In a nutshell, a recent observation of what appeared to be an outlier isolated ultra-diffuse galaxy with no dark matter phenomena was probably due to an underestimation of the systemic error they experienced in estimating its inclination relative to Earth from which we are observing it. It probably isn't a weird outlier after all.
A recent paper blogged here found that ultra-diffuse galaxy AGC 114905 appeared to show no signs of dark matter or MOND effects, despite the fact that MOND should have applied since nothing else was close enough to provide an external field effect, and despite the fact that there was no obvious mechanism in a dark matter particle based analysis by which the galaxy could have experienced tidal stripping of dark matter that left it with only stars and interstellar gas.
An analysis of the result by Stacy McGaugh, a leading MOND proponent among astronomers, also blogged here, argued that systemic error in one of the measurements made was greatly underestimated and probably explained the result.
A new paper by J.A. Sellwood and R.H. Sanders, which will be published in the same journal that published the original results, also strongly suggests that the same systemic error which McGaugh highlighted as likely being underestimated and which the authors of the original paper flagged as the biggest factor that could impact their "no dark matter" phenomena hypothesis is probably the explanation of this outlier result. Sellwood and Sanders note that in a hypothesis without dark matter or MOND and with the margin of systemic error in determining the inclination of the galaxy relative to an Earth observer in the original paper, that AGC 114905 would be unstable and fly apart, despite showing no other signs of being an unstable system.
The end result affirms that existing paradigms and also represents the scientific process in which scientists critique other scientists until a satisfactory conclusion is reached to explain what we observe.
The paper and its abstract are as follows:
Recent 21 cm line observations of the ultra-diffuse galaxy AGC~114905 indicate a rotating disc largely supported against gravity by orbital motion, as usual. Remarkably, this study has revealed that the form and amplitude of the HI rotation curve is completely accounted for by the observed distribution of baryonic matter, stars and neutral gas, implying that no dark halo is required. It is surprising to find a DM-free galaxy for a number of reasons, one being that a bare Newtonian disk having low velocity dispersion would be expected to be unstable to both axi- and non-axisymmetric perturbations that would change the structure of the disc on a dynamical timescale, as has been known for decades.
We present N-body simulations of the DM-free model, and one having a low-density DM halo, that confirm this expectation: the disc is chronically unstable to just such instabilities. Since it is unlikely that a galaxy that is observed to have a near-regular velocity pattern would be unstable, our finding calls into question the suggestion that the galaxy may lack, or have little, dark matter.
We also show that if the inclination of this near face-on system has been substantially overestimated, the consequent increased amplitude of the rotation curve would accommodate a halo massive enough for the galaxy to be stable.
2 comments:
It would be funny if the whole MOND saga one day became a standard example of how to do science, given how much controversy and accusation of bad science there has been. Science is trial and error, and that includes sociological and methodological error at the level of paradigms.:)
I think pretty much anyone who has followed the modified gravity v. dark matter debate sees the parallels to pre-Copernican revolution epicycles.
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