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Originally Posted by Cougar
The English is not always stellar.
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Good one!
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What do you think is meant when the authors say, "We should stress that we cannot exclude that those objects for which the available kinematical data do not give convincing evidence for dark matter are actually embedded in massive diffuse halos, as optical and radio studies of spiral galaxies have shown to be the case for disk galaxies..." [page 390]
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They continue on and refer back to the introduction. In the introduction they talk about the hydrogen studies of spiral galaxies. Spirals have an optical disk - which is what we see in visible light and for most spirals a hydrogen disk extends well beyond the optical disk. This is where the 21cm hydrogen linewidth profiles come from that provide one method of determining rotational velocity. Optical rotation curves provide the other method.
What is observed is that the rotation curves remain flat beyond the optical disk in most spirals. So this is the evidence for a dark halo in spirals. If it had a keplerian fall off then there would be no evidence for a dark halo. (In the baryonic dark matter thread I was simply pointing to some evidence that this halo could actually be composed of molecular hydrogen gas - but the gravitational effect is still the same.)
What they're referring to on page 390 is that their data is within the optical portion of the elliptical galaxies. So they cannot rule out the presence of a dark halo beyond the optical extent of the galaxy simply because their data does not extend out that far. However, the kinematic data that they have been able to recover only indicates the presence of a dark halo for NGC 7796. But there should be some signature of the dark halo on the optical kinematics of the galaxies too - which is why they conclude that they have no evidence for those galaxies that the dark halo is actually there.
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Also, I note the authors' final word on this matter states, "The few trends pointed out above have to be taken only as a simple practical way to direct our attention to possibly interesting clues within a very small and biased set of galaxies. We expect that future surveys will allow for a more objective selection of elliptical galaxies."
I must have missed that part. Why do they refer to their set as "biased"?
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Their sample was biased because they had to select very specific types of ellipticals to conduct their study.
They wanted high surface brightness ellipticals to improve signal to noise ratios. They wanted galaxies with angular diameters small enough so that they could sample the sky background because that light must be subtracted. They wanted objects with large central velocity dispersions and they wanted round ellipticals classified as E0 or E1.
So in selecting these galaxies they are not sampling the full range of elliptical galaxy morphology, surface brightness, central velocity dispersion and size. That is their selection bias and they have to acknowledge it because it cannot be certain that their results extend to all the different types of ellipticals.