Size Scales

Picture of the Galactic Center as seen from a dark site The Galactic Center has a diverse array of activity occurring on many different size scales. Lets begin with the above annotated image of the Galactic Center as seen by the unaided eye. The size of this image is about 10 by 15 degrees in size. For comparison, your closed fist held out at arm's length covers about 5 degrees on the sky. Don't let this image confuse you though. Believe it or not, the bright part of the image called Baade's window is actually not the location of the Galactic Center. The true location is labeled on the image to the right of Baade's window. Notice that there is not as much of starlight there. The reason for this is the large amount of dust and gas between us and the Galactic Center. The dust absorbs the optical light along the long way blocking much of our view of the Galactic Center at these wavelengths.

Now lets zoom in ...

0.5X0.5 degree infrared image taken by Ian Gatley (NOAO) Luckily, the dust is not as good at absorbing light at longer wavelengths such as infrared and radio waves. Therefore, much of our knowledge of the Galactic Center results from observations at these wavelengths rather than from studies of optical light. This is demonstrated by this image of the Galactic Center taken in the infrared (1.2-2.2 microns). The image is 0.5X0.5 degrees in size, where 0.5 degrees is about the size of the Moon and Sun. The image shows emission from the millions of stars located within the bulge and center of our galaxy. The dark patches are the densest dust clouds which obscure even the infrared light from the stars behind them. (Sky and Telescope June 1995)

VLA image of the GC, Kassim et al. 1999	Taken from Genzel, R., Hollenbach, D., & Townes, C., 1994, Reports of Progress in Physics, 57, 417

When we go to even longer wavelengths and zoom in even more, the Galactic Center comes alive with activity. This radio (6cm) image obtained at the VLA (Very Large Array) near Socorro NM shows the "mini-spiral" at the center of the Milky Way. The image covers about 4 arcminutes on the sky; corresponding to the inner 10 pc or so. (Yusef-Zadeh et al. 1998). The mini-spiral is composed of the Northern Arm, Eastern Arm and Western Arc. They are seen as both ionized gas emission in the radio and thermal dust emission in the mid-infrared .

The diagram on the right shows the prominent dust, stellar and gas features in the central 10 pc region. The numbered dots in the diagram represent prominent stars including IRS 7, the dot with a comet-like tail, and the AF star. The thick, dark ring surrounding the mini-spiral is the Circum-Nuclear Disk (CND). Collisions between clouds of material within the CND are thought to result in a loss of angular momentum allowing them to fall in toward or orbit around the central super-massive black hole located at the position of Sgr A*. The tidal stretching of the material during infall produces the streamers now observed as the mini-spiral.

4 arcminute 6cm radio image of the mini-spiral from the VLA	Diagram taken from Genzel, R., Hollenbach, D., & Townes, C., 1994, Reports of Progress in Physics, 57, 417

Zooming in even more ...

1.7 arcminute 2.2 micron image highlighting the stars

Above is a near-infrared (JHK) ~1.7 x 1.7 arcminute false color image of the Galactic Center. The image was taken with the Ohio State Infrared Imager/Spectrometer (OSIRIS) (Blum, Selgren, Ramirez, Ohio State U.). If you look hard, you can match up the pattern of stars in the center of this image with the diagram above.

Finally ...

A 5X5 arcsecond image of the Central Parsec at 2.2 microns taken with the Keck telescope using Adaptive Optics. The inset is about one square arcsecond.

Above is a 5X5 arcsecond (0.2 pc) near-infrared (2.2 micron) image of the very center of the Galaxy taken using adaptive optics on the 10 meter Keck telescope. The image is shows a cluster of stars known as the Central Parsec cluster. The inset is zooming in on the even smaller cluster of stars which are believed to be orbiting around the super-massive black hole, Sgr A* .

So in all, we have zoomed in from a scale of 5 degrees to 1 arcsecond - a factor 18,000!