Milky Way Doubles Its Mass

This week Robert Naeye, S&T's editor in chief, is dashing around the American Astronomical Society convention in Long Beach, California. Here the nation's professional astronomers are sharing their research results, projects, plans, and hopes. Buckets of astronomy news always come out at these meetings. Bob will be blogging about these announcements during the next few days. Here's his first report. Stay tuned!

---

I heard a lot of interesting results on Monday at the American Astronomical Society’s annual winter meeting. But three really caught my attention. The first concerns the Milky Way, and it looks like some textbooks are going to have to be revised.

An international team led by Mark Reid (Harvard-Smithsonian Center for Astrophysics) presented the most direct measurements yet of the rotation rate of our Milky Way Galaxy. Using a network of 10 radio telescopes stretching from Hawaii to New England known as the Very Long Baseline Array (VLBA), the group was able to measure the extremely precise parallaxes (distances) of pointlike radio sources in 18 of the Milky Way's major star-forming regions. The study revealed a total of four spiral arms, all of which contain young stars, and two of which consist of both young and old stars.

We reported on that a while ago. But there's more. The team has been able to map not just the distances of the radio sources, but their complete motions in three dimensions — by combining their sideways movement on the sky (proper motion) and movement toward or away from us (radial velocity). The result provides a map of the galaxy and its rotation directly from geometry, free from most astronomical assumptions.

The team finds that the galaxy's rotation rate is 270 kilometers per second (600,000 miles per hour). This is 15 percent faster than previously thought. And that in turn means the Milky Way is more massive than previously believed. In fact, it puts our galaxy on equal footing with our nearest large neighbor, the Andromeda Galaxy (M31).

Because Andromeda has more stars than the Milky Way, astronomers have generally assumed that it was about 50 percent more massive. But this new result agrees with a study published two years ago by Mark Wilkinson (Cambridge University, England) and his colleagues. By studying the motions of satellite dwarf galaxies and globular clusters, they found that the Milky Way is at least as massive, and maybe more so, than Andromeda.

“We should no longer think of the Milky Way as the little sister of the Local Group of galaxies,” says Reid. “The two galaxies are basically fraternal twins — equal in mass.”

Does this mean the Milky Way is richer in dark matter?

Reid also noted that the Sun takes about 225 million years to orbit the center of the galaxy, meaning that the last time the Sun was in its current position, dinosaurs were just starting to roam the Earth.

For more information, here's a press release from the National Radio Astronomy Observatory. And here's a paper about the distance measurements by members of the group.