arXiv subjects

Benoit Famaey

Publications and source records attributed to Benoit Famaey.

At least 19 recordsLinked to original sources

arXiv2026

A direct measurement of the tilting rate of the Milky Way disc

The Milky Way disc is typically described, at lowest order, as a flattened rotating stellar component with a fixed spin axis. However, interactions and mergers with smaller galaxies are expected to have an impact on the disc properties. Such interactions can leave long-lasting signatures within galactic discs, including warps and corrugation waves that have already been detected in the Milky Way, but also a slow reorientation of the disc known as tilting, which has never been directly measured up to now. Here, using the vertical proper motions of 110 million stars from the Gaia DR3 catalogue, we present the first-ever direct measurement of the tilting rate of our Galaxy's disc. The detected signal of 14 +/- 2 deg/Gyr around the Sun-Galactic Centre axis is statistically significant and is consistent with typical values found in cosmological simulations. By comparing our measurements with high-resolution tailored simulations, we infer that the observed tilting rate is likely the long-term consequence of a major accretion event that occurred around 8-10 billion years ago. The signal is consistent with the merger of a massive satellite galaxy on a retrograde orbit with a ratio of about 1:4, probably associated with the Gaia-Enceladus/Sausage event. These results provide independent evidence for a key episode in the Milky Way's formation history and introduce a new way to reconstruct the assembly histories of disc galaxies from their present-day motions.

astro-ph.GA↗

arXiv2026

Galactic tides in the Solar System within a non-axisymmetric Milky Way model adjusted to Gaia data

Context. Galactic tides are external differential forces acting on extended systems immersed in the Galactic potential. They play a key role in the dynamics of comets in the Oort cloud. Aims. We aim to establish the strength of Galactic tides from the non-axisymmetric potential of the Milky Way revealed by the Gaia mission, and how this strength evolves along the Galactic trajectory of the Solar System. Methods. We derived expressions for Galactic tide parameters independently of any simplified trajectory for a star in the Galaxy or any specific symmetry of the Galactic potential. We obtained six parameters, $G_1$ to $G_6$, that quantify the influence of tides. Using the most up-to-date Galactic potential model, extended here to three dimensions, we studied the time evolution of these parameters along the Sun's trajectory using a statistical approach. Results. Even for Galactic trajectories featuring modest radial and vertical excursions, as investigated here, the dominant Galactic parameter, $G_3$, is found to vary by an order of magnitude along the solar trajectory. $G_1$ and $G_2$ reach up to about one half and one quarter, respectively, of $G_3$ along the solar trajectory, whilst parameters $G_4$ to $G_6$ reach up to one tenth of $G_3$. Conclusions. Contrary to what is often assumed, all Galactic tide parameters vary widely along the Galactic trajectory of the Solar System. Two consequences can be expected: First, $G_3$ directly affects the flux of observable long-period comets and the extent of the fossilised Sednoids region; second, $G_1$ and $G_2$, and to a lesser extent, $G_4$ to $G_6$, break the integrability of the dynamics, potentially affecting the long-term structure of the Oort cloud. Additionally, the new parameters $G_4$ to $G_6$, while small for the Solar System, may strongly impact extrasolar systems with a large out-of-the-plane excursion in the Galaxy.

astro-ph.EP↗

arXiv2026

On the residual missing mass of the Bullet Cluster

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 centred on the galaxies of the Bullet Cluster.

astro-ph.CO↗

arXiv2026

Mati\`ere noire et (ou) gravitation modifi\'ee : une approche historique et \'epist\'emologique

The current standard model of cosmology assumes that the majority of matter in the Universe is made of dark matter, and that the latter is fundamentally different from ordinary matter. Dark matter can in principle explain the rotation of galaxies, the gravitational lensing from galaxy clusters or the appearance of the cosmic microwave background, the oldest light in the Universe. But does dark matter really exist? Here, we review the history of this concept and its implications for the formation and evolution of galaxies. We also consider the questions that remain, the limitations of the model, and present alternative theories, in particular modifications to the gravitional law that would -- perhaps -- make it possible to do without it.

