arXiv subjects

Benoit Famaey

Publications and source records attributed to Benoit Famaey.

At least 73 records · Page 4Linked to original sources

arXiv2021

Evolution of globular-cluster systems of ultra-diffuse galaxies due to dynamical friction in MOND gravity

(Abridged) Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution $N$-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). We test the MOND analog of the Chandrasekhar formula for the dynamical friction proposed by Sánchez-Salcedo on a single GC. We also explore whether MOND allows GC systems of isolated UDGs to survive without sinking into nuclear star clusters. The simulations are run using the adaptive-mesh-refinement code Phantom of Ramses. The mass resolution is $20\,M_\odot$ and the spatial resolution $50\,$pc. The GCs are modeled as point masses. Simulations including a single GC reveal that, as long as the apocenter of the GC is over about 0.5 effective radii, the Sánchez-Salcedo formula works excellently, with an effective Coulomb logarithm increasing with orbital circularity. Once the GC reaches the central kiloparsec, its sinking virtually stops, likely because of the core stalling mechanism. In simulations with multiple GCs, many of them sink toward the center, but the core stalling effect seems to prevent them from forming a nuclear star cluster. The GC system ends up with a lower velocity dispersion than the stars of the galaxy. By scaling the simulations, we extend these results to most UDG parameters, as long as these UDGs are not external-field dominated.

astro-ph.GA↗

arXiv2021

On the absence of backsplash analogues to NGC 3109 in the $Λ$CDM framework

The dwarf galaxy NGC 3109 is receding 105 km/s faster than expected in a $Λ$CDM timing argument analysis of the Local Group and external galaxy groups within 8 Mpc (Banik \& Zhao 2018). If this few-body model accurately represents long-range interactions in $Λ$CDM, this high velocity suggests that NGC 3109 is a backsplash galaxy that was once within the virial radius of the Milky Way and was slingshot out of it. Here, we use the Illustris TNG300 cosmological hydrodynamical simulation and its merger tree to identify backsplash galaxies. We find that backsplashers as massive ($\geq 4.0 \times 10^{10} M_\odot$) and distant ($\geq 1.2$ Mpc) as NGC 3109 are extremely rare, with none having also gained energy during the interaction with their previous host. This is likely due to dynamical friction. Since we identified 13225 host galaxies similar to the Milky Way or M31, we conclude that postulating NGC 3109 is a backsplash galaxy causes $>3.96σ$ tension with the expected distribution of backsplashers in $Λ$CDM. We show that the dark matter only version of TNG300 yields much the same result, demonstrating its robustness to how the baryonic physics is modelled. If instead NGC 3109 is not a backsplasher, consistency with $Λ$CDM would require the 3D timing argument analysis to be off by 105 km/s for this rather isolated dwarf, which we argue is unlikely. We discuss a possible alternative scenario for NGC 3109 and the Local Group satellite planes in the context of MOND, where the Milky Way and M31 had a past close flyby $7-10$ Gyr ago.

astro-ph.GA↗

arXiv2021

Charting the Galactic acceleration field I. A search for stellar streams with Gaia DR2 and EDR3 with follow-up from ESPaDOnS and UVES

We present maps of the stellar streams detected in the Gaia Data Release 2 (DR2) and Early Data Release 3 (EDR3) catalogs using the STREAMFINDER algorithm. We also report the spectroscopic follow-up of the brighter DR2 stream members obtained with the high-resolution CFHT/ESPaDOnS and VLT/UVES spectrographs as well as with the medium-resolution NTT/EFOSC2 spectrograph. Two new stellar streams that do not have a clear progenitor are detected in DR2 (named Hrid and Gunnthra), and seven are detected in EDR3 (named Gaia-6 to Gaia-12). Several candidate streams are also identified. The software also finds very long tidal tails associated with the 15 globular clusters NGC 288, NGC 1261, NGC 1851, NGC 2298, NGC 2808, NGC 3201, M 68, $ω$Cen, NGC 5466, Palomar 5, M 5, NGC 6101, M 92, NGC 6397 and NGC 7089. These stellar streams will be used in subsequent contributions in this series to chart the properties of the Galactic acceleration field on $\sim$ 100 pc to $\sim$ 100 kpc scales.

