It just refuses to go down...
Well, I'd play the innuendo card with this paper, at any rate.
Galaxy rotation curves are typically described as flat, meaning that as you go further away from their centres, the orbital speeds of the gas and stars don't change. This is the traditional evidence for dark matter. You need something more than the visible matter to accelerate material up to these speeds, especially the further away you go : if you use the observed matter, the prediction is that orbital speeds should steadily decrease a la Kepler. Unseen dark matter is a neat way to resolve this dilemma, along with a host of other observational oddities.
This paper claims to have extended rotation curves considerably further than traditional measurements and find that the damn curves remain flat no matter how far they go. They reach about 1 Mpc, about the size of our whole Local Group, and still don't show any sign of a drop. This is not at all expected, because eventually the curve should drop as you go beyond the bulk of the dark mass. It is, they say, much more in-keeping with the prediction of modified gravity theories that do away with dark matter, such as MOND.
I won't pretend I'm in any way expert in their methodology, however. A standard rotation curve directly measures the line of sight speed of gas and/or stars, which is relatively simple to convert into an orbital speed – and for qualitatively determining the shape of the curve, the corrections used hardly matter at all. But here the authors don't use such direct kinematic measurements, but instead use weak gravitational lensing. By looking at small distortions of background galaxies, the amount of gravity associated with a target foreground source can be determined. Unlike strong lensing, where distortions are easily and directly visible in individual sources, this is inferred through statistics of many small sources rather than from singular measurements.
Here they go even more statistical. Much more statistical, in fact. Rather than looking at individual lens galaxies they consider many thousands, dividing their sample into four mass bins and also by morphology (late-type spiral discs and early-type ellipticals). The lensing measurements don't give you orbital speed directly, but acceleration, which they then convert into velocity.
1 Mpc is really quite a long way in galactic terms, and it wouldn't be at all uncommon to find another similar-sized galaxy within such a distance : in our Local Group, which is not atypical, there are three large spiral galaxies. Measuring the rotation curve out to such distances then becomes intrinsically complicated (even if you had a direct observational tracer like the gas) because it's hard to know which source is contributing to it.
They say their sample is of isolated galaxies with any neighbours being of stellar mass less than 10% of their targets out to 4 Mpc away, but their isolation criterion uses photometric redshifts*. Here I feel on very much firmer footing in claiming that these are notoriously unreliable. Especially as the "typical" redshift of their lens galaxies is just 0.2, far too low for photometric measurements to be able to tell you very much. Their large sample means they understandably don't show any images, but it would have been nice if they'd said something about a cursory visual inspection, something to give at least some confidence in the isolation.
* These are measurements of the redshift based on the colour of the galaxy, which is extremely inexact. The gold standard are spectroscopic measurements, which can give precisions of a few km/s or even less.
If we take their results as given, they find that the rotation curves of all galaxies in all mass bins remain flat out to 1 Mpc, the limit of their measurement (although in one particular subset this doesn't look so convincing). They also show that in individual cases where they apparently can get good results from weak lensing, the results compare favourably with the direct kinematics they get from gas data.
As often with results questioning the dark matter paradigm, I'd have to describe the results as "intriguing but overstated". I don't know anywhere near enough about the core method of weak lensing to comment on the main point of the paper. But that this is normally in itself as result of statistical inference, and that here they use a very large sample of galaxies and convert the result from the native acceleration measurement to velocity, and that their isolation criteria seems suspect... I remain unconvinced. I'd need a lot more persuading that the weak lensing data is really giving meaningful results to such large distances from the lens galaxies.
What would have been nice to see is the results from simulations. If they could show that their photometric redshifts were accurate enough in simulated cases to give reliable results, and that the weak lensing should given something similar to this (or not, if the dark haloes in the simulations have a finite extent), then I'd find it all a lot more convincing. As it stands, I don't believe it. Especially given that so many galaxies are now known with significantly lower dark matter contents than expected : these "indefinitely flat" rotation curves seem at odds with galaxies with such low rotation speeds even in their innermost regions. Something very fishy's going on.
No comments:
Post a Comment