physics.pop-ph↗

arXiv2025

Asymmetries in stellar streams induced by a galactic merger

Stellar streams are sensitive to perturbations from, e.g., 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 galactic major 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 run 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 2.5 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.

astro-ph.GA↗

arXiv2025

A bar stability criterion distinguishing between modified gravity and dark matter in galaxies

This paper presents a study on the distinguishability of dark matter and Modified Newtonian Dynamics (MOND) at galactic scales based on the stability criterion proposed by Efstathiou, Lake, and Negroponte (ELN criterion). First, we test the statistical validity of this stability criterion against the presence of bars within the SPARC and CALIFA databases, successfully identifying $\sim 70\%$ of barred galaxies. Then, we employ a series of N-body galaxy simulations to exhibit a direct observable difference between the dark matter and MOND theoretical frameworks, at least in gas-poor galaxies. We present N-body models that satisfy the stability requirement of the ELN criterion, and so are stable against bar formation in the presence of a dark matter halo, and that do actually exhibit bar instabilities in MOND. On the other hand, the question of how to inhibit bar formation in gas-poor galaxies in MOND is posed, and requires a detailed investigation of the external field effect.

astro-ph.GA↗

arXiv2025

Mass models of galaxy clusters from a non-parametric weak-lensing reconstruction

We study the CLASH sample of galaxy clusters using a new deprojection method for weak gravitational lensing observations. This method is non-parametric, allowing us to infer mass profiles, or equivalently circular velocities, without having to assume a specific halo profile. While this method assumes spherical symmetry, we show that, on average, triaxiality is unlikely to significantly affect our results. We use this method to study the total mass profiles of the CLASH clusters, as well as the relation between their total and baryonic components: (1) We find that the implied circular velocities are consistent with being approximately flat at large radii, akin to the rotation curves of galaxies. (2) We infer radially resolved baryonic mass fractions, finding that these vary significantly from cluster to cluster and depend strongly on the details of the X-ray gas mass profiles. Since the gas mass profiles are poorly constrained at large radii, it is unclear whether the CLASH clusters reach the cosmic baryon fraction expected in $\Lambda$CDM. (3) The non-parametric masses are consistent with the stellar mass--halo mass relation expected in $\Lambda$CDM. (4) Galaxy clusters systematically deviate from the Baryonic Tully-Fisher Relation (BTFR) and the Radial Acceleration Relation (RAR) defined by galaxies, but the magnitude of the offset depends strongly on the gas mass extrapolation at large radii. Contrary to some previous results based on hydrostatic equilibrium, we find that galaxy clusters may fall on the same BTFR and RAR as galaxies if one adds a suitable positive baryonic mass component.

astro-ph.CO↗

arXiv2025

MUSE observations of dwarf galaxies and a stellar stream in the M83 group

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 (KK208) surrounding the nearby grand spiral M83, which resides together with the lenticular galaxy Cen A in the Centaurus group. This data complete the phase-space information for all known dwarf galaxies around M83 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 as defined by the Local Group dwarfs. For the stellar stream we serendipitously identify a previously unknown globular cluster, which is old and metal-poor. Two dwarf galaxies (NGC5264 and dw1341-29) may be a bound satellite of a satellite system due to their proximity and shared velocities. Having access to the positions and velocities of 13 dwarfs around M83, we estimate the mass of the group with different estimators. Ranging between 1.3 and $3.0 \times 10^{12}$ M$_\odot$ 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 co-rotating plane-of-satellites around M83.

astro-ph.GA↗

arXiv2025

A primordial origin to cosmic tensions: towards reconciling $H_0$ and $S_8$ with early dark energy and scale-dependent primordial non-Gaussianities