astro-ph.GA↗

arXiv2021

The ACTIONFINDER: An unsupervised deep learning algorithm for calculating actions and the acceleration field from a set of orbit segments

We introduce the "ACTIONFINDER", a deep learning algorithm designed to transform a sample of phase-space measurements along orbits in a static potential into action and angle coordinates. The algorithm finds the mapping from positions and velocities to actions and angles in an unsupervised way, by using the fact that points along the same orbit have identical actions. Here we present the workings of the method, and test it on simple axisymmetric models, comparing the derived actions to those generated with the Torus Mapping technique. We show that it recovers the Torus actions for halo-type orbits in a realistic model of the Milky Way to $\sim 0.6$% accuracy with as few as 1024 input phase-space measurements. These actions are much better conserved along orbits than those estimated with the Stäckel fudge. In our case, the reciprocal mapping from actions and angles to positions and velocities can also be learned. One of the advantages of the ACTIONFINDER is that it does not require the underlying potential to be known in advance, indeed it is designed to return the acceleration field. We expect the algorithm to be useful for analysing the properties of dynamical systems in numerical simulations. However, our ultimate goal with this effort will be to apply it to real stellar streams to recover the Galactic acceleration field in a way that is relatively agnostic about the underlying dark matter properties or the behavior of gravity.

astro-ph.GA↗

arXiv2021

Barred spiral galaxies in modified gravity theories

When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an $8σ$ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction $-$ which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for `fast' bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently `ultrafast' bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.

astro-ph.GA↗

arXiv2021

The Phantom of RAMSES user guide for galaxy simulations using Milgromian and Newtonian gravity

This document describes the general process of setting up, running, and analysing disc galaxy simulations using the freely available program Phantom of RAMSES (PoR). This implements Milgromian Dynamics (MOND) with a patch to the RAMSES grid-based $N$-body and hydrodynamical code that uses adaptive mesh refinement. We discuss the procedure of setting up isolated and interacting disc galaxy initial conditions for PoR, running the simulations, and analysing the results. This manual also concisely documents all previously developed MOND simulation codes and the results obtained with them.

astro-ph.IM↗

arXiv2020

The impact of the halo spin-concentration relation on disc scaling laws

Galaxy scaling laws, such as the Tully-Fisher, mass-size and Fall relations, can provide extremely useful clues on our understanding of galaxy formation in a cosmological context. Some of these relations are extremely tight and well described by one single parameter (mass), despite the theoretical existence of secondary parameters such as spin and concentration, which are believed to impact these relations. In fact, the residuals of these scaling laws appear to be almost uncorrelated with each other, posing significant constraints on models where secondary parameters play an important role. Here, we show that a possible solution is that such secondary parameters are correlated amongst themselves, in a way that removes correlations in observable space. In particular, we focus on how the existence of an anti-correlation between the dark matter halo spin and its concentration -- which is still debated in simulations -- can weaken the correlation of the residuals of the Tully-Fisher and mass-size relations. Interestingly, using simple analytic galaxy formation models, we find that this happens only for a relatively small portion of the parameter space that we explored, which suggests that this idea could be used to derive constraints to galaxy formation models that are still unexplored.

astro-ph.GA↗

arXiv2020

Kinematics and dynamics of Gaia red clump stars

We analyse the kinematics and dynamics of a homogeneous sample of red clump stars selected from the second Gaia data release catalogue in the direction of the Galactic poles. The level of completeness of the sample at heights between 0.6 and 3.5 kpc is asserted by comparison with the 2 Micron All Sky Survey catalogue. We show that both the density distribution and velocity dispersion are significantly more perturbed in the North than in the South, in all analysed regions of our Galactic neighbourhoods. We provide a detailed assessment of these North-South asymmetries at large heights. We then proceed to evaluate how such asymmetries could affect determinations of the dynamical matter density under equilibrium assumptions. We find that a Jeans analysis delivers relatively similar vertical forces and integrated dynamical surface densities at large heights above the plane in both hemispheres. At these heights, the densities of stars and gas are very low and the surface density is largely dominated by dark matter, which allows to estimate, separately in the North and South, the local dark matter density derived under equilibrium assumptions. In the presence of vertical perturbations, such values should be considered as an upper limit. This Jeans analysis yields values of the local dark matter density above 2~kpc, $ρ_{\rm DM} \sim 0.013 \, {\rm M}_\odot/{\rm pc}^3$ ($ \sim 0.509 \, {\rm GeV/cm}^3$) in the perturbed Northern hemisphere, and $ρ_{\rm DM} \sim 0.010 \, {\rm M}_\odot/{\rm pc}^3$ ($ \sim 0.374 \, {\rm GeV/cm}^3$) in the much less perturbed South. As a comparison, we determine the local dark matter density by fitting a global phase-space distribution to the data. We end up with a value in the range of $ρ_{\rm DM} \sim 0.011 - 0.014 \, {\rm M}_\odot/{\rm pc}^3$ in global agreement with Jeans analysis.

