The central bars in spiral galaxies are known to be very efficient mechanisms for driving spiral structure, much like a rotating rod of steel in water. If indeed more conspicuous (stronger) spirals are driven by more conspicuous (stronger) bars, the intriguing question arises as to what drives a spiral arm pattern in a galaxy without a central oval or elongated bar.
Admittedly these situations are not that common, but such galaxies certainly do exist: the beautiful spiral NGC 2997 discussed in earlier chapters, is one example of a galaxy which does appear to be almost devoid of any bar in its old population of stars (Figure 21).
A key point here lies in the actual shape of the halo of dark matter in which a galaxy is embedded. If the dark halo is somewhat football-shaped, and the football is rotating, then the angular pull or torque on the gas from the halo of dark matter may be large enough to induce a spiral structure in the gas! Football-shaped halos of dark matter; clearly stuff for the next decades of astronomy, and beyond!
We wish to allude to an important point here: it is often not appreciated how extensive the disks of spiral galaxies can be, outside of their optical boundaries. Figures 162-168 produced by the famous Australian astrophotographer David Malin show the exquisite structure in the outer disks of several famous spirals: NGC 2997, NGC 1566, Messier 83 and others. David Malin has overlaid in color, the normal optical image of each galaxy in these accompanying figures.
These outer disks are believed to contain the same population of (principally old) stars which we see when we image galaxies in the near-infrared, behind their masks of dust. The reason we cannot capture these amazing outer disks is that although their dust content must be low and almost devoid of brilliant young blue stars, they are exceedingly faint: far too faint to be detected using traditional infrared detectors. Historically, astronomers resorted to special methodologies in their darkrooms.
In the photographic era, it was common practice at certain observatories to "stack" the images. What we mean by this, is that several long exposures photographs of a galaxy
would be secured. The grain structure in every photographic plate is different - and one can think of the grain structure as the "noise." Photons from the galaxy are "the signal." To increase the signal, one needs to decrease the noise by "stacking" the images: in simple terms, one sequentially projects several positive images of a galaxy onto a sheet of paper - or film. Prior to projection, careful registration (alignment) of every photograph is, of course, absolutely crucial.
To enhance the faint outer disks further, David Malin would "stack" photographs of galaxies which had each been "photographically-amplified" before the "multi-image" or "stacking" process, as in Figures 162-168. With the advent of digital camera-arrays, the combination of many separate long-exposure images is done on computers - without the need to spend hour upon hour in a darkroom. We used the same "stacking" procedure at the darkrooms of the European Southern Observatory in Garching (near Munich) to record extremely faint details in the outer regions of a stellar nursery in our Galaxy, known as the Rosette Nebula (Figure 9).
Nowadays, with digital detectors on giant telescopes, astronomers can image individual stars in the faint outer disks, even beyond the photographic boundaries seen in these figures. In a recent study by Ken and colleagues using the 8-meter Gemini South telescope in Chile, stars in the outer disk of the galaxy NGC 300 were studied which lie seventy percent further in radius than those seen in conventional optical images of that galaxy.
It is intriguing to contemplate that the old stars in these faint outer disks must be responding to the dark matter halo in each galaxy in which they are embedded - the halos are dynamically "live" and the interaction of stars and dark matter must be continuously taking place. We know that the dark matter distribution must be following or responding to the gravitational field of the stars within the optical boundary, but the reverse is true outside of their optical limits, where the dark matter content dominates. There, the stars in the exceedingly faint outer disks (so brilliantly captured by David Malin in Figures 162-168) must be following the dark matter halo. These images serve to highlight the dynamic interaction between the structure of spiral galaxies which we see in optical images and their mysterious halos of Eyes to the Future nonluminous matter. 311
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