The Hubble ($H_0$) tension between direct measurements of the expansion rate and the prediction of the $\Lambda$CDM cosmological model calibrated on the Cosmic Microwave Background (CMB), is a strong motivation to explore alternative cosmological models. A popular class of such models includes an additional component of dark energy relevant in the early Universe, which solves the Hubble tension. These Early Dark Energy (EDE) models however typically overpredict the value of the $S_8$ parameter. Here, we show how combining EDE with scale-dependent primordial non-Gaussianities (sPNG) can in principle both increase $H_0$ and decrease $S_8$ at once, even conceivably allowing to solve the potential $S_8$ tension between measurements of weak gravitational lensing and the $\Lambda$CDM expectation. Such sPNG are related to non-trivial inflationary physics, and observationally affect the non-linear regime of structure formation while leaving the linear regime mostly untouched. Depending on the amplitude of the sPNG, such models can either yield back the $\Lambda$CDM expectation for the power spectrum in the non-linear regime, and hence an $S_8$ parameter compatible with $\Lambda$CDM, or can even reconcile the value of $S_8$ from old weak-lensing measurements with the CMB, while solving the Hubble tension in all cases. In such models, both tensions would then be entirely related to pre-CMB physics of the early Universe.

astro-ph.CO↗

arXiv2025

Modified Newtonian Dynamics (MOND)

This chapter presents Modified Newtonian Dynamics (MOND), the proposal that, below a certain acceleration scale $a_0$, dynamics departs from the Newtonian expectation. In that context, the determining factor for the emergence of apparent missing matter in galactic systems is predicted to be the acceleration, and not the mass or size of the system. MOND enables, for example, the prediction of rotation curves from only the baryonic distribution of galaxies. The simple rule is that the acceleration observed in the low-acceleration regime is the square root of the Newtonian expectation times $a_0$. Immediately, the flatness of rotation curves follows, as well as the proportionality of the fourth power of the asymptotic circular speed to only the baryonic mass of the galaxy. While the asymptotic circular speed is predicted not to depend on the baryonic surface density of galaxies of fixed baryonic mass, the inner shape of rotation curves is predicted to strongly depend on it. More generally, MOND implies an algebraic relation between the acceleration expected from Newtonian gravity and the total observed acceleration, at any radius in a galaxy. This is known, observationally, as the Radial Acceleration Relation. For galaxy clusters, it is commonly accepted that MOND fails, needing a stronger gravitational force (or more baryonic mass than observed) to account for the thermodynamic state of galaxy clusters, their lensing and kinematics. MOND, however, is not a complete theory, but a phenomenological non-relativistic paradigm in the limit of low accelerations, in need of embedding in a more fundamental theory. While various non-relativistic field theories of MOND exist, the search for a relativistic theory that recovers general relativity for high accelerations and MOND for low accelerations in the quasi-static limit, as well as a cosmology compatible with observations, is still on-going.

astro-ph.GA↗

arXiv2025

A cosmic degeneracy story: structure formation with warm dark matter and scale-dependent primordial non-Gaussianities

It has been recently shown that cosmological models with scale-dependent primordial non-Gaussianities (sPNG) could provide a possible path to solve current cosmic tensions. Moreover, it has been pointed out that some of these models might mimic the effects of Warm Dark Matter (WDM) for several observables at low redshift. Here, we confirm the qualitative similarity of the matter power spectrum for sPNG and WDM models, but also point out differences in the halo mass function and void size function. We then jointly simulate WDM and sPNG together. Such simulations allow us to demonstrate that the joint impact of WDM and sPNG is close to the linear superposition of their respective effects at low redshift, at the percent level. We finally propose a model with mixed hot and cold dark matter together with sPNG, that reproduces the $\Lambda$CDM power spectrum at redshifts $z \leq 3$ but is still distinct in terms of halo statistics.

astro-ph.CO↗

arXiv2024

Measures of luminous and dark matter in galaxies across time

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.

astro-ph.GA↗

arXiv2024

On the nature of the missing mass of galaxy clusters in MOND: the view from gravitational lensing

