Mass of the Milky Way

[junglelord]

This is something interesting to ponder.

Scientists have dramatically revised the mass of the Milky Way, saying our home galaxy is half again as heavy as previously thought. The Milky Way is now on par with the nearby Andromeda Galaxy in terms of heft. The Milky Way spins a lot faster than was thought, too.

http://www.space.com/scienceastronomy/0 ... -mass.html

[tholden]

Of course, somebody who believed that electromagnetic forces held spiral galaxies together would be free to accept the new findings regarding the speed of rotation of the galaxy without having to believe that nobody ever had a clue as to the mass of the galaxy prior to January 2009........

[Solar]

Density Wave Theory is from 1964. Shouldn't this 50% increase in the Milky Way's mass have a direct affect on the galaxy rotation curve problem?

Which then wold have a direct effect on how much "dark matter" is theoretically in the Milky Way necessitating a revision of computer models?

These guys just sent everyone back to the drawing board. Oh wait ... you just plug the 50% increase into the computer model and away you go.

[StefanR]

Breaking News

On January 5, 2009, astronomers using the Very Long Baseline Array (VLBA) radio telescope announced that the rotation speed of the galaxy at the location of the Solar System around 28,000 light-years from the galactic center is roughly 100,000 miles per hour faster than previously estimated, at 568 million miles (or 914 million kilometers) per hour (CfA press release). In findings presented at the 213th American Astronomical Society meeting, the astronomers also noted that the increased rotation speed implies that the Milky Way is some 50 percent more massive than previously estimated, making it about as large as the nearby Andromeda Galaxy. The Milky Way's larger mass estimate means that its gravitational pull is greater, which increases the likelihood of collisions with neighboring galaxies.

http://www.solstation.com/x-objects/cenbulge.htm

Taking just from this bit, I would take it to be that a different rotation speed is measured. Full stop and don't add more. The rest, mass, size, gravitational pull and likelihood of collisions (maybe even Global Warming ), is just added to save the frame-work. What is more interesting is maybe how they got their velocity measurements.

[junglelord]

If infact the velocity is higher then anticipated, that would mean that the spin ratios would be different as the universe is in a harmonic relationship. Therefore the anticipated velocity of the earth, sun, spiral arm, local group, great attractor, need to be re-examined. It was also recently pointed out that the structure of the Milky Way was not as previously thought and that new data showed a different formation and that there were only two major arms making us a barred spiral and that we were not a full spiral, which would mean less stars I would think. Also we are not in a major arm as orignally thought so thats all new news.
http://www.universetoday.com/2008/06/03 ... iral-arms/
But they also claim as many stars between the arms so no difference.

occasionally you may find alternative names being used for the spiral arms. This table lists some of the alternative names:

Common Name Alternative Name
Norma Arm 3 kiloparsec Arm
Scutum-Crux Arm Centaurus Arm
Sagittarius Arm Sagittarius-Carina Arm
Orion Arm Local Arm
Perseus Arm -
Cygnus Arm Outer Arm

Note also that the Orion Arm is not a major spiral arm, but only an enhancement of stars and gas between the Sagittarius and Perseus arms.

It should be emphasized that there are almost as many stars between the spiral arms as in the spiral arms. The reason why the arms of spiral galaxies are so prominant is that the brightest stars are found in the spiral arms. Spiral arms are the major regions of star formation in spiral galaxies and this is where most of the major nebulae are found.
http://www.atlasoftheuniverse.com/milkyway.html

Radio Telescopes are used to measure neutral hydrogen

The Shape of the Milky Way - The Evidence
There are two methods traditionally used to map the spiral structure of our Galaxy. The first method is to study the density of the neutral hydrogen (HI) in the plane of the Galaxy which is enhanced in the spiral arms. This was first attempted by Jan Oort, Frank Kerr, and Gart Westerhout in 1958. They studied the galactic system as a spiral nebula by using radio-telescopes in the Netherlands and Australia. The early version of their map (incomplete on the left side) shows various sections of the spiral arms. The second method is to plot the giant HII regions (bright nebulae of ionised hydrogen) which are usually formed in the spiral arms. This was attempted by Yvonne and Yvon Georgelin in 1976. They studied the spiral structure of our Galaxy determined from H II regions. Their map allowed them to determine where the spiral arms are.

For a recent attempt at mapping the Milky Way in neutral hydrogen (although only the outer parts) see The Spiral Structure of the Outer Milky Way in Hydrogen by Levine, Blitz, and Heiles, (2006). For a recent map of the HII regions in the Milky Way see Star-forming complexes and the spiral structure of our Galaxy by Delphine Russeil, (2003). These various maps can be analysed to show the precise spiral form of the Galaxy, see Jacques Vallée's various studies of the Milky Way ( 1, 2, 3). The Milky Way is probably a four-arm logarithmic spiral.

Our Galaxy is also a barred-spiral galaxy. For an analysis of this central bar see The Long Bar in the Milky Way by López-Corredoira, Cabrera-Lavers, Mahoney, Hammersley, Garzón, and González-Fernández, (2006), (and also their earlier 2001 paper). The central bar of the Milky Way looks like the bar in the spiral galaxy M95
http://www.atlasoftheuniverse.com/milkyway.html

Of course they claim a lot of things.
Wonder if that data jives with this data....then they say only two arms which reduces mass I would think, then they say more velocity, they claim 50% more mass, then they say what ever they need too...

