Benoit Famaey

Modified Newtonian Dynamics (MOND) is a paradigm that can do away with dark matter at galaxy scales, but displays a residual missing mass discrepancy in galaxy clusters. Prompted by the updated JWST-based gravitational lens model of the Bullet Cluster, I confirm here that this cluster exhibits the same residual missing mass discrepancy as other clusters of similar mass in the MOND context. Moreover, this missing mass should be mostly collisionless, since it is centered on the galaxies of the Bullet Cluster.

Dark matter is one of the pillars of the current standard model of structure formation: it is assumed to constitute most of the matter in the Universe. However, it can so far only be probed indirectly through its gravitational effects, and its nature remains elusive. In this focus meeting, we discussed different methods used to estimate galaxies’ visible and dark matter masses in the nearby and distant Universe. We reviewed successes of the standard model relying on cold dark matter, confronted observations with simulations, and highlighted inconsistencies between the two. We discussed how robust mass measurements can help plan, perform, and refine particle dark matter searches. We further exchanged about alternatives to cold dark matter, such as warm, self-interacting, and fuzzy dark matter, as well as modified gravity. Finally, we discussed prospects and strategies that could be implemented to reveal the nature of this crucial component of the Universe.

We report an updated constraint on the Solar System quadrupole parameter Q 2 , which encodes the external field effect predicted by modified gravity versions of the modified Newtonian dynamics (MOND) paradigm. Using the dataset employed to compute the DE440 planetary ephemerides and estimating it simultaneously with other parameters included in the planetary ephemerides, we find Q 2 = ( 1.6 ± 1.8 ) × 10 − 27 s − 2 ( 1 σ ), representing an improvement of 40% over previous estimates. We also show explicitly that the contribution to the MOND prediction of Q 2 from the Solar System’s largest planet, Jupiter, is at the 0.05% level, validating the approximation of retaining only the Sun in theoretical calculations. With this new constraint on Q 2 , we update previously acknowledged tensions with external galaxy rotation curves, now leading to discrepancies at the 3 − 15 σ level depending on the detailed mass modeling or the subset of galaxies considered. Within the Milky Way itself, the Q 2 constraint imposes an upper bound of only 2% (at 95% confidence) on the MOND boost to the galactic radial acceleration (i.e., the ratio of the observed over baryonic Newtonian acceleration) at the position of the Sun, in strong tension with current observational limits. The updated Q 2 posterior finally confirms that Solar System measurements provide stronger constraints than current wide-binary data on classical modified gravity versions of MOND.

Context. The radial acceleration relation (RAR) is a tight empirical correlation between the observed radial acceleration ( a _tot ) and the baryonic radial acceleration ( a _bar ) measured across galaxy radii; these two accelerations start to deviate significantly from each other below a characteristic acceleration scale, a ₀ . To date, observational studies of the RAR have predominantly focused on galaxies in the local Universe, leaving its evolution with cosmic time largely unexplored. Aims. Using high signal-to-noise data from the MUSE Hubble Ultra Deep Field survey, we investigated the RAR with a sample of 79 star-forming galaxies (complete above M _★ > 10 ^8.8 M _⊙ ) at intermediate redshifts (0.33 < z < 1.44). Methods. We estimated the observed intrinsic acceleration ( a _tot ) and the baryonic acceleration ( a _bar ) from a disk-halo decomposition that incorporates stellar, gas, and dark matter components, with corrections for pressure support, using 3D forward modelling. Results. We find a RAR in our intermediate- z sample offset from the local relation, with a higher characteristic acceleration scale ( a ₀ | _{z ∼1} = 2.38 ^+0.12 _−0.10 × 10 ⁻¹⁰ m/s ² ) and a larger intrinsic scatter (∼0.17 dex). Dividing the sample into redshift bins and refitting the RAR in each bin, we find a characteristic acceleration scale that systematically increases with z . Parametrising the z -dependence as a ₀ ( z ) = a ₀ (0)+ a ₁ ⋅ z , we obtain a ₁ = 1.59 ^+0.11 _−0.10 × 10 ⁻¹⁰ m/s ² , providing evidence for a z -evolution. We find similar results using various dark matter halo profiles as well as the modified Newtonian dynamics framework in our 3D forward modelling. Conclusions. Our results show that the RAR persists at intermediate redshifts, with statistically significant redshift evolution of the characteristic acceleration, pointing to a possible evolution of the baryon-missing mass connection over cosmic time.

Stellar streams are sensitive to perturbations from, for example, giant molecular clouds, bars and spiral arms, infalling dwarf galaxies, or globular clusters which can imprint gaps, clumps, spurs, and asymmetries in tails. In addition to these effects, the impact of a major galactic merger on a population of stellar streams remains to be explored. Here, we focus on the emergence and longevity of asymmetries between the leading and trailing tails of streams caused by such interactions. We ran collisionless N -body simulations of a Milky-Way-like galaxy hosting 36 globular cluster streams and merging with a perturber galaxy. We propose a new asymmetry metric to quantify the structural differences between both tails from their respective cumulative density profiles. We find that the over- and under-densities along streams induced by the merger depend on the orbital characteristics of their progenitors. The non-simultaneity of this effect from stream to stream implies that global asymmetry signatures are less prominent than in individual cases. These population-averaged imprints remain detectable over only 2 Gyr, but asymmetric signatures can persist over much longer periods for individual streams with wide orbits that have been perturbed prior to coalescence. We thus caution that the interpretation of streams’ morphology in the context of dark matter mapping is strongly subject to degeneracies and should be performed considering the merger history of the host.

