Short on time at the moment, so I'll leave deeper analysis for later.
The first article discusses the strong evidence for an external field effect from prior work which I have blogged, suggestive of a MOND-like gravitational issue and providing a distinguishing observation to separate cold dark matter theories from modified gravity theories.
The second article purposes to find a no dark matter galaxy that can't be explained by MOND either (presumably meaning that it is not a good external field effect candidate, although I'll need to look closer to dope that out).
The external field effect in cold dark matter models
Aseem Paranjape, Ravi K. Sheth
Comments: 10 pages, 6 figures, submitted to MNRAS
In general relativity (GR), the internal dynamics of a self-gravitating system under free-fall in an external gravitational field should not depend on the external field strength. Recent work has claimed a statistical detection of an `external field effect' (EFE) using galaxy rotation curve data. We show that large uncertainties in rotation curve analyses and inaccuracies in published simulation-based external field estimates compromise the significance of the claimed EFE detection. We further show analytically that a qualitatively similar statistical signal is, in fact, expected in a Λ-cold dark matter (ΛCDM) universe without any violation of the strong equivalence principle. Rather, such a signal arises simply because of the inherent correlations between galaxy clustering strength and intrinsic galaxy properties. We explicitly demonstrate the effect in a baryonified mock catalog of a ΛCDM universe. Although the detection of an EFE-like signal is not, by itself, evidence for physics beyond GR, our work shows that the sign of the EFE-like correlation between the external field strength and the shape of the radial acceleration relation can be used to probe new physics: e.g., in MOND, the predicted sign is opposite to that in our ΛCDM mocks.
No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905
Pavel E. Mancera Piña, et al.
Comments: Accepted for publication in MNRAS
We present new HI interferometric observations of the gas-rich ultra-diffuse galaxy AGC 114905, which previous work, based on low-resolution data, identified as an outlier of the baryonic Tully-Fisher relation. The new observations, at a spatial resolution ∼2.5 times higher than before, reveal a regular HI disc rotating at about 23 km/s. Our kinematic parameters, recovered with a robust 3D kinematic modelling fitting technique, show that the flat part of the rotation curve is reached. Intriguingly, the rotation curve can be explained almost entirely by the baryonic mass distribution alone. We show that a standard cold dark matter halo that follows the concentration-halo mass relation fails to reproduce the amplitude of the rotation curve by a large margin. Only a halo with an extremely (and arguably unfeasible) low concentration reaches agreement with the data. We also find that the rotation curve of AGC 114905 deviates strongly from the predictions of Modified Newtonian dynamics. The inclination of the galaxy, which is measured independently from our modelling, remains the largest uncertainty in our analysis, but the associated errors are not large enough to reconcile the galaxy with the expectations of cold dark matter or Modified Newtonian dynamics.
10 comments:
So both MOND and cold dark matter are missing something?
@Ryan
"So both MOND and cold dark matter are missing something?"
We already knew that.
MOND underestimates dark matter phenomena in galaxy clusters, and needs a relativistic generalization to work in other circumstances.
CDM is a disaster at explaining all sorts of galaxy scale phenomena in a predictive manner and behaves in ways correlated with ordinary matter distributions to an extent that you need a quite baroque epicycle of the CDM theory to explain.
But we didn't know that in the particular instance of small galaxies with no apparent dark matter, only two other cases of which have previously been documented. Both theories have superficially plausible ways to deal with the other two cases.
Other similar cases have been explained in MOND with the external field effect, and have been explained by CDM theories primarily with tidal stripping from a nearby galaxy.
If this galaxy with no apparent dark matter in it doesn't have another galaxy nearby to provide either an external field effect or tidal stripping, then both of MOND and CDM are inadequate to explain this particular case.
I have to read it more closely to see if there really is no nearby galaxy for purposes of these solutions.
But, if that is the case, my first instinct would be to turn to Deur's approach and see if the geometry of this galaxy with no apparent dark matter could explain this result since Deur's theory implies that dark matter-like phenomena should be absent in cases where the mass distribution is close to a homogeneous spherically symmetric one, even if the strength of the acceleration from the gravitational field is less than MOND's a(0) threshold.
Of course, it is entirely possible that all of these theories are contrary to this data point, or that this data point is wrong due to some observational error. Right now it is a single data point outlier so cautious skepticism is in order.
After looking more closely at the paper, I can confirm that they at least made a serious effort to consider the proximity of other galaxies and to find an isolated one. They also made a good case that the principal source of systemic error in the observation would have to be unreasonably large to produce such an anomalous result.
While they characterize the galaxy (or at least its star component) as disk-like, but is notable for being gas rich, rather than star dominated. Interstellar gas rather than stars are the predominant source of mass in this galaxy outside its core, and maybe is also less disk-like. So this does tend to favor a Deur-like analysis of its geometry.
MOND underestimates dark matter phenomena in galaxy clusters, and needs a relativistic generalization to work in other circumstances.
CDM is a disaster at explaining all sorts of galaxy scale phenomena in a predictive manner and behaves in ways correlated with ordinary matter distributions to an extent that you need a quite baroque epicycle of the CDM theory to explain.
why not both sides are correct
there are both MOND and dark matter
@neo - that's possible (maybe likely even) but I think most researchers are hoping for a simple answer rather than a complex one.
@Ryan While it is possible, I don't think it is remotely likely.
@ Ryan - that's possible (maybe likely even) but I think most researchers are hoping for a simple answer rather than a complex one.
conformal gravity + sterile neutrinos + axion + micro black hole
@andrew
why not ?
occam's razor
@neo
Because it would be virtually impossible to devise a form of supplemental dark matter that would go where it would need to be for a MOND-like theory.
@andrew
it would only need to be in galaxy clusters
maybe it's black holes or warm dark matter
My preferred explanation is a dark regime that is as complex or more complex than baryonic matter. But that's for aesthetic reasons (ie the mediocrity principle) more than anything.
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