[StefanR]

Making such calculations can be very tricky of course, especially if the precise whole workings of the galaxy is still shrouded in fog

Probably the measurements of the VLBA are accurate in a certain way but in this study there seem to be not so many datapoints yet:

The scientists observed regions of prolific star formation across the Galaxy. In areas within these regions, gas molecules are strengthening naturally-occuring radio emission in the same way that lasers strengthen light beams. These areas, called cosmic masers, serve as bright landmarks for the sharp radio vision of the VLBA. By observing these regions repeatedly at times when the Earth is at opposite sides of its orbit around the Sun, the astronomers can measure the slight apparent shift of the object's position against the background of more-distant objects.

"The new VLBA observations of the Milky Way are producing highly-accurate direct measurements of distances and motions," said Karl Menten of the Max Planck Institute for Radio Astronomy in Germany, a member of the team. "These measurements use the traditional surveyor's method of triangulation and do not depend on any assumptions based on other properties, such as brightness, unlike earlier studies."

http://www.nrao.edu/pr/2009/mwrotate/

Compared to an article linked by Jungelord:

If we put all this data together then we get a map like this one below. There is not much data available about the far side of the Galaxy but spiral galaxies are usually fairly symmetrical. Features on one side of a galaxy are often repeated on the other side.

http://www.atlasoftheuniverse.com/milkyway.html

They also seem to infer elliptical orbits for "star-forming"-regions;

The VLBA can fix positions in the sky so accurately that the actual motion of the objects can be detected as they orbit the Milky Way's center. Adding in measurements of motion along the line of sight, determined from shifts in the frequency of the masers' radio emission, the astronomers are able to determine the full 3-dimensional motions of the star-forming regions. Using this information, Reid reported that "most star-forming regions do not follow a circular path as they orbit the Galaxy; instead we find them moving more slowly than other regions and on elliptical, not circular, orbits."

The researchers attribute this to what they call spiral density wave shocks, which can take gas in a circular orbit, compress it to form stars, and cause it to go into a new, elliptical orbit. This, they explained, helps to reinforce the spiral structure.

http://www.nrao.edu/pr/2009/mwrotate/

But with so little data-points, I do hope that the difficulties described here were dealt with;

5 Conclusion

A new catalogue of star-forming complexes has been established from a multiwavelength study of the Galactic plane. Each complex is a grouping of HII regions, diffuse ionized gas, molecular clouds and OB stars. For each complex the systemic velocity, the kinematic distance, the stellar distance and the excitation parameter have been determined. A rotation curve has been fitted through those complexes for which it has been possible to calculate the stellar distances. Our curve is in good agreement with that of Brand & Blitz (1993). We also performed a study of velocity departures relative to the circular rotation model. We find that such departures exist in large parts of the arms, with different values from one arm to another. This can explain, at least partly, the quite large dispersion of the complexes around the mean rotation curve. The Galactic warp is also found, in good agreement with previous studies. In addition we note that circular rotation departure and warp do not seem obviously correlated.

link to article

Thirdly they are concluding that :

Reid and his colleagues found other surprises, too. Measuring the distances to multiple regions in a single spiral arm allowed them to calculate the angle of the arm. "These measurements," Reid said, "indicate that our Galaxy probably has four, not two, spiral arms of gas and dust that are forming stars." Recent surveys by NASA's Spitzer Space Telescope suggest that older stars reside mostly in two spiral arms, raising a question of why the older stars don't appear in all the arms. Answering that question, the astronomers say, will require more measurements and a deeper understanding of how the Galaxy works.

http://www.cfa.harvard.edu/press/2009/pr200903.html

And that when just in June 2008 :

Spiral Arms surrounding the Bulge

On June 3, 2008, astronomers using the infrared Spitzer Space Telescope announced that only two "major" spiral arms (Scutum-Centaurus and Perseus) of the Milky Way extend from the large bar of stars dominating the central bulge of the galaxy, instead of four as previously believed. In findings presented at the 212th American Astronomical Society meeting, the Milky Way's two other spiral arms (Norma and Sagittarius) were demoted to "minor" status as less distinct arms between the two major arms that have lower densities of both young and old stars but abundant gas with pockets of star-forming activity.

http://www.solstation.com/x-objects/cenbulge.htm

So now we are back to four arms only with two active and two not? Confusing

[MGmirkin]

Density Wave Theory is from 1964. Shouldn't this 50% increase in the Milky Way's mass have a direct affect on the galaxy rotation curve problem?

Did they actually FIND additional mass, or is this another "dark matter" assumption? IE, we found the rotational speed issue, but NOT the actual matter/mass to back it up gravitationally? Just wondering. Have they stated they FOUND 50% more matter or simply that it must exist SOMEWHERE (in the form of dark matter or undiscovered normal matter) to account for the rotation issue? I tended to read the latter, but I could be mistaken.

Regards,
~Michael Gmirkin

[StefanR]

Have they stated they FOUND 50% more matter or simply that it must exist SOMEWHERE (in the form of dark matter or undiscovered normal matter) to account for the rotation issue? I tended to read the latter, but I could be mistaken.

That's what I got out of it too. Faster spinning means that to hold it all together would mean a bigger gravitational pull and to get that from the standard view would mean more mass. Very odd.

[junglelord]

Density Wave Theory is from 1964. Shouldn't this 50% increase in the Milky Way's mass have a direct affect on the galaxy rotation curve problem?

Did they actually FIND additional mass, or is this another "dark matter" assumption? IE, we found the rotational speed issue, but NOT the actual matter/mass to back it up gravitationally? Just wondering. Have they stated they FOUND 50% more matter or simply that it must exist SOMEWHERE (in the form of dark matter or undiscovered normal matter) to account for the rotation issue? I tended to read the latter, but I could be mistaken.

Regards,
~Michael Gmirkin

It was the velocity that I ponder, not their meaning less talk of "mass" which they do not understand anyway.