Like a man traveling in foggy weather, those at some distance before him on the road he sees wrapped up in the fog, as well as those behind him, and also the people in the fields on each side, but near him all appears clear, though in truth he is as much in the fog as any of them.
The setting: the Mauna Kea Observatories in Hawaii. The galaxy: one of the largest spiral galaxies yet identified, known as NGC 309. We had, for years, known that this galaxy was so large, that neighboring spiral galaxies (such as Messier 81) could comfortably fit in between its gargantuan spiral arms (see Figure 131). The challenge: to image this galaxy using near-infrared camera arrays, which had then only recently been declassified by the US military and made available for general use.
When the dust penetrated image of NGC 309 first appeared on our computer screens, we questioned whether the telescope had actually been pointed at the correct object! Figure 131 shows an optical image of the galaxy and Figure 132 the near-infrared view. We simply could not believe our eyes! What emerged from behind the shroud was a two-arm grand design spiral, with a small central bar, barely discernable on optical images. (Grand design spiral galaxies are famous for their prominent and well-defined spiral arms, which can be traced over large angles and which cover a large fraction of the disk of the galaxy. They truly are grand in design. In contrast, flocculent spiral arms as seen in Figure 140 and Figure 152 are short, fleece-like and the multi-arms in these galaxies cover only small areas of the disk.) Could it be that NGC 309 was hinting at a fundamental new and general Hidden Symmetry in Nature: that behind the dust shrouds of many spiral galaxies could lie beautifully symmetric stellar backbones?
Upon writing up our findings (which subsequently appeared on the cover of the British scientific journal Nature), we noted that NGC 309 seemed to change its Hubble type, when imaged through its Shroud of the Night. Could it be true that Nature was to present astronomers with a daunting new view of galaxy structure, which may be unrelated to their optical Hubble types? The Dust Penetrated Universe
The Dust Penetrated Universe
Figure 132  221
The possibility of such a result had been anticipated by astronomer Vera Rubin and her collaborators. Dr Rubin and her team had studied dozens of spiral galaxies, and had noted that Sa, Sb and Sc spiral galaxies could all have the same shape of their rotation curve, and so belong to the same "form family." The implications of Rubin's studies are profound: galaxies of Hubble types spanning Sa to Sc may have similar backbones of stars in the near-infrared; for it is the stellar backbones of galaxies to which Rubin's "form families" of rotation curves are inextricably tied.
Earlier, in 1957, Fritz Zwicky commented on a comparison of blue and yellow plates of the Whirlpool Galaxy, Messier 51, and its companion NGC 5195. The plates were taken using the 200-inch reflector at Mt. Palomar. In a famous book entitled Morphological Astronomy., Zwicky's mind panned the future:
We are confronted with the possible case of being a normal spiral when seen in the light of its blue stars [young stars] and a barred spiral in the light of the yellow-green stars [older stars].
The photographic work of Zwicky was extended by astronomers Stewart Sharpless and Otto Franz, who published an important paper in 1963, entitled "Composite Photography of Galaxies." In 1964, the astronomer Merle Walker (then at the Lick Observatory, California) produced composite photographs of our nearby spiral galaxy Messier 33, using negatives taken in blue and infrared light with the 120-inch reflector at the Lick Observatory.
Composite photography (Figure 133) uses photographs taken in two different spectral regions (for example, blue and yellow), and contact positives are made from these. What changes as one images galaxies from blue to yellow light? There is less star formation and less obscuring effects of dust seen in the yellow images, so that more light is seen from the older stars. The positive corresponding to the one spectral region was then superposed upon the negative corresponding to the other. The composite photographic technique was a first, early step to "penetrate the dust mask;" to see old stars which do Shrouds of the Night not contribute a major share of the light, but which do contribute a major portion of the
222 mass in the optical extent of a galaxy. "Such a method is potentially an important one
for the study of the morphology of galaxies" wrote Sharpless and Franz. The methodology used by Sharpless and Franz was first described by Charles Fehrenbach and Alice Daudin, in 1945.
Clearly, we desperately needed images of other galaxies. In 1993, Harry van der Laan, then Director-General of the European Southern Observatory, invited David to work at their Headquarters in Garching (near Munich) for several months, and, during that time, we had access to their telescopes at La Silla. One telescope in particular had been fitted with a special near-infrared camera. As we image galaxies further and further toward the near-infrared, the pervasive influence of dust grains gradually declines and the contribution of older stars increases. What emerges through the dusty Shrouds of the Night is the dominance of an older population of stars: it is this older population which forms the backbone of these spirals.
As noted in Chapter 3, we spent one entire night imaging NGC 2997 in the Atacama desert in Chile, soaking up near-infrared photons (Figure 21). It was indeed painstaking to construct the mosaic from multiple images, but so worthwhile, for we were successfully penetrating its dusty shroud (Figure 22).
The duality of spiral structure was, once again, seen to full advantage: our infrared view of this galaxy (Figure 21) reveals two smooth arms but one arm appears to be more dominant than the other; moreover, the famous optical spur simply does not have any prominent counterpart at all. It was also intriguing to note that remnants of light from some dust-enshrouded younger ionized hydrogen regions actually remained visible behind the Shroud of NGC 2997.
We well remember our excitement after examining infrared images of the companion galaxy to the Whirlpool Galaxy, Messier 51. After all, the Whirlpool Galaxy and its companion (designated NGC 5195) have been photographed by both professional and amateur astrono-Shrouds of the Night mers countless numbers of times over the years.
Was this article helpful?