arXiv subjects

B. Famaey

Publications and source records attributed to B. Famaey.

At least 19 recordsLinked to original sources

arXiv2026

MUSE-DARK III: The evolution of the radial acceleration relation at intermediate redshifts

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, a0. So far, observational studies of the RAR have predominantly focused on galaxies in the local Universe, leaving its evolution with cosmic time largely unexplored. Using high signal-to-noise data from the MUSE Hubble Ultra Deep Field survey, we investigate the RAR with a sample of 79 star-forming galaxies (complete above M* >10^8.8 Msun) at intermediate redshifts (0.33 <z <1.44). We estimate the observed intrinsic acceleration and the baryonic acceleration from a disk-halo decomposition that incorporates stellar, gas, and dark matter components, with corrections for pressure support, using 3D forward modelling. We find a RAR in our intermediate-z sample offset from the local relation, with a higher characteristic acceleration scale, a0(z~1) = 2.38+/-0.1* 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. Parametrizing the z-dependence as a0(z)= a0(0) + a1 * z, we obtain a1 = 1.59+/-0.1 *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. Our results show that the RAR persists at intermediate redshift, with statistically significant redshift evolution of the characteristic acceleration, pointing to a possible evolution of the baryon-missing mass connection over cosmic time.

astro-ph.GA↗

arXiv2026

Improved constraints on modified Newtonian gravity from Cassini radio tracking data

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 $Q_2 = (1.6 \pm 1.8) \times 10^{-27}\,\mathrm{s}^{-2}$ (1-$\sigma$), 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 $3$-$15\sigma$ 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.

gr-qc↗

arXiv2024

A non-axisymmetric potential for the Milky Way disk

We provide a purely dynamical global map of the non-axisymmetric structure of the Milky Way disk. For this, we exploit the information contained within the in-plane motions of disk stars from Gaia DR3 to adjust a model of the Galactic potential, including a detailed parametric form for the bar and spiral arms. We explore the parameter space of the non-axisymmetric components with the backward integration method, first adjusting the bar model to selected peaks of the stellar velocity distribution in the Solar neighbourhood, and then adjusting the amplitude, phase, pitch angle and pattern speed of spiral arms to the median radial velocity as a function of position within the disk. We check a posteriori that our solution also qualitatively reproduces various other features of the global non-axisymmetric phase-space distribution, including most of the moving groups and phase-space ridges despite those not being primarily used in the adjustment. This fiducial model has a bar with pattern speed $37~{\rm kms}^{-1}{\rm kpc}^{-1}$ and two spiral modes that are two-armed and three-armed respectively. The two-armed spiral mode has a $\sim 25~\%$ local contrast surface density, a low pattern speed of $13.1~{\rm kms}^{-1}{\rm kpc}^{-1}$, and matches the location of the Crux-Scutum, Local and Outer arm segments. The three-armed spiral mode has a $\sim 9~\%$ local contrast density, a slightly higher pattern speed of $16.4~{\rm kms}^{-1}{\rm kpc}^{-1}$, and matches the location of the Carina-Sagittarius and Perseus arm segments. The Galactic bar, with a higher pattern speed than both spiral modes, has recently disconnected from those two arms. The fiducial non-axisymmetric potential presented in this paper, reproducing most non-axisymmetric signatures detected in the stellar kinematics of the Milky Way disk, can henceforth be used to confidently integrate orbits within the Galactic plane.

