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The companion looks so irregular on optical photographs (Figures 134 and 135), as if severely disturbed by a close encounter with the majestic spiral Messier 51. Could it not, however, be that NGC 5195 is proudly wearing its mask of cosmic dust and gas?

Penetrate the Cosmic Shroud of NGC 5195, and behold its magnificent, symmetric structure (Figure 136): no longer does chaos reign supreme, but its backbone of old stars is stunningly symmetric. Together with its symmetric disk is a hint of an incipient two-armed spiral galaxy.

If one were to first show a student an infrared image of this galaxy as seen behind its cosmic shroud, and then ask the student to determine what the galaxy might look like optically - this would be an impossible task, because of the duality of spiral structure. NGC5195 just looks so dramatically different when penetrating its dusty Shrouds of the Night.

An object of intense astrophysical interest is our closest active radio galaxy, known as Centaurus A (Figure 137). Its complex and intriguing optical structure was noted by Sir John Herschel who observed it from the Cape of Good Hope. In 1954, the astronomers Baade and Minkowski suggested that Centaurus A actually represented two galaxies in collision.

What is so striking in Figure 137 is the colossal mask of dust striding across the central region of the galaxy. What might be lurking behind the dust mask of Centaurus A? In the year 1999, a group of French astronomers led by F. Mirabel generated an infrared view of Centaurus A. In collaboration with French astronomer M. Sauvage, we generated specialized dust maps, to probe the actual distribution of the dust grains - in order words, to effectively probe the shape of the mask.

The Mirabel images and our dust maps beheld a great secret: right in the central region of Centaurus A lay what appears to be the remnant of a spiral galaxy (Figure 138). Baade and

Minkowski were correct; it is conceivable that optical images show that which has remained The Dust Penetrated Universe

of a spiral galaxy after colliding with a large elliptical galaxy, forming the imposing structure which we see today.

Worlds in collision ... galaxies in collision. It is no wonder that Centaurus A shows the most exquisite outer loops of stars when images of this merging system are made. Figure 139 shows one such view, generated by a team of astronomers led by E. Peng and which included one of the authors (Ken). Centaurus A truly spawns vast arrays of shells of stars, as these two worlds collided .

How vastly different do infrared images of Centaurus A appear to optical ones; the dust mask of Centaurus A carefully retained its inner secrets, securely kept locked in time and space for epochs of millions of years, until instruments on our Earth revealed the inner spiral galaxy not even two decades ago (Figure 138). The grand dust shroud of Centaurus A had at last been penetrated.

One of the most famous spiral galaxies in the southern skies is the galaxy designated NGC 253 in the constellation of Sculptor. An optical image (Figure 140) shows a plethora of short, fleece-like spiral arms, with no apparent regularity whatsoever. As with NGC 309, a grand revelation awaits the investigator behind its dusty Shroud of the Night (Figure 141). In infrared light, NGC 253 exquisitely reveals a hidden symmetry of two dominant spiral arms and a bar beneath the galactic froth.

Enter the world's grandest infrared space telescope, the Spitzer Space Telescope, named after one of Princeton's foremost astrophysicists, the late Lyman Spitzer, Jr. The Spitzer Space Telescope is the last in NASA's Great Observatories Programme; the first is the Hubble Space Telescope, deployed by the Space Shuttle Discovery in 1990. The second Great Observatory was launched and deployed by the Space Shuttle Atlantis in 1991 and was known as the Shrouds of the Night Compton Gamma-Ray Observatory. This mission collected data on some of the most violent

230 physical processes in the Universe, characterized by their extremely high energies. It was

The Dust Penetrated Universe

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