astro-ph.GA↗

arXiv2020

The hidden past of M92: Detection and characterization of a newly formed 17° long stellar stream using the Canada-France Imaging Survey

We present an analysis of the structure, kinematics and orbit of a newly found stellar stream emanating from the globular cluster M92 (NGC 6341). This stream was discovered in an improved matched-filter map of the outer Galaxy, based on a "color-color-magnitude" diagram, created using photometry from the Canada-France Imaging Survey (CFIS) and the Pan-STARRS 1 3$π$ survey (PS1). We find the stream to have a length of 17° (2.5 kpc at the distance of M92), a width dispersion of $0.29$°(42 pc) and a stellar mass of $[3.17 \pm{0.89}] \times 10^{4} $ M$_\odot$ ($10 \%$ of the stellar mass of the current main body of M92). We examine the kinematics of main sequence, red giant and blue horizontal branch stars belonging to the stream and that have proper motion measurements from the second data release of Gaia. N-body simulations suggest that the stream was likely formed very recently (during the last $\sim 500$ Myr) forcing us to question the orbital origin of this ancient, metal-poor globular cluster.

astro-ph.GA↗

arXiv2020

The bar resonances and low angular momentum moving groups in the Galaxy revealed by stellar ages

We use the second Gaia data release in combination with the catalog of Sanders & Das (2018) to dissect the Milky Way disc in phase-space and relative ages. We confirm and report the existence of multiple velocity moving groups at low azimuthal velocities and angular momenta, below Arcturus, regularly separated by $\sim~18-20\,\rm{km s^{-1}}$ in azimuthal velocity. Such features were predicted to exist more than ten years ago from the reaction of the Milky Way to a perturbation in the disc undergoing phase-mixing. These structures appear slightly younger than their phase-space surroundings, arguing against an extra-galactic origin. We also identify in relative age many of the classical ridges in the plane of azimuthal velocity vs. Galactocentric radius, traditionally associated with resonance features. These ridges are also younger than their phase-space surroundings in accordance with predictions from recent state-of-the-art cosmological hydrodynamical simulations of Milky Way-like galaxies. We study the response of dynamically young and old stellar disc populations to resonances from an analytic model of a large bar, which, remarkably, qualitatively reproduces the trends seen in the data. Our results re-inforce the idea that the Galactic disc is currently being shaped by both internal and external perturbations, and that, while absolute isochrone ages have to be taken with great care, exploring the dynamical structure of the disc in stellar ages, especially with future asteroseismic data, will provide much stronger constraints than metallicity/abundance trends alone.

astro-ph.GA↗

arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances

We present part 2 of the 6th and final Data Release (DR6 or FDR) of the Radial Velocity Experiment (RAVE), a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A) and span the complete time frame from the start of RAVE observations on 12 April 2003 to their completion on 4 April 2013. In the second of two publications, we present the data products derived from 518387 observations of 451783 unique stars using a suite of advanced reduction pipelines focussing on stellar atmospheric parameters, in particular purely spectroscopically derived stellar atmospheric parameters (Teff, log(g), and the overall metallicity), enhanced stellar atmospheric parameters inferred via a Bayesian pipeline using Gaia DR2 astrometric priors, and asteroseismically calibrated stellar atmospheric parameters for giant stars based on asteroseismic observations for 699 K2 stars. In addition, we provide abundances of the elements Fe, Al, and Ni, as well as an overall [alpha/Fe] ratio obtained using a new pipeline based on the GAUGUIN optimization method that is able to deal with variable signal-to-noise ratios. The RAVE DR6 catalogs are cross matched with relevant astrometric and photometric catalogs, and are complemented by orbital parameters and effective temperatures based on the infrared flux method. The data can be accessed via the RAVE Web site (http://rave-survey.org) or the Vizier database.

