New Hubble Paper On Lensing In Galactic Clusters

[ohwilleke]

TL;DR Summary

Hubble telescope observations of gravitational lensing in galactic clusters have show results not expected in the lambdaCDM "standard model of cosmology". I'm interested in commentary on the result.

A new report based on Hubble Space Telescope observations is a big deal because it presents a new and independent apparent disparity between the lambdaCDM predictions for dark matter phenomena in galactic clusters and what is observed via gravitational lensing. The paper and its summary and abstract are as follows (paragraph breaks inserted editorially):

The large mass of a galaxy cluster deflects light from background objects, a phenomenon known as gravitational lensing. The large-scale gravitational lens caused by the whole cluster can be modified by smaller-scale mass concentrations within the cluster, such as individual galaxies.

Meneghetti et al. examined these small-scale gravitational lenses in observations of 11 galaxy clusters. They found small lenses that were an order of magnitude smaller than would be expected from cosmological simulations. The authors conclude that there is an unidentified problem with either prevailing simulation methods or standard cosmology.

From Science. The abstract of the article states that:

Cold dark matter (CDM) constitutes most of the matter in the Universe. The interplay between dark and luminous matter in dense cosmic environments, such as galaxy clusters, is studied theoretically using cosmological simulations. Observations of gravitational lensing are used to characterize the properties of substructures—the small-scale distribution of dark matter—in clusters.

We derive a metric, the probability of strong lensing events produced by dark-matter substructure, and compute it for 11 galaxy clusters. The observed cluster substructures are more efficient lenses than predicted by CDM simulations, by more than an order of magnitude. We suggest that systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain our results.

Massimo Meneghetti, et al., "An excess of small-scale gravitational lenses observed in galaxy clusters" 369 (6509) Science 147-1351 (September 11, 2020). DOI: 10.1126/science.aax5164

The article is closed access but an article based on a NASA press release discusses the findings at greater length. Full text pre-print available at arXiv. From the body text (emphasis mine, references omitted):

We performed several tests to investigate potential sources for this discrepancy. The results remain unchanged even when one key ingredient - energy feedback from active-galacticnuclei powered by SMBH accretion - which alters the internal structure of halos is disabled in the simulations. This feedback suppresses star formation in substructures, altering the slope of their inner density profiles, making them less centrally concentrated and, hence, weaker gravitational lenses. Even without feedback, we are unable to bridge the gap between simulations and observations completely. In addition, simulations without feedback would be grossly discrepant from observations for other well measured quantities like the total fraction of baryons in clusters converted into stars. The mass and spatial resolutions of our simulations are sufficiently high to resolve the typical substructures included in the lensing mass models. We also exclude the possibility that the computed GGSL probability could be enhanced by unassociated halos along the line-of-sight (LOS) to these clusters. Including multiple lens planes in the models generated using cosmological simulations, we find that the substructure critical lines and caustics are negligibly affected by halos along the LOS. The observationally constrained lens models reproduce the shapes and sizes of the observed GGSL events. For instance, the model predicted image positions match within ∼ 0.5 arc sec with what is seen.

The discrepancy between observations and simulations may be due to issues with either the CDM paradigm or simulation methods. Gravitational lensing has previously been used to probe detailed properties of dark-matter halos associated with individual cluster galaxies. Simulations show that the mass and radial distributions of sub-halos are nearly universal. Varying results have been reported for the level of agreement between lens model predictions and simulations for other derived quantities, e.g. the mass distribution functions of substructure derived from lensing data agree with simulations but their radial distributions are more centrally concentrated in observed clusters than seen in simulations. Strong lensing clusters also contain more high-circular-velocity sub-halos (i.e. sub-halos with maximum circular velocities Vcirc > 100 km s−1 ) compared with simulations. The maximum circular velocity is given by

Vcirc = maxs GM(r) r , (1)

where G is the gravitational constant, M(r) is the galaxy mass profile and r is the distance from the galaxy center. Fig. 4 shows that, in our lens models, observed galaxies have larger circular velocities than their simulated analogs at a fixed mass. This implies that dark matter sub-halos associated with observed galaxies are more compact than theoretically expected. Observed substructures also appear to be in closer proximity to the larger scale cluster critical lines. Explaining this difference requires the existence of a larger number of compact substructures in the inner regions of simulated clusters. Baryons and dark matter are expected to couple in the dense inner regions of sub-halos, leading to alterations in the small-scale density profile of dark matter, it could be that our current understanding of this interplay is incorrect. Alternatively, the difference could arise from incorrect assumptions about the nature of dark matter.

Previous discrepancies between the predictions of the standard cosmological model and data on small scales have arisen from observations of dwarf galaxies and of satellites of the Milky Way, namely the so called missing satellite, cusp-core, too-big-to-fail problems, and planes of satellite galaxies. The discrepancy that we report is unrelated to these other issues. Previous studies revealed that observed small satellite galaxies were fewer in number and were less compact than expected from simulations; here, we find the opposite for cluster substructures. The GGSL events that we observe show that subhalos are more centrally concentrated than predicted by simulations i.e. there is an excess not a deficit. Hypotheses advocated to solve previous controversies on dwarf galaxy scales would only serve to exacerbate the discrepancy in GGSL event numbers that we report here.

Our results therefore require alternative explanations. One possibility is numerical effects arising from the resolution limits of simulations. However, currently known numerical artefacts are not effective enough at disrupting satellites. We investigated this issue thoroughly and found that it can change the predicted GGSL event rate at most by a factor of 2, which is insufficient to explain the nearly order of magnitude discrepancy that we find. These numerical artefacts would also appear on galactic scales, where they would in turn worsen the missing satellite problem.

Learned fellow contributors and readers, what are you reactions to this?

Do you think it is real or a methodological issue?

Are you aware of hypotheses that would explain these results or predicted them?

Have you seen other publications or commentary or letters analyzing theses results?

Was this expected from prior less complete or high quality studies?

 

[Tendex]

It looks as real as it gets an totally unexpected. I wouldn't be surprised if it's totally ignored though exactly because of this.

(Maybe this post would get more replies in the relativity subforum. Easier to get locked too).

 

[ohwilleke]

I don't think this gets totally ignored. Anomalies in experimental results from big time experiments like Hubble catch people's attention, especially when published in a big, peer reviewed journal.