Modified Newtonian Dynamics (MOND) has long been known to fail in galaxy clusters, implying a residual missing mass problem for clusters in this context. Here, using mass profiles derived from strong- and weak-lensing shear, as well as magnification data, for a sample of clusters from the CLASH survey, we characterize the density profile of this residual MOND missing mass in the central Mpc of galaxy clusters. In line with results obtained in the literature from the hydrostatic equilibrium of hot gas, we find that an inner constant density core and an outer power-law slope sharper than $-3.5$ provides a good description within $\sim 1$~Mpc. We also show that the data in the central parts of clusters can be even better represented by a `dark mass-follows-gas' profile with an exponential cut-off. Clusters in the sample display a remarkable uniformity for the missing-to-hot-gas density ratio in the inner parts, of order $\sim$10, as well as for the exponential cut-off radius, of order $\sim 400$~kpc. These lensing results can in principle serve as a crucible for relativistic theories of MOND in galaxy clusters, or for any other tentative hypothesis regarding the nature of the clusters residual missing mass in the MOND context.

astro-ph.CO↗

arXiv2024

Simulations of cluster ultra-diffuse galaxies in MOND

Ultra-diffuse galaxies (UDGs) in the Coma cluster have velocity dispersion profiles that are in full agreement with the predictions of Modified Newtonian Dynamics (MOND) in isolation. However, the external field effect (EFE) from the cluster seriously deteriorates this agreement. It has been suggested that this could be related to the fact that UDGs are out-of-equilibrium objects whose stars have been heated by the cluster tides or that they recently fell onto the cluster on radial orbits, such that their velocity dispersion may not reflect the EFE at their instantaneous distance from the cluster center. Here, we simulate UDGs within the Coma cluster in MOND, using the Phantom of Ramses (\textsc{por}) code, and show that if UDGs are initially at equilibrium within the cluster, tides are not sufficient to increase their velocity dispersions to values as high as the observed ones. On the other hand, if they are on a first radial infall onto the cluster, they can keep high velocity dispersions without being destroyed until their first pericentric passage. We conclude that, without alterations such as a screening of the EFE in galaxy clusters or much higher baryonic masses than currently estimated, in the MOND context UDGs must be out-of-equilibrium objects on their first infall onto the cluster.

astro-ph.GA↗

arXiv2024

Scale-dependent local primordial non-Gaussianity as a solution to the $S_8$ tension

For the last decade, several probes have pointed to a cosmological tension between the amplitude of density fluctuations extrapolated from the cosmic microwave background within the standard cosmological model and the one encapsulated by the $S_8$ parameter from large scale structure. The origin of this $S_8$ tension has not yet been elucidated and may hint at systematics in the data, unaccounted effects from baryonic physics, or new physics beyond the standard model of cosmology. Baryonic physics may in principle provide a nonlinear solution to the tension by suppressing the matter power spectrum more strongly on nonlinear scales than is traditionally assumed. Such a solution would not worsen the Hubble tension, contrary to many other proposed solutions to the $S_8$ tension. However, no realistic baryonic feedback in hydrodynamical simulations provides the needed suppression as a function of redshift. Here, we point out that a scale-dependence of local-type primordial non-Gaussianities (PNG), with significant PNG at scales of a few Mpc, can provide the needed suppression, since such PNG can suppress the power spectrum at slightly larger scales than baryons do. We demonstrate this by devising collisionless numerical simulations of structure formation in boxes of 0.5 Gpc/$h$ with scale-dependent local-type PNG. Our simple models show that, as a proof of principle, scale-dependent PNG, with a Gaussian random field for primordial density fluctuations on large scales and $f_{\rm NL} \simeq -300$ at $\lesssim 10$ Mpc scales, together with state-of-the-art baryonification of the matter power spectrum, can in principle solve the $S_8$ tension. The $S_8$ tension would then be a smoking-gun of non-trivial inflationary physics.

astro-ph.CO↗

arXiv2024

Radial and azimuthal gradients of the moving groups in Gaia DR3: The slow/fast bar degeneracy problem