The orbital dynamics of small Solar System bodies beyond Neptune are affected by the gravitational attraction of the Sun's galactic neighborhood [1,2]. This effect is stronger for the most distant small bodies in the Scattered Disc and Oort cloud, with semi-major axes of thousands of astronomical units. The Gaia mission reveals the structure of our Galaxy with an unprecedented level of details; it is therefore possible now to investigate the variety of trajectories that may have been followed by the Sun in the Galaxy, and how these trajectories have affected the dynamics of distant trans-Neptunian objects [3,4]. We focus on the effects of the galactic tides, which are caused by the difference between the gravitational attraction from the galactic medium exerted on the Sun and on distant small Solar System bodies. Locally, the potential of the galactic tides has the following form:V(r) = (G1x2 + G2y2 + G3z2)/2 ,where r = (x, y, z)T is the position vector of the small body with respect to the Sun in a rotating galactic frame, and G1, G2 and G3 are the parameters of the galactic tides. The so-called normal component about the z-axis, depending of G3, is usually the dominant one. It is directly related to the local mass density ρ0 of the Galaxy. The parameters G1, G2 and G3 depend on the instantaneous position of the Sun in the Galaxy. Prior to Gaia, most studies considered very simplified models in which the Sun follows a circular orbit in the plane of an axi-symmetric galactic potential [5]. These rough approximations led to constant values for G1, G2 and G3. In a more realistic model of the Galaxy, the Sun follows a complex trajectory that depends on the galactic structure, including its bulge, halo, bar, and evolving spiral arms [6]. Khalil et al. (2024) [7] have recently used the Gaia data to adjust a new, state-of-the-art model of the Milky Way. Here, we investigate the variety of possible past trajectories of the Sun produced by this refined model, and disentangle the dynamical features produced by each component of the galactic potential. Along the Sun's trajectory, we compute the parameters G1, G2 and G3 of the galactic tides and the local mass density ρ0 of the Galaxy. Figure~1 shows an example of trajectory produced by the core axi-symmetric component of the potential of Khalil et al. (2024)—it includes the galactic bulge, disks and halo. The local mass density ρ0 of the Galaxy varies by almost a factor two, which result in large variations of the external forcing felt by distant small Solar System bodies beyond Neptune.Figure~1: Evolution of the galactic mass density ρ0 in the (R, z) plane, where R and z are the distance of the Sun with respect to the galactic center and with respect to the galactic disc. acknowledgement: This work was supported by the Programme de Planétologie (PNP) of CNRS/INSU, co-funded by CNES. References[1] H. Rickman, Ch. Froeschlé, Cl. Froeschlé, G.~B. Valsecchi, 2004, A&A 428, 673[2] M. Saillenfest, M. Fouchard, T. Ito, A. Higuchi, 2019, A&A 629, A95[3] N. Kaib, R. Roškar, T. Quinn, 2011, Icarus 215, 491[4] C.~A. Martínez-Barbosa, L. Jílková, S. Portegies Zwart, A.~G.~A. Brown, 2016, MNRAS 464, 2290[5] J. Heisler, S. Tremaine, 1986, Icarus 65, 13[6] E. Gardner, P. Nurmi, C. Flynn, S. Mikkola, 2011, MNRAS 411, 947[7] Y. R. Khalil, B. Famaey, G. Monari, M. Bernet, A. Siebert, R. Ibata, G. F. Thomas, P. Ramos, T. Antoja, C. Li, S. Rozier, M. Romero-Gómez, 2024, A&A, in press

Spectroscopy for faint dwarf galaxies outside of our own Local Group is challenging. Here, we present MUSE spectroscopy to study the properties of four known dwarf satellites and one stellar stream (KK 208) surrounding the nearby grand spiral M 83, which resides together with the lenticular galaxy Cen A in the Centaurus group. This data completes the phase-space information for all known dwarf galaxies around M 83 down to a completeness of −10 mag in the V band. All studied objects have an intermediate to old and metal-poor stellar population and follow the stellar luminosity-metallicity relation defined by the Local Group dwarfs. For the stellar stream, we serendipitously identify a previously unknown globular cluster that is old and metal-poor. Two dwarf galaxies (NGC 5264 and dw1341-29) may be a bound satellite of a satellite system due to their proximity and shared velocities. With our access to the positions and velocities of 13 dwarfs around M 83, we estimated the mass of the group with different estimators. Ranging between 1.3 and 3.0 × 10¹² M_⊙ for the halo mass, we find it to be larger than previously assumed. This may impact the previously reported tension for cold dark matter cosmology with the count of dwarf galaxies. In contrast to Cen A, we do not find a corotating plane of satellites around M 83.