astro-ph.GA↗

arXiv2021

The Sagittarius stream in Gaia eDR3 and the origin of the bifurcations

The Sagittarius dwarf spheroidal (Sgr) is a dissolving galaxy being tidally disrupted by the Milky Way (MW). Its stellar stream still poses serious modelling challenges, which hinders our ability to use it effectively as a probe of the MW gravitational potential. Our goal is to construct the best possible sample of stars with which we can advance our understanding of the Sgr-MW interaction, focusing on the characterisation of the bifurcations. We improve on previous methods based on the use of the wavelet transform to systematically search for the kinematic signature of the Sgr stream in the Gaia data. We refine our selection via the use of a clustering algorithm on the statistical properties of the colour-magnitude diagrams. Our final sample contains > 700k candidate stars, 3x larger than previous samples. We have been able to detect the bifurcation of the stream in both the northern and southern hemispheres, requiring four branches to fully describe this system. We present the detailed proper motion distribution of the trailing arm as a function of Lambda showing the presence of a sharp edge (on the side of the small proper motions) beyond which there are no Sgr stars. We also characterise the correlation between kinematics and distance. The chemical analysis of our sample shows a significant difference between the faint and bright branches. We provide analytical descriptions for the proper motion trends as well as for the sky distribution of the four branches of the stream. We interpret the bifurcations as the misaligned overlap of the material stripped at the antepenultimate pericentre (faint branches) with the stars ejected at the penultimate pericentre (bright branch). The source of this misalignment is still unknown but we argue that models with some internal rotation in the progenitor, at least during the time of stripping of the stars in the faint branches, are worth exploring.

astro-ph.GA↗

arXiv2021

Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies

The tight radial acceleration relation (RAR) obeyed by rotationally supported disk galaxies is one of the most successful a priori predictions of the modified Newtonian dynamics (MOND) paradigm on galaxy scales. Another important consequence of MOND as a classical modification of gravity is that the strong equivalence principle (SEP) -- which requires the dynamics of a small, free-falling, self-gravitating system not to depend on the external gravitational field in which it is embedded -- should be broken. Multiple tentative detections of this so-called external field effect (EFE) of MOND have been made in the past, but the systems that should be most sensitive to it are galaxies with low internal gravitational accelerations residing in galaxy clusters within a strong external field. Here, we show that ultra-diffuse galaxies (UDGs) in the Coma cluster do lie on the RAR, and that their velocity dispersion profiles are in full agreement with isolated MOND predictions, especially when including some degree of radial anisotropy. However, including a breaking of the SEP via the EFE seriously deteriorates this agreement. We discuss various possibilities to explain this within the context of MOND, including a combination of tidal heating and higher baryonic masses. We also speculate that our results could mean that the EFE is screened in cluster UDGs. The fact that this would happen precisely within galaxy clusters, where classical MOND fails, could be especially relevant to the nature of the residual MOND missing mass in clusters of galaxies.

astro-ph.GA↗

arXiv2021

Perturbed distribution functions with accurate action estimates for the Galactic disc

In the Gaia era, understanding the effects of the perturbations of the Galactic disc is of major importance in the context of dynamical modelling. In this theoretical paper we extend previous work in which, making use of the epicyclic approximation, the linearized Boltzmann equation had been used to explicitly compute, away from resonances, the perturbed distribution function of a Galactic thin-disc population in the presence of a non-axisymmetric perturbation of constant amplitude. Here we improve this theoretical framework in two distinct ways in the new code that we present. First, we use better estimates for the action-angle variables away from quasi-circular orbits, computed from the AGAMA software, and we present an efficient routine to numerically re-express any perturbing potential in these coordinates with a typical accuracy at the per cent level. The use of more accurate action estimates allows us to identify resonances such as the outer 1:1 bar resonance at higher azimuthal velocities than the outer Lindblad resonance (OLR), and to extend our previous theoretical results well above the Galactic plane, where we explicitly show how they differ from the epicyclic approximation. In particular, the displacement of resonances in velocity space as a function of height can in principle constrain the 3D structure of the Galactic potential. Second, we allow the perturbation to be time dependent, thereby allowing us to model the effect of transient spiral arms or a growing bar. The theoretical framework and tools presented here will be useful for a thorough analytical dynamical modelling of the complex velocity distribution of disc stars as measured by past and upcoming Gaia data releases.

astro-ph.GA↗

arXiv2021

The phantom dark matter halos of the Local Volume in the context of modified Newtonian dynamics

