The Galactic Center at Different Wavelengths

One problem we saw earlier was that we can't look at the Galactic Center in visible light (the light that our eyes can see) because there is dust in the way. In order to study the Galactic center, we need to look at it at different wavelengths of light that our eyes are not sensitive to. This can also help us uncover what is happening at this very exciting part of our Galaxy.

The Very Large Array (VLA). Credit: NRAO

As you can see from the above images, while the optical light is dark, the radio, sub-millimeter, mid-infrared, near-infrared, and X-ray light can all penetrate through the dust. Different kinds of telescopes are necessary to take data at these different wavelengths. The quality of the data one gets from a telescope is often measured by the resolution of the telescope. This is proportional to the wavelength the data is taken at divided by the diameter of the telescope (Resolution ~ Wavelength/Diameter). For instance, in order to obtain high-resolution images at radio (long) wavelengths, we must use an array of telescopes that have a long effective baseline, but can be broken up into smaller telescopes. One example of this is the Very Large Array (VLA) in Soccoro, New Mexico. However, high resolution infrared and optical images are being produced with telescopes containing a single 10 meter mirror diameter, such as the twin Keck telescopes in Mauna Kea in Hawaii. Let's take a look at some high-resolution images of the Galactic Center at various wavelengths.


The Galactic Center was first detected in 1932 at radio wavelengths by the pioneer of radio astronomy, Karl Jansky. However, at that point in time, radio astronomy was largely done with a single large dish-like telescope, not arrays like the VLA. These large dishes could just detect a strong radio source toward the Galactic Center and not mch else. Since then, some of the most striking images of the Galactic Center have been produced by the VLA. The image to the left shows an example of Galactic Center radio data. The two most prominent features are the non-thermal and thermal radio arcs or "filaments" and a collection of ring-shaped supernova remnants (SNRs). The brightest part is towards the Galactic Center. Note that the wavelength of these observations are 90 centimeters. The wavelength of light coming from these observations is about 1.8 million times longer than the wavelengths of light our eyes can see!

An image of the Galactic Center taken with the VLA.

The whispy radio filaments that are visible in the above image wre first imaged by UCLA Astronomy Professor Mark Morris and his coleague Farhad Yusef-Zadeh in the mid 1980's. These unique structures have only been observed toward the Galactic Center and are very strongly magnetized. The emission coming from these filaments happens as a result of charged particules spiraling at relativistic speeds around magnetic field lines.

VLA 20 cm image of the radio arcs (Lang, C. and Morris, M.)


A Planck image of the Galaxy. Credit: ESA

Microwave data shows emission from cool gas and dust. The to the left shows recent results from Planck, the European Space Agency's (ESA) recent microwave space-based mission. Since Earth's atmosphere absorbs microwaves so efficiently, all observations at this wavelength must be done in outer space with satellites (like COBE and Planck). These results show previously unknown groups of cold gas and a "haze" of microwaves. This is really important because the wavelength that this image is taken at traces carbon monoxide (CO). CO is a cold molecule that is easily detectable and traces molecular hydrogen (two hydrogen atoms bound together), which is required for stars to form. The "haze" could come from synchrotron emission.


The mid-infrared wavelengths trace cold objects and are incredibly efficient in cutting through the dust that obscures our view to the Galactic Center. In this Chandra mid-image, we are able to see cold dust, dust-covered stars, faint stars, and dense arcs of dust. To observe in the mid-infrared, however, astronomers have to either send satellites into space or observe in the stratosphere, like the SOFIA observatory.

The GC in the Mid-Infrared. Credit: Chandra


Keck high-resolution image of the Galactic Center. Sgr A* corresponds to the central black hole.

Near-infrared wavelengths are a very popular region of the electromagnetic spectrum for observing the Galactic Center due to both thier ability to pass through the dust along the plane as well as our ability to get high angular resolution images at these wavelengths. Also, near-infrared detectors (which are the near-infrared version of the CCDs that are in cameras) are progressively getting better. Near-infrared images of the Galactic Center 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 taken in the Galactic Center Group at UCLA) could only resolve groups of stars.

4 micron image of the Galactic Center.

In fact, high angular resolution near-infrared images by the Keck telescope have been able to resolve a tiny cluster of stars orbiting the supermassive blackhole at the very center of our Galaxy. We have also been able to obtain spectra of the stars. This means that we take the near-infrared light and separate it by wavelength. This allows us to determine what kinds of stars exist in the Galactic Center, and we have found blue supergiants, red giant stars, dust obscured stars, and so on. With bigger telescopes on the way (like the Thirty Meter Telescope), there is much more near-infrared research ahead!


An optical image of the Galactic plane

It is nearly impossible to observe anything at the Galactic Center at optical wavelengths. While our telescopes to have the resolution to do so, the large amount of starlight-obscuring dust along the plane of the Galaxy causes any optical light from the Galactic Center to be extincted by 30 orders of magnitude (1 X 10^30).


An X-ray image of the Galactic Center. Credit: Chandra and APOD

The image to the left shows X-ray emission from the Galactic Center taken with the Chandra X-Ray observatory. The points of light seen in this image show hot white dwarfs, neutron stars, and black holes, all swimming in a hot gas many millions of degrees. The brightest patch in the center is the supermassive black hole. Star clusters are also visible that host massive, x-ray emitting stars. The gas here 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. The Galactic Center is truly an energetic region.