You are here

Sagittarius RIP

Submitted by Kevin Jardine on 6 June, 2008 - 08:59

In addition to the release of their astounding infrared mosaic of the inner galaxy, the Spitzer team had a separate release this week to announce some conclusions about the Milky Way spiral structure, including a larger bar and a reduction to two major spiral arms.

There is nothing that new about the conclusion that the Milky Way has two major spiral arms. As I pointed out when I first started this site, there is no major consensus about the number of arms the Milky Way has. In particular, the conclusion that no major arm runs through Sagittarius has already been strongly made by the Russian astronomer Anna Mel'nik in papers published in 2001 and 2005. I linked to these papers as part of my commentary on the inner galaxy in hydrogen-alpha.

Still, it's useful to have the NASA stamp of approval on the idea. I've long been sceptical about the Sagittarius arm because my face-on map of the galaxy shows no major spiral structure in that direction - just a jumble of different objects at different distances.

To illustrate their conclusions, the Spitzer team released a new face-on image of the Milky Way. This is an update of the image produced by NASA's scientist-illustrator Robert Hurt in 2005. Although the new image does an excellent job of illustrating the Spitzer conclusions about the inner galaxy, it is sadly inaccurate elsewhere. In particular, the image shows major Perseus arm star associations in the third quadrant (180° - 270°) that are known not to exist (the Perseus arm is mostly diffuse gas and dust in that quadrant) and ignores important new information on the Outer arm and the Orion spur.

Although this new information has only been recently published, it has been available on the main astronomical preprint server for at least 6 months, so it is very disappointing that it was not taken into account when producing the illustration.

As a result, there is still no decent illustration that draws together the most current information about the Milky Way's structure.

Spitzer scientist Robert Benjamin was quoted by New Scientist as saying "Trying to create a picture of the Milky Way is about 40% hard science and 60% imagination." That's true, but in this case, I wish the illustrator had used a bit more science and a bit less imagination.