The Black Hole
Andromeda Galaxy - M31
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
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
|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
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.
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 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.