We explore the predictions of Milgromian gravity (MOND) in the Local Universe by considering the distribution of the `phantom' dark matter (PDM) that would source the MOND gravitational field in Newtonian gravity, allowing an easy comparison with the dark matter framework. For this, we specifically deal with the quasi-linear version of MOND (QUMOND). We compute the `stellar-to-(phantom)halo-mass relation' (SHMR), a monotonically increasing power-law resembling the SHMR observationally deduced from spiral galaxy rotation curves in the Newtonian context. We show that the gas-to-(phantom)halo-mass relation is flat. We generate a map of the Local Volume in QUMOND, highlighting the important influence of distant galaxy clusters, in particular Virgo. This allows us to explore the scatter of the SHMR and the average density of PDM around galaxies in the Local Volume, $Ω_{\rm pdm} \approx 0.1$, below the average cold dark matter density in a $Λ$CDM Universe. We provide a model of the Milky Way in its external field in the MOND context, which we compare to an observational estimate of the escape velocity curve. Finally, we highlight the peculiar features related to the external field effect in the form of negative PDM density zones in the outskirts of each galaxy, and test a new analytic formula for computing galaxy rotation curves in the presence of an external field in QUMOND. While we show that the negative PDM density zones would be difficult to detect dynamically, we quantify the weak lensing signal they could produce for lenses at $z \sim 0.3$.

astro-ph.GA↗

arXiv2021

The proper motion of Andromeda from Gaia eDR3: confirming a nearly radial orbit

We present an analysis of the proper motion of the Andromeda galaxy (M31), based on the Early Third Data Release of the Gaia mission. We use the Gaia photometry to select young blue main sequence stars, and apply several quality cuts to obtain clean samples of these tracers. After correcting the proper motion measurements for the internal rotation of the M31 disk motion, we derive an apparent motion of 52.5 +/- 5.8 muas/yr with respect to the Gaia reference frame, or 61.9 +/- 9.7 muas/yr after applying a zero-point correction determined from quasars within 20 degrees from M31 and a correction from systemic biases. Accounting for the Solar reflex motion we deduce a relative velocity between Andromeda and the Milky way (in a non-rotating frame at the current location of the Sun) of 42.2 +/- 39.3 km/s along right ascension (40.0 +/- 39.3 km/s along galactic longitude) and -59.4 +/- 30.3 km/s along declination (-60.9 +/- 30.3 km/s along galactic latitude), with a total transverse velocity of V_trans = 82.4 +/- 31.2 km/s. These values are consistent with (but more accurate than) earlier Hubble Space Telescope measurements that predict a future merger between the two galaxies. We also note a surprisingly large difference in the derived proper motion between the blue stars in M31 and samples of red stars that appear to lie in that galaxy. We propose several hypotheses to explain the discrepancy but found no clear evidence with the current data to privilege any one of them.

astro-ph.GA↗

arXiv2020

The outer disc in shambles: blind detection of Monoceros and ACS with Gaia's astrometric sample

The astrometric sample of Gaia allows us to study the outermost Galactic disc, the halo and their interface. It is precisely at the very edge of the disc where the effects of external perturbations are expected to be the most noticeable. Our goal is to detect the kinematic substructure present in the halo and at the edge of the Milky Way (MW) disc, and provide observational constraints on their phase-space distribution. We download, one HEALpix at a time, the proper motion histogram of distant stars, to which we apply a Wavelet Transformation to reveal the significant overdensities. We then analyse the large coherent structures that appear in the sky. We reveal a sharp yet complex anticentre dominated by Monoceros (MNC) and the Anticentre Stream (ACS) in the north, which we find with an intensity comparable to the Magellanic clouds and the Sagittarius stream, and by MNC south and TriAnd at negative latitudes. Our method allows us to perform a morphological analysis of MNC and ACS, both spanning more than 100$^\circ$ in longitude, and to provide a high purity sample of giants with which we track MNC down to latitudes as low as $\sim$5$^\circ$. Their colour-magnitude diagram is consistent with extended structures at a distance of $\sim$10-11 kpc originated in the disc, with a very low ratio of RR Lyrae over M giants, and kinematics compatible with the rotation curve at those distances or only slightly slower. We present a precise characterisation of MNC and ACS, two previously known structures that our method reveals naturally, allowing us to detect them without limiting ourselves to a particular stellar type and, for the first time, using only kinematics. Our results allow future studies to model their chemo-dynamics and evolution, thus constraining some of the most influential processes that shaped the MW.

