A JOURNEY TO THE GALACTIC CENTER

What does our Galaxy look like?

We live in the Milky Way Galaxy, which is a collection of stars, gas, dust, and a supermassive black hole at its very center. Our Galaxy is a spiral galaxy, which are rotating structures that are flat (disk-like) like a DVD when looked upon edge-on. There is also a bulge in the middle that consists of mostly old stars. When you look at a spiral galaxy face-on, you can see beautiful spiral arms where stars are being born. Our solar system is in the Orion arm, and we are about 25,000 light years (2.5 X 10^17 miles) from the very center of the Galaxy.

Since our solar system lies in one of the spiral arms, we live in the flat plane of the Milky Way. We can actually see the dense plane of the Milky Way stretch across the sky in dark places that do not have a lot of surrounding light pollution. The diffuse light is the combined light from millions of stars. Some of the light from these stars are obscured by large clouds of dust, which is why there are dark patches. Dust and gas are necessary to form stars, and most stars are formed within the spiral arms. Note that we can't really see the center of the galaxy with our eyes because there is dust in the way!

What is happening to the gas in the Galactic Center?

As we saw before, we can trace ionized gas at radio wavelengths. High-resolution observations of the Galactic Center at radio wavelengths have revealed a structure called the "mini-spiral." The mini-spiral has several different parts--the Northern Arm, the Wester Arc, and the Eastern Arm

Motion of the gas

The motion of the gas in the mini-spiral can be determined using the Doppler shift of emission lines, particularly Ne II and Bracket-gamma hydrogen lines. These studies show that the gas along the Northern Arm is falling in toward, or obriting around, the central black hole. Current evidence suggests that the three arms are orbiting the supermassive black hole according to Kepler's laws, and that the gas in the Northern Arm is actually interacting with a nearby source. The Eastern Arm is actually colliding with the Northern arm flow, and that the gas is photoionized--electrons are being stripped off (Zhao et al. 2009).

The Circumnuclear Disk (CND)

In addition to having different arcs and arms, the mini-spiral is surrounded by a thick ring of molecular material called the Circumnuclear Disk (CND). The origin of this disk is unclear, but it is a ring, or a torus, with an inner radius of about 1 parsec and an outer radius of several parsecs (Note: 1 parsec is 1.917 X 10^13 miles!). This disk's rotation follows Kepler's laws, and it is heated by both central region of old stars and the ultraviolet (short wavelength) flux of bright young stars that are near the black hole.

The young, massive stars

The young, massive, early-type (O, A, and B spectral class) stars are the brightest stars in the Galactic Center and contribute most of the ultraviolet radiation that ionizes the surrounding gas seen in the mini-spiral. These stars have Helium (He I) and Hydrogen (H I) emission lines in their spectra, which are come from the fast winds flowing away from the stars. The most noticeable cluster of these stars are in the IRS 16 cluster, which are located close to Sgr A*. T hese stars are also 3 - 7 million years old. The Galactic Center tends to host, on average, more massive stars than other clusters found in the local neighborhood. This is probably due to the extreme conditions found at the Galactic Center created by the supermassive black hole, the large magnetic field, and the UV radiation from early-type stars.

As we zoom into the very core of the Galactic Center, our field of view shrinks to a mere 5 arcseconds (one thousandth of a degree). At radio wavelengths, the brightest feature of this region is the point-like radio source Sagittarius A* (pronounced "Sag A star"). This source is a compact object, and approximately one Astronomical Unit (1 AU is about 93 million miles) in size, which is much smaller than our solar system (Neptune is 2.8 billion miles from the Sun). At near-infrared wavelengths, this point source in the radio is not clearly seen. Astronomers have seen pulsation near the radio position of Sgr A* in the near-infrared, which they attribute to this radio source flaring.

In 1974, Sir Martin Rees proposed the idea that supermassive black holes could exist within the centers of active galactic nuclei or quasars. In that same year, Balick and Brown made the conenction between their radio detection of Sgr A* and other known active galactic nuclei.

In the past 20 years, astronomers have collected enough evidence through the observed motions of gas and stars to convince ourselves that something very massive lurks at the center of our galaxy. The first dynamical evidence came from the motions of the ionized gas streamers of the mini-spiral orbiting Sgr A*. Using the velocities of the gas estimated from the Doppler shift of spectral lines, astronomers estimated that a mass of six million solar masses must lie within 10 arcseconds of Sgr A*. This did not explicitly prove the existence of a black hole since that amount of matter could be accounted for by a high density of stars within such a large volume.

Stellar Orbits about the Black Hole.

Very accurate stellar positions can be estimated in order to keep track of the motions of the stars in the compact central cluster, which are zipping around Sgr A* at speeds up to 3 million miles per hour! Using Kepler's laws of motion, the orbital velocities and the positions of the bright stars an be used to estimate the mass that must be contained within their orbits. The resulting enclosed mass is 4.6 ± 0.7 X 10^6 solar masses--4.6 million times the mass of our Sun! This large mass combined with the minute size of Sgr A* in radio emission suggests taht the stars must be swiftly circling around a supermassive black hole.

Black Holes in other Galaxies

Recent observations of nearby galaxies reveal that such supermassive black holes are not unique to the MIlky Way. The formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood. In the case of Sgr A*, there is a mysterious absence of the high energy emission (X-rays and UV radiation) often observed from active galactic nuclei. The image to the left is of an active galactic nucleus in Cen A. There is a lot of activity at long wavelengths and short, energetic wavelengths. This is completely different than our galactic nucleus, Sgr A*. Sgr A* may have been more energetic in the past.

Hope you have enjoyed this Galactic Journey and now have some understanding of what's going on in the center of our Galaxy!

Thank You - The Galactic Center Group.