New arm discovered in outer edge of the Milky Way Galaxy

May 23, 2011 by Bob Yirka report
Barred Spiral Milky Way. Illustration Credit: R. Hurt (SSC), JPL-Caltech, NASA

(PhysOrg.com) -- In a surprising twist, if you will, Thomas Dame and Patrick Thaddeus, both of the Harvard-Smithsonian Center for Astrophysics, have put forth in a paper to be published in an upcoming issue of Astrophysical Journal Letters, the notion that a cluster of gas clouds they've discovered, that lies far from what is currently believed to be the center of the Milky Way Galaxy, is likely the extension of one of the great arms that form our galaxy.

The researchers made their discovery by thinking outside of the box, so to speak; most research on our galaxy starts with the assumption that the spiral that swirls out from the center, amounts to what most see as an almost a two dimensional plane. Dame and Thaddeus, looked beyond that plane, though not by much, and came upon a mass of giant molecular that they believe is an extension of what is known as the Scutum-Centaurus arm; and thus have dubbed the new the Outer Scutum-Centaurus arm.

Ever since Stephen Alexander first came up with the idea that our galaxy existed in the shape of a spiral way back in 1852, stargazers have been studying the vast gas clusters that fill the around us, and since that time, have thus far concluded that there are six “arms” that make up the Milky Way galaxy, with a central core chock full of stars. The new arm extension would fill in a bit of a gap on one side that would make the entire look more even around the edges.

Further research will have to be conducted, of course, before a true consensus is reached, but if what these two researchers have found is accepted by the astrophysics community, it will mean that our galaxy is actually two mirror images of itself, and that it’s a bit warped as well, seeing as how it’s not as flat as once thought; an interesting parallel, for those that recall how in the early days of science, the world as we knew it was thought to be flat as well, and we all know how that turned out. Thus, it does seem conceivable, what with our inability to see a lot of the due to trying to look at it from an embedded position, that one day we’ll find the whole thing is actually not what we think it is at all, but something much more complex.

Explore further: Swirling electrons in the whirlpool galaxy

More information: A Molecular Spiral Arm in the Far Outer Galaxy, T. M. Dame, P. Thaddeus, Astrophysical Journal Letters, in press.
Available on ArXiv at arXiv:1105.2523v1 [astro-ph.GA]

Abstract
We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center. After tracing the arm in existing 21 cm surveys, we searched for molecular gas using the CfA 1.2 meter telescope and detected CO at 10 of 220 positions. The detections are distributed along the arm from l = 13 deg, v = -21 km/s to l = 55 deg, v = -84 km/s and coincide with most of the main H I concentrations. One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 Mo. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way. The new arm appears to be the continuation of the Scutum-Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.

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jjoensuu
4.1 / 5 (11) May 23, 2011
how we know it is an arm and not a leg?
spectator
1 / 5 (1) May 23, 2011
Further research will have to be conducted, of course, before a true consensus is reached, but if what these two researchers have found is accepted by the astrophysics community, it will mean that our galaxy is actually two mirror images of itself


The bar gives it away, in that there are actually two major orbital planes which intersect.

Then you have average internal gravity and it's vector sum with average net external gravity. This could cause some arms to be warped in directions which are not even co-planar with their initial orbital planes.

this is actually pretty obvious stuff which ought to have been noticed decades ago. If you have a companion galaxy, then that galaxy warps the Milky Way in it's direction, and vice versa, since some portions of the Milky Way would be a hundrd thousand light years closer than other portions of the Milky Way (or 10,000 to 15,000 light years closer if it's above or below the orbital plane of the Milky Way's stars).
spectator
1 / 5 (1) May 23, 2011
This is to be expected even in isolated galaxies.

I hope you don't "really" think spiral galaxies form with all of their matter on the same orbital plane.

Matter starts on different orbital planes which intersect through the CoG. Over time, collisions and local "stellar range" gravitational influences would cause some stuff to fall into the CoG, and other things would obtain average angular momentum approaching an "average" orbital plane.