astro-ph.GA↗

arXiv2020

Fluctuations in galactic bar parameters due to bar-spiral interaction

We study the late-time evolution of the central regions of two Milky Way-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, R_b, measurements fluctuate on a dynamical timescale by up to 100%, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15%, correlating with R_b. The Tremaine-Weinberg-method estimates of the bars' instantaneous pattern speeds show variations around the mean of up to ~20%, typically anti-correlating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range ~60-200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50% of bar measurements in Milky Way-mass external galaxies, the bar lengths of SBab type galaxies are overestimated by ~15% and those of SBbc types by ~55%. Consequently, bars longer than their corotation radius reported in the literature, dubbed "ultra-fast bars", may simply correspond to the largest biases. Given that the Scutum-Centaurus arm is likely connected to the near half of the Milky Way bar, recent direct measurements may be overestimating its length by 1-1.5 kpc, while its present pattern speed may be 5-10 km/s/kpc smaller than its time-averaged value.

astro-ph.GA↗

arXiv2020

Massive disc galaxies in cosmological hydrodynamical simulations are too dark matter-dominated

We investigate the disc-halo connection in massive (Mstar/Msun>5e10) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compare it with that inferred from the study of HI rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves (SPARC) dataset. We find that discrepancies between the the simulated and observed discs arise both on global and on local scales. Globally, the simulated discs inhabit halos that are a factor ~4 (in EAGLE) and ~2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also use synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict dark-matter dominated systems with stellar-to-total enclosed mass ratios that are a factor of 1.5-2 smaller than real galaxies at all radii. This is an alternative manifestation of the `failed feedback problem', since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly due to an overly efficient stellar and/or AGN feedback implementation.

astro-ph.GA↗

arXiv2020

Globular Clusters in the Sagittarius stream. Revising members and candidates with Gaia DR2

We reconsider the case for the association of Galactic globular clusters (GCs) to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), using Gaia DR2 data. We use RR Lyrae to trace the stream in 6D and we select clusters matching the observed stream in position and velocity. In addition to the clusters residing in the main body of the galaxy (M 54, Ter 8, Ter 7, Arp 2) we confirm the membership of Pal 12 and Whiting 1 to the portion of the trailing arm populated by stars lost during recent perigalactic passages. NGC 2419, NGC 5634 and NGC 4147 are very interesting candidates, possibly associated to more ancient wraps of the stream. We note that all these clusters, with the exception of M 54, that lies within the stellar nucleus of the galaxy, are found in the trailing arm of the stream. The selected clusters are fully consistent with the [Fe/H] vs. [Mg/Fe], [Ca/Fe] patterns and the age-metallicity relation displayed by field stars in the main body of Sgr dSph.

astro-ph.GA↗

arXiv2019

Exploring the outskirts of globular clusters: the peculiar kinematics of NGC 3201

The outskirts of globular clusters (GCs) simultaneously retain crucial information about their formation mechanism and the properties of their host galaxy. Thanks to the advent of precision astrometry both their morphological and kinematic properties are now accessible. Here we present the first dynamical study of the outskirts of the retrograde GC NGC 3201 until twice its Jacobi radius (< 100 pc), using specifically-selected high-quality astrometric data from Gaia DR2. We report the discovery of a stellar overdensity along the South-East/North-West direction that we identify as tidal tails. The GC is characterized globally by radial anisotropy and a hint of isotropy in the outer parts, with an excess of tangential orbits around the lobes corresponding to the tidal tails, in qualitative agreement with an N-body simulation. Moreover, we measure flat velocity dispersion profiles, reaching values of $3.5\pm0.9$ km/s until beyond the Jacobi radius. While tidal tails could contribute to such a flattening, this high velocity dispersion value is in disagreement with the expectation from the sole presence of potential escapers. To explain this puzzling observation, we discuss the possibility of an accreted origin of the GC, the presence of a dark matter halo --leftover of its formation at high redshift -- and the possible effects of non-Newtonian dynamics. Our study uncovers a new path for the study of GC formation and of the properties of the Milky Way potential in the era of precision astrometry.

astro-ph.GA↗

arXiv2019

Young stars raining through the Galactic Halo: the nature and orbit of Price-Whelan 1

