Another MOND Success

In another fail for the LambdaCDM model, MOND explains the observation that pairs of isolated galaxies tend to differ in orbital velocities by a preferred amount (as demonstrated in a large observational sample), something that the "Standard Model of Cosmology" does not predict.

Examining a catalogue of isolated galaxy pairs, a preferred orbital intervelocity of ~150 km/s was recently reported. This discovery is difficult to reconcile with the expectations from Newtonian numerical simulations of cosmological structure formations. 

In a previous paper we have shown that a preferred intervelocity for galaxy pairs is expected in Modified Newtonian Dynamics (MOND). Here a detailed analysis of the MOND predictions is presented, showing that a remarkable agreement with the observations can be achieved for a mass to light ratio M/L~1 in solar units. This agrees with the expectations for a typical stellar population, without requiring non-baryonic dark matter for these systems.

Riccardo Scarpa, Renato Falomo, Aldo Treves "On the Orbital Velocity of Isolated Galaxy Pairs: II Accurate MOND Predictions" arXiv:2202.13766 (February 28, 2022) (Accepted for publication on MNRAS) 

https://doi.org/10.48550/arXiv.2202.13766

The body text explains that:

Galaxy pairs represent an important probe to investigate the behaviour of gravity under particular conditions that cannot be found in the laboratory or in the very local Universe. For typical masses of ∼ 10^11 M⊙ and separation of & 100 kpc the acceleration of gravity between the two galaxies is indeed below 10^−10 cm s^−2 , a regime difficult to probe in the solar system or other stellar structures. 

Nottale and Chamaraux (2018a) constructed a catalog of ∼ 13000 close-by (0.01 < z < 0.05) isolated galaxy pairs (IGP), extracted from the HyperLeda extragalactic database (Makarov et al. 2014). Starting from the observed radial velocity difference (intervelocity hereafter) of the components of the pair, and using a technique of statistical deprojection described in Nottale and Chamaraux (2018b), they were able to statistically reconstruct the 3D velocity distribution. The remarkable result was found, that a region of preferred intervelocities with a peak at ∼ 150 km/s does exist (Nottale and Chamaraux 2020). Considering a larger version of the IGP catalog, containing ∼ 16500 pairs, extracted from a more recent version of HyperLeda, Scarpa Falomo and Treves (2022), hereafter Paper I, confirmed the presence and position of the peak (see Fig.1). 

Current Newtonian simulations of galaxy structure formation do not seem to predict a narrow range of intervelocities for galaxy pairs (e.g. Moreno et al. (2013) and references therein). However, this is a recently discovered feature and dedicated numerical simulations are required to fully address the issue. 

Meanwhile, in paper I it was shown that a narrow region of orbital velocities of galaxy pairs is expected within the framework of modified Newtonian dynamics (MOND, Milgrom (1983a,b,c)). This because the orbital velocity V does not depend on the separation of the galaxies within the pair, and is only mildly linked to the mass M of the galaxies (V ∝ M^1/4 , see below). 

Here we revisit these findings and present a detailed discussion of the MOND predictions. We focus on the treatment of the two body problem, moving from the initial approximate formula proposed in Milgrom (1983c), to the rigorous formulation discussed in Milgrom (1994). The peak position of the 3D intervelocity distribution is then used to constrain the mass-to-light M/L ratio of the population under examination.

. . .

Flat rotation curves of galaxies indicate that the mass distribution within galaxies is such that these structures do enter in MOND regime at distances from their center smaller than their size. Because of this, the interaction between pair of galaxies always occur in the so called deep MOND regime, in which all relevant accelerations are well below the MOND constant a(0) = 10^−8 cm s^−2 . This is important because it removes the uncertainties on the unknown interpolation function needed in the Newtonian-MOND intermediate regime, and because in the deep MOND limit the two body problem can be solved exactly (Milgrom 1994), making galaxy pairs particularly interesting for the study of dynamics in the low acceleration limit.

. . .  

We presented a strict consistency of the position and width of the peak of 3D intervelocities of isolated galaxy pairs with the deep MOND predictions, when M/L∼ 1 is assumed, and therefore no dark matter is required in these systems. This consistency reinforces the proposal that MOND is the key for interpreting the peak, rather than hypothesize a complex process of structure formation within Newtonian dynamics that force galaxies pairs to preferentially end up with a well defined orbital velocity. 

We recall, however, that these results are based on the IGP catalog defined by Nottale and Chamaraux (2018a) and its extension using the updated HyperLeda database as presented in paper I. The two samples largely overlap, therefore to further confirm the presence and the properties of the intervelocity peak an independent data set of galaxy pairs is required. This would imply to extend the study to higher redshifts and fainter magnitude of galaxies to ensure a good level of homogeneity in the sample (see also discussion in Nottale and Chamaraux (2020)). 

Another possibility would be to focus on galaxies of specific Hubble types, studying separately spirals from ellipticals. These galaxies are thought to have gone through different dynamical evolutionary paths. However, masses being equal, within the MOND framework, spirals and ellipticals pairs should exhibit the same preferred intervelocities. 

Finally, if a large sample of pair of quasars could be build, their massive host galaxy would extend the present result to even larger masses, also ensuring the perturbing effect of other nearby galaxies on the orbital velocity would be less important.