Example:

Any two stars in the galaxy attract one another gravitationally, although very weakly. So over very long time periods, due to conservation of both linear momentum and angular momentum, the stars eventually moderate one another's orbits, and end up on orbital planes closer and closer together.

But in a real world system, this process would take eons and eons and eons to ever run to completion and "really" get everything onto the same real orbital plane as the average orbital plane.
A_Paradox
4 / 5 (1) May 23, 2011
@spectator,

I tend to think that stars, per se, are not necessarily going to bring other stars into "neat" orbits around the galactic COG, not near neighbours anyway. Reason: stars are effectively gravitational "points" to each other due to the relatively vast distances between compared to their respective physical cross sections. I think this means that, unlike gas particles which will hit and bounce off each other, they have near misses which randomly redirect the participants in any such fly-bys.

I think galactic discs come about because gas and dust clouds are composed of gazillions of minute particles which *do* experience impacts, which should be able to slow and amalgamate previously separate clouds. This should be aided also by the distributed mass of each cloud which would allow small particles to wander around within the cloud but attract back the vagrant particles which happen to get knocked clear of the main mass.
Husky
5 / 5 (3) May 23, 2011
what i would like to know is if besides the average orbital plane of our milky way, would patterns in statistics show that there could exist a preffered orbital plane for all galaxies around us, in other words did the universe rotate when it was born?
Husky
not rated yet May 23, 2011
put that in a colorcoded panoramic graph like WMap and we could see how orbital momentum shapes on cosmic scale...
yyz
5 / 5 (2) May 23, 2011
To add a bit to Paradox's post above, dark matter halos are also intimately involved with disk formation and dynamics of disks in galaxies. DM halos of both the MWG and the MCs appear to be (partly) involved in the outer warping of the HI disk of our galaxy: http://scholarwor...+1957%22

Computer models of disk warping of the MWG by the MCs in support of their paper can be found here: http://www.astro....ies.html

"this is actually pretty obvious stuff which ought to have been noticed decades ago"

Warping of the outer HI disk of the MWG was discovered through radio surveys in 1957 (see refs in paper above).

Question
1 / 5 (1) May 23, 2011
what i would like to know is if besides the average orbital plane of our milky way, would patterns in statistics show that there could exist a preffered orbital plane for all galaxies around us, in other words did the universe rotate when it was born?

My bet would be that it is completely random.
Na_Reth
1 / 5 (2) May 23, 2011
what i would like to know is if besides the average orbital plane of our milky way, would patterns in statistics show that there could exist a preffered orbital plane for all galaxies around us, in other words did the universe rotate when it was born?

My bet would be that it is completely random.


There is no such thing as random in this universe.
The only reason something becomes "random" to a human perspective is when that human cannot calculate or expect the outcome.
Question
5 / 5 (3) May 23, 2011

quote: "There is no such thing as random in this universe.
The only reason something becomes "random" to a human perspective is when that human cannot calculate or expect the outcome."
I thought that was the meaning of random.
eachus
5 / 5 (1) May 25, 2011

quote: "There is no such thing as random in this universe.
The only reason something becomes "random" to a human perspective is when that human cannot calculate or expect the outcome."
I thought that was the meaning of random.


Um, in computer science that is the meaning of "pseudo-random." If you really need random, you buy a (relatively cheap) random noise source that depends on quantum effects, and apply a mathematical transform to make the random data "white."

Is the random data so generated really random? That is a question that theoretical physicists try from time to time to answer. So far the best answer comes from Werner Heisenburg. He believed that there was a clockwork level in atoms and other quantum mechanical systems that his uncertainty principle prevents us from seeing.

Recent experiments though, show that if quantum mechanics says that something is not black or white but grey, the grey state can be directly observed.
eachus
5 / 5 (1) May 26, 2011
What cannot be determined is when the indeterminate state will collapse into a binary state. Physics is getting closer to defining how that clockwork behaves, but we still won't be able to see it.