TR: Milky Way Is Square, According To New Galactic Map

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TR: Milky Way Is Square, According To New Galactic Map

Postby bystander » Wed Oct 13, 2010 5:06 pm

Milky Way Is Square, According To New Galactic Map
Technology Review : the physics arXiv blog | 13 Oct 2010
Some of our galaxy's spiral arms are straight rather than curved, giving the Milky Way a distinctly square look, say astronomers.

APOD: 2003 March 3 - M101: The Pinwheel Galaxy
(Credit: Jean-Charles Cuillandre, Hawaiian Starlight, CFHT)
The structure of nearby galaxies such as Andromeda is relatively straightforward to see. But the Milky Way presents an entirely different kind of challenge.

The problem is that we see the Milky Way edge on, so that nearer stars and clouds are superimposed on more distant ones. Telling these apart is tricky because working out the distance of any astronomical object is hard. And that makes the overall structure a real head scratcher.

That's not to say astronomers haven't got a few tricks up their sleeves to help. The conventional way to work out the structure is a two step process. Astronomers first create a model of the galaxy and work out how each part of ought to be moving relative to us.

Then they scour the Milky Way for clouds of ionised hydrogen. Astronomers can work out the velocity of these clouds by studying the emission spectra and looking for the tell tile shifts in spectral lines that movement causes.

By matching this measured velocity to the calculated values, astronomers can work out where in the galaxy any cloud should be.

But this method is notoriously ambiguous, not least because nobody is quite sure how fast the galaxy is rotating, so the model probably has all kinds of errors. Another problem is that stars orbiting the centre of the galaxy at the same distance as us (a large portion of the galaxy, as it turns out) all have a similar velocity. So working out where they are is tricky.

It's no surprise, then, that there is little consensus on the exact structure of the Milky Way's spiral arms.

Today Jaques Lepine at the University of Sao Paulo in Brazil and a few buddies add a little spice to this mix.

They've studied the spectra produced by clouds of carbon monosulphide, a relatively common component of our galaxy, rather than ionised hydrogen. This gave them velocity information for 870 regions of the Milky Way which they've used to create a new map of the galaxy with detail never seen before.

One conclusion is that the Milky Way has an additional spiral arm, not seen in previous surveys of the galaxy. The new arm is about 30,000 light years from the galactic core at a longitude of between 80 and 140 degrees.

But a bigger surprise is their conclusion that some of the arms in the Milky Way are not curved in the traditional way, but are straight instead. This gives the Milky Way a distinctly squarish look.

That's not as outrageous as it sounds. Astronomers know of many galaxies with straight arms, such as M101, the Pinwheel Galaxy, shown above.

So according to Lepine and co, anybody looking at us from M101 will see a similar kind of squarish structure. Fascinating stuff!

The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance - JRD Lepine et al

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Re: TR: Milky Way Is Square, According To New Galactic Map

Postby Ann » Thu Oct 14, 2010 1:16 am

Indeed, many galaxies have rather straight arms. Look at M 61, for example.


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Re: TR: Milky Way Is Square, According To New Galactic Map

Postby Ann » Thu Oct 14, 2010 6:00 am

By the way, if the Milky Way really has squarish arms, then that suggests that our galaxy has quite a lot of star formation. It is basically only bright star formation regions and associations that give spiral arms a "not-rounded" appearance. Spiral galaxies with little star formation typically have gracefully rounded arms.


NGC 488 has little star formation and gracefully rounded arms.

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Re: TR: Milky Way Is Square, According To New Galactic Map

Postby neufer » Thu Oct 14, 2010 8:10 pm

      Dante's Paradiso: Canto 33
    As the geometrician, who endeavours
    To square the circle, and discovers not,
    By taking thought, the principle he wants,
    Even such was I at that new apparition;
    I wished to see how the image to the circle
    Conformed itself, and how it there finds place;
    But my own wings were not enough for this,
    Had it not been that then my mind there smote
    A flash of lightning, wherein came its wish.
    Here vigour failed the lofty fantasy:
    But now was turning my desire and will,
    Even as a wheel that equally is moved,
    The Love which moves the sun and the other stars.

      James Joyce's _Ulysses_ "Ithaca"
    Going to dark bed there was a square round Sinbad the Sailor roc’s auk’s egg
    in the night of the bed of all the auks of the rocs of Darkinbad the Brightdayler.
Art Neuendorffer

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