safely de-orbited in 2000. The third member of the Great Observatory family, the Chandra X-Ray Observatory (CXO), was deployed from the Space Shuttle Columbia and was boosted into a high-Earth orbit, in July 1999. This observatory is observing such objects as black holes, quasars, and high-temperature gases throughout the x-ray portion of the spectrum. Finally, the Spitzer Space Telescope, launched in 2003 on a Delta-II rocket.
The marvel of the Spitzer Space Telescope is that astronomers can actually see the radiation emitted from tiny molecules and minute grains of dust - instead of dark dust rifts and patches, one views glowing shrouds of dust! We have been privileged to study images of galaxies secured through the eyes of the Infrared Array Camera (abbreviated as IRAC) onboard the Spitzer Space Telescope. IRAC is the brainchild of pioneer Giovanni Fazio (Figure 142) at the Harvard-Smithsonian Center for Astrophysics, who together with his highly dedicated team, devoted over 15 years from the first conceptual phases of the camera to its final construction prior to launch. Fazio and team are truly to be saluted on the work of a masterpiece.
Turn the eyes of IRAC toward our closest spiral galaxy, the majestic Andromeda Spiral (Figure 143), which is located at a distance of about 2.5 million light years from us, with an estimated diameter of 140 000 light years.
In the historic photographs by Barnard, Roberts, Keeler, Hubble and others, the Andromeda Spiral Galaxy looks so quiescent; so tranquil - so calm. However, appearances can be so deceiving.
Images secured with IRAC reveal two glowing rings of fire (Figures 144 and 145). The outer ring has a diameter of approximately 65 000 light years. The second ring of dust immediately beckoned our attention; it betrayed a crucial secret. A feature kept secret for over 200 million years. The dimensions of this ring are some 4900 light years by 3300 light years. Why had it never been discovered before? The amazing inner ring is completely hidden, in optical light, by the luminous stars in the central bulge of the Andromeda Galaxy!
What event could have caused these extraordinary set of rings, whose centers do not coincide with the center of the Andromeda Galaxy (Figure 146)?
The penny dropped. Key insights were provided by French astronomers Françoise Combes and Frederic Bournaud.
A head-on galaxy collision of the Andromeda Spiral Galaxy with one of its companion galaxies, Messier 32! One smaller galaxy heading for an almost direct hit with a much larger one. The Andromeda Galaxy has over two dozen companion galaxies, one of which was catalogued by Messier as number 32 in his list (hence the name, Messier 32).
The culprit in this galactic "hit and run" is M32 colliding with the Andromeda Spiral! One may think of a simple analogy - that of tossing a stone into a pond of water. Rings or ripples are created, traveling outward with time.
Our team included members from South Africa, the United States and France. French team members Frederic Bournaud and Francoise Combes began to simulate the history of the Andromeda Spiral. Using highly sophisticated computer codes, it was discovered that Messier 32 has indeed collided almost head-on with the Andromeda Galaxy, creating a pair of off-centered rings observed by the Spitzer Space Telescope.
Messier 32 had impacted the disk at over 250 kilometers per second, creating two rings of fire, whose outward expansion velocities are about 50 and 18 kilometers each second, respectively. In cosmologi-
cal terms, the time of collision is remarkably short: only 200 million years ago. On Earth, the continents had not yet separated but dinosaurs indeed roamed freely.
It was an astronomical task for Pauline Barmby at Harvard University and her team to image the Andromeda Spiral Galaxy with the Infrared Array Camera on-board the Spitzer Space Telescope. That galaxy covers such a huge angular size in the sky: over six full moons. The field of view with the infrared camera IRAC is reasonably small, and so the telescope was moved in 700 different positions to cover the entire galaxy.
Exquisite hidden structures are revealed behind the mask of dust, so beautifully drawn by Trouvelot in 1874 (Figure 147). The Empress Andromeda is found to wear new clothes, when examined not in the optical, but in the infrared section of the spectrum. Seeming
© OUTER RING CENTER ©INNER RING CENTER © GALAXY CENTER
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