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Conclusions on mapping the Milky Way

Submitted by Kevin Jardine on 7 June, 2013 - 15:26

A few days ago I announced the Velocity Explorer, an interactive tool for exploring the velocity of gas in the galactic plane. As part of that announcement I mentioned that I had used Velocity Explorer to create a model of the Milky Way:

and even a partial map:

Maps derived from velocity data can't be treated very seriously for many reasons that essentially boil down to the fact that there is no straightforward relationship between velocity and distance. A more reliable map will be possible only when many more maser parallax observations become available, especially from the southern hemisphere.

Nevertheless, I think I can derive several interesting conclusions from the exercise. Of course since these conclusions are based on my (unreliable) map, they may be incorrect. However, in most cases they don't depend upon a distance-velocity relationship and so may very well be real.

The spur system is elaborate

The map shows many short bridges or "feathers" between the spiral arms. This is not surprising as such structures are common in spiral galaxies. What is a bit more unusual is the elaborate system of large spurs on the near side of the galaxy. These include the Orion, Vela and Cygnet spurs as well as the Perseus bridge and at least four other structures. This means that as maser parallax data becomes available the process of mapping objects located between the spiral arms will not be simple as we cannot assume that the masers are associated with one or two large spurs. They will in fact be part of an elaborate hydrogen web.

The velocity data shows a double ring

Instead of a simple ring (sometimes called the "near and far 3kpc arms"), surrounding the bar, the LAB velocity data clearly shows a more complex double ring structure. It is not clear at this point how this maps into a physical structure but it seems unlikely that the bar is surrounded by a simple elliptical ring.

There is anomalous velocity in the anticentre too

The Velocity Explorer shows at least two distinct bands of clouds with strongly negative velocities towards the outer galaxy.

There are major kinks in the Perseus and Sagittarius arms

The velocity data as described in the section on the spiral arms suggests that the Perseus and Sagittarius arms are not simple logarithmic spirals but have more elaborate shapes. In particular, a major bend in the Perseus arm between about 220° - 235° means that we are looking down rather than across the arm in this direction and will likely see an overdensity of spiral tracers.

There appears to be a large complex of clouds in the outer first quadrant

There is a continuous velocity spread between the Norma and Centaurus arms from 37° to 47° as can be seen in this Velocity Explorer image. I have labelled this region the Centaurus confluence on the map. It appears to be a region where the Milky Way's spiral structure breaks down and the Norma and Centaurus arms merge into a large flocculent cloud complex.