The structure and dynamics of the central bar of the Milky Way are still under debate whilst being fundamental ingredients for the evolution of our Galaxy. The recent Gaia DR3 offers an unprecedented detailed view of the 6D phase-space of the MW. We aim to characterise the dynamical moving groups across the MW disc, and use their large-scale distribution to help constrain the properties of the Galactic bar. We used wavelet transforms of the azimuthal velocity ($V_\phi$) distribution in bins of radial velocity to robustly detect the kinematic substructure in the Gaia DR3 catalogue. We then connected these structures across the disc to measure the azimuthal ($\phi$) and radial ($R$) gradients of the moving groups. We simulated thousands of perturbed distribution functions using Backwards Integration of feasible Galaxy models that include a bar, to compare them with the data and to explore and quantify the degeneracies. The radial gradient of the Hercules moving group ($\partial V_\phi/\partial R$ = 28.1$\pm$2.8 km$\,$s$^{-1}\,$kpc$^{-1}$) cannot be reproduced by our simple models of the Galaxy which show much larger slopes both for a fast and a slow bar. This suggests the need for more complex dynamics (e.g. spiral arms, a slowing bar, external perturbations, etc.). We measure an azimuthal gradient for Hercules of $\partial V_\phi/\partial \phi$ = -0.63$\pm$0.13$\,$km$\,$s$^{-1}$deg$^{-1}$ and find that it is compatible with both the slow and fast bar models. Our analysis points out that using this type of analysis at least two moving groups are needed to start breaking the degeneracies. We conclude that it is not sufficient for a model to replicate the local velocity distribution; it must also capture its larger-scale variations. The accurate quantification of the gradients, especially in the azimuthal direction, will be key for the understanding of the dynamics governing the disc. (ABR)

astro-ph.GA↗

arXiv2024

From inflation to dark matter halo profiles: the impact of primordial non-Gaussianities on the central density cusp

It has recently been shown that local primordial non-Gaussianities (PNG) with significant amplitude ($|f_{\rm NL}| \sim 1000$), at small (Mpc) scales, can help in forming simulated galaxies with more disky baryonic kinematics than in the Gaussian case, while generating matter power spectra that can differ by up to 20% from the Gaussian case at non-linear scales. Here, we explore in detail the consequences of such small-scale PNG on the dark matter halo profiles. We show in particular that, for negative $f_{\rm NL}$, dark matter halos formed in collisionless simulations are not always well described by the traditional Navarro-Frenk-White (NFW) profiles, as supported by their sparsity distribution. We conclude that NFW profiles are not as clear attractors for the density profiles of dark matter halos in the presence of PNG than in the case of a Gaussian contrast density field. We show how alternatives to the NFW profile can describe halos both in the Gaussian and non-Gaussian cases. From the combination of our sparsity analysis and the quality of the adjustments of the density profiles with a minimal extension to NFW, we conclude that $z=1$ halos carry the most interesting information about PNG

astro-ph.CO↗

arXiv2024

On the tension between the Radial Acceleration Relation and Solar System quadrupole in modified gravity MOND

Modified Newtonian Dynamics (MOND), postulating a breakdown of Newtonian mechanics at low accelerations, has considerable success at explaining galaxy kinematics. However, the quadrupole of the gravitational field of the Solar System (SS) provides a strong constraint on the way in which Newtonian gravity can be modified. In this paper we assess the extent to which the AQUAL and QUMOND modified gravity formulations of MOND are capable of accounting simultaneously for the Radial Acceleration Relation (RAR), the Cassini measurement of the SS quadrupole and the kinematics of wide binaries in the Solar neighbourhood. We achieve this by inferring the location and sharpness of the MOND transition from the SPARC RAR under broad assumptions for the behaviour of the interpolating function and external field effect. We constrain the same quantities from the SS quadrupole, finding that this requires a significantly sharper transition between the deep-MOND and Newtonian regimes than is allowed by the RAR (an 8.7$\sigma$ tension under fiducial model assumptions). This may be relieved somewhat by allowing additional freedom in galaxies' mass-to-light ratios -- which also improves the RAR fit -- and more significantly (to 1.9$\sigma$) by removing galaxies with bulges. For the first time, we also apply to the SPARC RAR fit an AQUAL correction for flattened systems, obtaining similar results. Finally we show that the SS quadrupole constraint implies, to high precision, no deviation from Newtonian gravity in nearby wide binaries, and speculate on possible resolutions of this tension between SS and galaxy data within the MOND paradigm.

astro-ph.GA↗