The Galactic Center at Different Wavelengths

It is already evident from zooming in to the very center of our Galaxy that there are many things occurring on a wide range of size scales. Also, as the above diagram shows, different physical processes produce light at different wavelengths providing us with even more information to understand the phenomena we observe. Stars emit a continuum of light either at optical (visible) or infrared wavelengths (see below). Gas emits light at very specific wavelengths that correspond to electron transitions within the atomic structure of the atoms composing the gas. It also emits a continuum of X-rays when heated to extremely hot temperatures. Dust, which is usually much cooler than stars or gas, emits a continuum of light mostly in the infrared and longer wavelengths.

Due to the nature of light and optics, different kinds of telescopes are needed to measure the emission of objects at different wavelengths. The resolution, R, of a telescope (Resolution is the amount of detail that can be seen by the telescope.) is limited by the size, D, of its primary mirror and the wavelength, lam, observed. The relationship is R~D/lam. For instance, in order to obtain high resolution images at radio wavelengths we must use an array of telescopes to simulate one telescope with an extra large mirror. The Very Large Array (VLA) in Socorro, NM, is a good example of this type of telescope which is called an interferometer. One the other hand, high resolution infrared and optical images are being produced with telescopes containing a single 10 meter diameter mirror. Examples of these telescope are the Keck twin telescopes and the Gemini telescope on the top of Maunea Kea in Hawaii.

Despite recent developments in telescopes with large primary mirrors (~10 meters), observations are still blurred due to turbulence in the Earth's atmosphere (This is called "seeing".). To counteract this effect, astronomers use a method called adaptive optics (AO) to correct for the blurring in the atmosphere by mimicking the distorted light wave and then reconstructing it so that it seems like there is no atmosphere at all. It is this seeing problem which prompted the existence of the Hubble Space Telescope which orbits above the Earth's atmosphere thus avoiding the problem altogether. Now lets have a look at the Galactic Center beginning at the longest wavelengths (radio) and proceeding to the shortest wavelengths (X-rays).

RADIO

The Galactic Center was first detected in 1932 at radio wavelengths by the pioneer of radio astronomy, Karl Jansky. However, back then, radio astronomy pretty much involved large single dish detectors which could just detect a strong radio source toward the direction of the Galactic Center and not much else. Since then, some of the most striking images of the Galactic Center have been produced by the Very Large Array (VLA) telescope. The image below was produced by the VLA and is a good representation of the wide variety of phenomena occurring at the Galactic Center. The two most prominent features are the non-thermal and thermal radios arcs or "filaments" and a collection of ring-shaped supernova remnants (SNRs).

VLA image of the GC taken at a wavelength of 90 cm	Diagram from Genzel, R., Hollenbach, D., & Townes, C., 1994, Reports of Progress in Physics, 57, 417

VLA 20 cm image of the radio arcs (Lang, C., Morris, M.) These radio filaments were first imaged by UCLA Astronomy Professor Mark Morris and his colleague Farhad Yusef-Zadeh in the mid 1980's. These unique structures have only been observed toward the Galactic Center and are strongly magnetized. The radio emission arising from these filaments is the result of synchrotron radiation - the radiation from charged particles which are spiraling at relativistic speeds around magnetic field lines. In fact, these filaments, which are coherent and highly ordered over hundreds of light years and are oriented perpendicular to the plane of the galaxy, are believed to trace the Galactic Center's large scale magnetic field which has strengths 100 to 1000 times stronger than those found in the Galactic disk.

6cm VLA image of the mini-spiral

MICROWAVE

COBE Image of the Galactic Plane

The above image was taken with the Cosmic Microwave Background Explorer (COBE) at microwave wavelengths and show emission from the gas and dust along the plane of the Galaxy. Since the Earth's atmosphere absorbs microwaves so efficiently, all observations at this wavelength must be done in outer space with satellites like COBE or SIRTF or at least at very high altitudes like the future airborne observatory, SOFIA.

NEAR-INFRARED

1.7X1.7 arcminute JHK micron image (Sellgren et al., Ohio State)	30X30 arcsecond HK micron image taken with the 8-m Gemini telescope

Near-infrared wavelengths have always been a popular region of the EM spectrum for observing the Galactic Center due to both their ability to pass though the dust along the plane as well as our ability to get high resolution images at these wavelengths. Also, near-infrared detectors have been around for a long time allowing most 3-10 meter class telescopes to collect electronic images in the near-infrared. Near-infrared images of the Galactic Center mostly probe the stellar content showing an abundant amount of stars. The earliest near-infrared images of the Galactic Center could only resolve groups of stars while recent observations like those shown above can resolve the individual stars within the clusters and be able to see the faintest members of the clusters.

In fact, high resolution near-infrared observations by the Keck telescope have been able to resolve a tiny cluster of stars orbiting around the supermassive black hole at the very center of our galaxy. However, only in the past few years have we begun to resolve the stars enough to obtain their stellar spectra which will reveal the kinds of stars they are - blue supergiants of red giant stars, for instance. So there is much more near-infrared research ahead using new 10-meter class telescopes like Gemini and the VLT as well as Keck.

MID-INFRARED

OPTICAL

Optical image of the galactic plane

No matter how much we want to, it is next to impossible to observe anything at the Galactic Center at optical wavelengths. While our telescope do have the resolution, the copious amount of dust along the plane of the galaxy causes any optical light from the center to be extincted by 30 orders of magnitude. Maybe in the far future when we put planet size telescopes in outer space to collect the few photons which do escape the dust clouds or just travel the 24,000 light year distance and have a look for ourselves!

X-RAY

X-ray image taken with the Chandra X-ray satellite

The image has been smoothed to bring out the X-ray emission from an extended cloud of hot gas surrounding the supermassive black-hole candidate Sagittarius A* (larger white dot at the very center of the image- a little to the left and above the smallest white dot). This gas glows in X-ray light because it has been heated to a temperature of millions of degrees by shock waves produced by supernova explosions and perhaps by colliding winds from young massive stars.