More Galaxy Dynamics Data Test Dark Matter and Modified Gravity Theories (And More)

Observations v. Modified Gravity and DM Models

The confirmation of modified gravity theories over a large range of scales is generically true of any modified gravity theory that can reproduce the core conclusions of MOND, such as MOG and Deur's approach, even though just two were specifically tested in this paper.

I would like to know if the early- and late-type galaxies have different shapes, although a difference in interstellar gas concentrations does provide a plausible way to reconcile a disparity between the two galaxy types. This could provide another Deur motivated distinction between the two groups of galaxies. 

The paper does suggest that there is a shape difference between the two types of galaxies:

The earlier type bulge dominated galaxies have larger inferred halo masses relative to ordinary mass, which is the opposite of what I had expected (as the paper also notes, spiral galaxies tend to have more inferred dark matter than elliptical galaxies and bulge dominated galaxies would seem to be closer to elliptical galaxies) but this could be (as the paper suggests), a function of unobserved interstellar gas being present to a larger degree in these galaxies than in older type galaxies (which by that time is exhausted in star formation). The paper states:

The higher values of gobs for red and bulge-dominated galaxies that we find in Fig. 8 are in qualitative agreement with earlier GGL studies. A recent KiDS-1000 lensing study by Taylor et al. (2020) found that, within a narrow stellar mass range near the knee of the SHMR (M* ∼ 2 − 5 × 1010 h −2 70M), galaxy halo mass varied with galaxy colour, specific star formation rate (SSFR), effective radius Re and Sérsic index n. Although not explicitly mentioned, their figures 1 and 6 reveal that their early-type (red, low-SSFR) galaxies have larger halo masses than their late-type (blue, low-n, high-SSFR) galaxies of the same stellar mass. Sérsic parameter coupling between n and Re, for a fixed galaxy luminosity, may also contribute towards the trends seen among the early-type galaxies in their Mhalo–n and Mhalo–Re diagrams. Much earlier Hoekstra et al. (2005) measured the GGL signal of a sample of ‘isolated’ Red-sequence Cluster Survey galaxies as a function of their rest-frame B-, V-, and R-band luminosity, and found that early-type galaxies have lower stellar mass fractions. In contrast, Mandelbaum et al. (2006) found no dependence of the halo mass on morphology for a given stellar mass below M* < 10^11 M, although they did find a factor of two difference in halo mass between ellipticals and spirals at fixed luminosity.

Finding a significantly different RAR at equal M* would have interesting implications for galaxy formation models in the ΛCDM framework. In the ΛCDM framework it is expected that the galaxy-to-halo-mass relation, and therefore the RAR, can be different for different galaxy types through their galaxy formation history (Dutton et al. 2010; Matthee et al. 2017; Posti et al. 2019; Marasco et al. 2020). Two parameters that correlate heavily with galaxy formation history are Sérsic index and colour.

Current MG theories do not predict any effect of galaxy morphological type on the RAR, at least on large scales [ed. not true for Deur's approach]. The MOND paradigm gives a fixed prediction for the relation between gbar and gobs given by Eq. 11. Since the RAR is the observation of exactly this relation, in principle MOND gives a fixed prediction, independent of any galaxy characteristic. As discussed in Section 2.3, the main exception is the EFE that could be caused by neighbouring mass distributions. However, Fig. 4 shows that an increase in the EFE only predicts an increase in steepness of the downward RAR slope at low accelerations (gbar < 10^−12 m s^−2 ), while the observed RAR of both early- and late-type galaxies follow approximately the same slope across all measured accelerations. It is therefore unlikely that their amplitude difference can be explained through the EFE.

. . .

In conclusion, unless early-type galaxies have significant circumgalactic gaseous haloes while late types (of the same stellar mass) do not, the difference we find in the RARs of different galaxy types might prove difficult to explain within MG frameworks. In MOND, gbar and gobs should be directly linked through Eq. 11 without any dependence on galaxy type. 

