Earlier this year I wrote about a diffuse band of gamma rays coming from regions above and below the Milky Way. The regions spanned about 25,000 light years above and below the galactic plane, and are thought to have formed from an active period of our galaxy’s supermassive black hole about 2 million years ago. While we could determine the size of these regions from their x-ray and gamma ray emissions, it has been difficult to determine their motion. But yesterday at the American Astronomical Society Meeting, new results from the Hubble telescope are measuring the motion of these regions using an interesting trick.
Holding It Together
In the center of our Milky Way galaxy is a supermassive black hole. We can’t see this black hole directly because there is too much dust in the direction of galactic center, but radio waves can penetrate that dust, so we can observe the radio signals of hot stars and gas near galactic center. We’ve been observing these signals over several years, and we’ve noticed how the stars near galactic center orbit the region very quickly. From their orbital motion and a simple use of Kepler’s laws we can get a pretty good idea of the mass of the black hole. It turns out to be about 4 million solar masses. While this is a huge black hole, most of the stars orbiting it aren’t too terribly close. So for the time being they aren’t at risk of being ripped apart by the intense forces near the black hole. But there was one object recently that did make a very close approach.
Energy Bubble
Yesterday I talked about the Fermi gamma ray telescope, and how it allowed us to make much more precise observations of gamma rays in the universe. Part of the purpose of the Fermi telescope is to observe gamma ray bursts, but its broader purpose is to make a sky survey of gamma ray sources in the universe. Already it has found something quite interesting.
A Shapley Galaxy
In the 1700s, William Herschel mapped the distribution of visible stars to determine the shape our galaxy. The result was a rather irregular distribution, and while Herschel had no way to determine the center of the Milky Way, he assumed the Sun was near the center. It was Shapley who, in 1918, demonstrated the Sun was not at the center of the Milky Way.
Ups and Downs
The Sun orbits the center of the Milky way at a speed of about 230 km/s, taking about 250 million years to go around the galaxy once. It is a period of times sometimes referred to as a galactic year. But the Sun does not move in a simple circle or ellipse as the planets move around the Sun. This is due to the fact that the mass of the galaxy is not concentrated at a single point, but is instead spread across a plane with spiral arms and such. As a result, while the Sun orbits the galaxy it also moves up and down across the galactic plane. While the Sun is above the plane, the mass of the galaxy works to move it downward, and when below the plane the mass pulls it upward. As a result the Sun oscillates through the galaxy, crossing the galactic plane once every 30 million years or so.
Behind the Veil
Last September a planet-massed gas cloud known as G2 will made a close approach to the supermassive black hole at the center of our galaxy. At minimum distance it will pass within about 260 AU of the black hole, which is about a third as close as any other object so observed. It will be close enough that it will enter the hot accretion region of the black hole, and may provide the first observation of matter as it is absorbed by the black hole.
The Far Side
While we’re quite familiar with our side of the Milky Way galaxy, the far side of our galaxy is still a bit of a mystery. The reason for this is that the center of the Milky Way is filled with gas, dust and stars, so it is very difficult to see the other side of our galaxy. The central region is so cluttered with material that it sometimes referred to as the Zone of Avoidance, since we have to exclude that region from observations beyond our galaxy.
Hot and Cold
Part of the evidence we have for dark matter is through its gravitational effect on the motion of stars. The first evidence for dark matter came from motion of stars in our galaxy, which indicated there must be a large quantity of unseen mass in our galaxy. So why is it that when we look for the gravitational effect of dark matter on nearby stars, we don’t see anything? It turns out that tells us something very interesting about the nature dark matter.
A Short Period
In the center of our galaxy, only 27,000 light years away, lies a supermassive black hole. Some of the strongest evidence of this black hole is the observation of stars closely orbiting it. Recently in Science a new star, S0-102, was announced with an period of 11.5 years.