We present radial velocities for five member stars of the recently discovered young (age= 100-150 Myr) stellar system Price-Whelan 1 (PW 1), that is located far away in the Galactic Halo (D~ 29 kpc, Z~ 15 kpc), and that is probably associated to the Leading Arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, V_r=275 +/- 10 km/s, significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the HI in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 deg (5~kpc) away from PW 1 and the velocity difference between the gas and the stars become larger as gas closer to the stars is considered. We discuss the possibilities that (a) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (b) that the parent cloud is the high velocity cloud HVC287.5+22.5+240, lagging behind the stellar system by ~25 km/s and ~10 deg.

astro-ph.GA↗

arXiv2019

Tracing Hercules in Galactic azimuth with Gaia DR2

The second data release of the Gaia mission has revealed, in stellar velocity and action space, multiple ridges, the exact origin of which is still debated. Recently, we demonstrated that a large Galactic bar with pattern speed 39 km/s/kpc does create most of the observed ridges. Among those ridges, the Hercules moving group would then be associated to orbits trapped at the co-rotation resonance of the bar. Here we show that a distinctive prediction of such a model is that the angular momentum of Hercules at the Sun's radius must significantly decrease with increasing Galactocentric azimuth, i.e. when getting closer to the major axis of the bar. We show that such a dependence of the angular momentum of trapped orbits on the azimuth would on the other hand not happen close to the outer Lindblad resonance of a faster bar, unless the orbital distribution is still far from phase-mixed, namely for a bar perturbation younger than ~ 2 Gyr. Using Gaia DR2 and Bayesian distances from the StarHorse code, and tracing the average Galactocentric radial velocity as a function of angular momentum and azimuth, we show that the Hercules angular momentum changes significantly with azimuth as expected for the co-rotation resonance of a dynamically old large bar.

astro-ph.GA↗

arXiv2018

Coma Berenices: first evidence for incomplete vertical phase-mixing in local velocity space with RAVE - confirmed with Gaia DR2

Before the publication of the Gaia DR2 we confirmed with RAVE and TGAS an observation recently made with the GALAH survey by Quillen ey al. concerning the Coma Berenices moving group in the Solar neighbourhood, namely that it is only present at negative Galactic latitudes. This allowed us to show that it is coherent in vertical velocity, providing a first evidence for incomplete vertical phase-mixing. We estimated for the first time from dynamical arguments that the moving group must have formed at most ~ 1.5 Gyr ago, and related this to a pericentric passage of the Sagittarius dwarf satellite galaxy. The present note is a rewritten version of the original arXiv post on this result now also including a confirmation of our finding with Gaia DR2.

astro-ph.GA↗

arXiv2018

MOND and the dynamics of NGC1052-DF2

The dwarf galaxy NGC1052-DF2 has recently been identified as potentially lacking dark matter. If correct, this could be a challenge for MOND, which predicts that low surface brightness galaxies should evince large mass discrepancies. However, the correct prediction of MOND depends on both the internal field of the dwarf and the external field caused by its proximity to the giant elliptical NGC1052. Taking both into consideration under plausible assumptions, we find $σ_{\rm MOND} = 13.4^{+4.8}_{-3.7}\;\mathrm{km}\,\mathrm{s}^{-1}$. This is only marginally higher than the claimed 90\% upper limit on the velocity dispersion ($σ< 10.5\;\mathrm{km}\,\mathrm{s}^{-1}$), and compares well with the observed root mean square velocity dispersion ($σ= 14.3\;\mathrm{km}\,\mathrm{s}^{-1}$). We also discuss a few caveats on both the observational and theoretical side. On the theory side, the internal virialization time in this dwarf may be longer that the time scale of variation of the external field. On the observational side, the paucity of data and their large uncertainties call for further analysis of the velocity dispersion of NGC1052-DF2, to check whether it poses a challenge to MOND or is a success thereof.

astro-ph.GA↗

arXiv2018

Is the Milky Way still breathing? RAVE-Gaia streaming motions

We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution catalogue (TGAS) to compute the velocity fields yielded by the radial (VR), azimuthal (Vphi) and vertical (Vz) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of Vz (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in VR and Vz. Below the plane we find the largest radial gradient to be dVR / dR = -7.01+- 0.61 km\s kpc, in agreement with recent studies. Above the plane we find a similar gradient with dVR / dR= -9.42+- 1.77 km\s kpc. By comparing our results with previous studies, we find that the structure in Vz is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.

astro-ph.GA↗