In EG the effect might be a consequence of yet unexplored aspects of the theory, such as a non-symmetric mass distribution or the effect of large-scale dynamics. To explore whether this is the case, however, more theoretical work is needed. Through the derivative in Eq. 14, EG does include a dependence on the slope of the baryonic density distribution. A shallower slope of Mbar(r) increases MADM and thus gobs, which might solve the current tension if early-type galaxies have significantly shallower baryonic mass distributions that extend far beyond 30 h −1 70 kpc, such as gaseous haloes (although Brouwer et al. 2017 did not find evidence for a significant effect of the baryonic mass distribution on the EG prediction; see their section 4.3). In addition, EG is currently only formulated for spherically symmetric systems. It would be interesting to investigate whether discs and spheroidal galaxies yield different predictions, and whether these differences would extend beyond 30 h −1 70 kpc. 

In a ΛCDM context, our findings would point to a difference in the SHMR for different galaxy types. Recently Correa & Schaye (2020) used SDSS data with morphological classifications from Galaxy Zoo to find that, at fixed halo mass (in the range 10^11.7 − 10^12.9 M), the median stellar mass of SDSS disc galaxies was a factor of 1.4 higher than that of ellipticals. They found this to be in agreement with the EAGLE simulations, where haloes hosting disc galaxies are assembled earlier than those hosting ellipticals, therefore having more time for gas accretion and star formation.

Also, I would suggest that in the absence of a clear reason to prefer a ΛCDM model, that modified gravity explanations are preferred as more economical and not facing so many challenges in other areas. Also, the ΛCDM comes across much more ad hoc and hasn't made ex ante predictions.

The new paper and its abstract are as follows:

We present measurements of the radial gravitational acceleration around isolated galaxies, comparing the expected gravitational acceleration given the baryonic matter with the observed gravitational acceleration, using weak lensing measurements from the fourth data release of the Kilo-Degree Survey. 

These measurements extend the radial acceleration relation (RAR) by 2 decades into the low-acceleration regime beyond the outskirts of the observable galaxy. We compare our RAR measurements to the predictions of two modified gravity (MG) theories: MOND and Verlinde's emergent gravity. We find that the measured RAR agrees well with the MG predictions. In addition, we find a difference of at least 6σ between the RARs of early- and late-type galaxies (split by Sérsic index and u−r colour) with the same stellar mass. Current MG theories involve a gravity modification that is independent of other galaxy properties, which would be unable to explain this behaviour. The difference might be explained if only the early-type galaxies have significant (Mgas≈M∗) circumgalactic gaseous haloes. 

The observed behaviour is also expected in ΛCDM models where the galaxy-to-halo mass relation depends on the galaxy formation history. We find that MICE, a ΛCDM simulation with hybrid halo occupation distribution modelling and abundance matching, reproduces the observed RAR but significantly differs from BAHAMAS, a hydrodynamical cosmological galaxy formation simulation. Our results are sensitive to the amount of circumgalactic gas; current observational constraints indicate that the resulting corrections are likely moderate. 

Measurements of the lensing RAR with future cosmological surveys will be able to further distinguish between MG and ΛCDM models if systematic uncertainties in the baryonic mass distribution around galaxies are reduced.

Margot M. Brouwer, et al., "The Weak Lensing Radial Acceleration Relation: Constraining Modified Gravity and Cold Dark Matter theories with KiDS-1000" (June 22, 2021) (650 Astronomy & Astrophysics A113 (2021)

DOI: 10.1051/0004-6361/202040108).

More Primordial Black Hole Exclusions

Meanwhile, primordial black hole dark matter theories suffer another blow, although in a size range much greater than the asteroid sized PBHs most commonly considered as a dark matter candidate.

The possibility that primordial black holes (PBHs) form a part of dark matter has been considered over a wide mass range from the Planck mass (10^−5 g) to the level of the supermassive black hole in the center of the galaxy. Primordial origin might be one of the most important formation channel of massive black holes. We propose the lensing effect of very long baseline interferometer observations of compact radio sources with extremely high angular resolution as a promising probe for the presence of intergalactic PBHs in the mass range ∼10^2-10^9 M⊙. 

For a sample of well-measured 543 compact radio sources, no millilensing multiple images are found with angular separations between 0.2 milliarcsecond and 50 milliarcseconds. From this null search result, we derive that the fraction of dark matter made up of PBHs in the mass range ∼10^4-10^8 M⊙ is ≲0.56% at 68% confidence level.

Huan Zhou, et al., "Constraints on the abundance of massive primordial black holes from lensing of compact radio sources" arXiv (June 22, 2021).

Disintegrating Open Galaxy Clusters

Finally, evidence that "open" galaxy clusters sometimes fall apart.