a less accurate estimate of the thickness 
of the galactic disc, said the researchers.

The gradual accumulation of new data has yielded 
a larger number of pulsars directly above or 
below us, so Gaensler's team was able to focus on 
20 to 30 of these. "We used less data, but it's much more reliable."

Coincidentally, a recent study from the Max 
Planck Institute for Radio Astronomy in Bonn, 
Germany, suggested that there is around five 
times as much magnetism in the Milky Way as would 
be predicted for a galaxy of our size. Without 
any prior knowledge of his work, Gaensler said 
that the study proposed that a Milky Way twice as 
thick as we thought might be one possible way to explain the discrepancy.

He added that it's not just magnetism, but also 
heat, pressure and many other factors about our 
understanding of the Milky Way that could be 
knocked out of kilter if the new theory proves to 
be correct. And by extension, this could effect 
our understanding of all galaxies, which itself 
is built on knowledge of the Milky Way.

Rate of star formation

"The study is intriguing and it could be an 
important result," commented Quentin Parker of 
the Anglo-Australian Observatory in Sydney.

He said the findings could mean that either the 
galaxy itself is wider than we thought, or the 
ionised gas of the WIM simply goes much further 
beyond the stars that make up the edge of the 
galaxy than we thought. "It's all a matter of 
semantics. And depends whether you're talking 
about the size of the galaxy in terms of dust, gas or stars."

However, said Parker, even if the results 
indicate that the ionised gas goes out further 
than we thought, this would mean that the 
star-forming region of the galaxy is much bigger 
than we thought. "If we've underestimated star 
formation in our own galaxy, we've underestimated 
it in other galaxies too 
 and this could affect 
our understanding of the rate of star formation 
in the history of the universe."

"Some colleagues have come up to me and have said 
'That wrecks everything!'" concluded Gaensler. 
"And others have said 'Ah! Now everything fits together!'"