Catching up on news from the recently concluded American Astronomical Society meeting, I want to be sure to mention RAVE — the Radial Velocity Experiment — which is an all-sky spectroscopic survey of as many as a million stars passing near the Sun. Observations of stars in other galaxies show that only dark matter can explain their movement since there is not enough visible mass to keep them gravitationally bound. Early results from RAVE confirm that dark matter dominates the mass of the Milky Way as well. The survey will help to firm up our picture of the Milky Way’s own dark matter and, by extension, its mass.
“One important early application of RAVE aims to measure just how much stuff there is in our Milky Way galaxy — the collection of stars, gas and dark matter that is the home of our sun,” said Rosemary Wyse (Johns Hopkins). “Newton’s Law of Gravity allows us to figure out from the orbital motions of stars how much mass is holding them together. Faster motions need more mass. We know from analyzing the motions in other galaxies that there is a lot more mass than we can see and this dark matter appears to dominate. But we are not sure exactly how much dark matter is needed in our own galaxy, and we don’t know what the dark matter is made up of. That information is important, and the RAVE survey is going to help us answer some of those questions.”
Another fascinating report on dark matter at AAS came from a study of VIRGOHI 21, a cloud of hydrogen in the Virgo Cluster some 50 million light years from Earth. New evidence suggests that VIRGOHI 21 is actually a dark galaxy; i.e., a galaxy emitting no starlight. What a team of astronomers from Arecibo Observatory and Cardiff University is arguing is that the hydrogen within this object is rotating in a way that implies a dark galaxy with ten billion times the mass of the Sun. Radio waves from the neutral hydrogen are what allow the object to be studied.
And if only one percent of the mass of the object is now thought to be hydrogen, the balance appears to be dark matter. That would explain why nearby galaxy NGC 4254 seems lopsided, with one spiral arm much longer than the rest. The companion galaxy that might cause this deformity has never been identified unless it be the dark VIRGOHI 21. We are taking small but significant steps toward a deeper understanding of dark matter, which should tell us much about how galaxies form and are distributed in the universe.