astro-ph.SR↗

arXiv2020

The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- I: Survey Description, Spectra and Radial Velocities

The Radial Velocity Experiment (RAVE) is a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A). The 6th and final data release (DR6 or FDR) is based on 518387 observations of 451783 unique stars. RAVE observations were taken between 12 April 2003 and 4 April 2013. Here we present the genesis, setup and data reduction of RAVE as well as wavelength-calibrated and flux-normalized spectra and error spectra for all observations in RAVE DR6. Furthermore, we present derived spectral classification and radial velocities for the RAVE targets, complemented by cross matches with Gaia DR2 and other relevant catalogs. A comparison between internal error estimates, variances derived from stars with more than one observing epoch and a comparison with radial velocities of Gaia DR2 reveals consistently that 68% of the objects have a velocity accuracy better than 1.4 km/s, while 95% of the objects have radial velocities better than 4.0 km/s. Stellar atmospheric parameters, abundances and distances are presented in subsequent publication. The data can be accessed via the RAVE Web (http://rave-survey.org) or the Vizier database.

astro-ph.SR↗

arXiv2020

A panoramic landscape of the Sagittarius stream in Gaia DR2 revealed with the STREAMFINDER spyglass

We present the first full six-dimensional panoramic portrait of the Sagittarius stream, obtained by searching for wide stellar streams in the Gaia DR2 dataset with the STREAMFINDER algorithm. We use the kinematic behavior of the sample to devise a selection of Gaia RR Lyrae, providing excellent distance measurements along the stream. The proper motion data are complemented with radial velocities from public surveys. We find that the global morphological and kinematic properties of the Sagittarius stream are still reasonably well reproduced by the simple Law & Majewski (2010) model (LM10), although the model overestimates the leading arm and trailing arm distances by up to $\sim 15$%. The sample newly reveals the leading arm of the Sagittarius stream as it passes into very crowded regions of the Galactic disk towards the Galactic Anticenter direction. Fortuitously, this part of the stream is almost exactly at the diametrically opposite location from the Galactic Center to the progenitor, which should allow an assessment of the influence of dynamical friction and self-gravity in a way that is nearly independent of the underlying Galactic potential model.

astro-ph.GA↗

arXiv2020

Detection of Strong Epicyclic Density Spikes in the GD-1 Stellar Stream. An Absence of Evidence for the Influence of Dark Matter Subhalos?

The density variations in thin stellar streams may encode important information on the nature of the dark matter. For instance, if dark matter aggregates into massive sub-halos, these perturbers are expected to scatter stars out of dynamically cold stellar streams, possibly leading to detectable gaps in those structures. Here we re-examine the density variations in the GD-1 stream, using Gaia DR2 astrometry, Pan-STARRS photometry, together with high precision radial velocities measured with the CFHT/ESPaDOnS and VLT/UVES instruments and complemented with public radial velocity catalogs. We show that after correcting for projection effects, the density profile exhibits high contrast periodic peaks, separated by $2.64\pm0.18$ kpc. An N-body simulation is presented that reproduces this striking morphology with simple epicyclic motion in a smooth Galactic potential. We also discuss the reliability of measuring density variations using ground-based photometric surveys, and for the particular case of GD-1 we highlight some of the artifacts present in the Gaia DR2 catalog along its track. Massive dark subhalos do not appear to be required to explain the density clumping along GD-1.

astro-ph.GA↗

arXiv2020

The formation of exponential disk galaxies in MOND

The formation and evolution of galaxies is highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies takes place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses (POR) code. These are the first-ever galaxy formation simulations done in MOND with detailed hydrodynamics, including star formation, stellar feedback, radiative transfer and supernovae. These models start from simplified initial conditions, in the form of isolated, rotating gas spheres in the early Universe. These collapse and form late-type galaxies obeying several scaling relations, which was not a priori expected. The formed galaxies have a compact bulge and a disk with exponentially decreasing surface mass density profiles and scale lengths consistent with observed galaxies, and vertical stellar mass distributions with distinct exponential profiles (thin and thick disk). This work thus shows for the first time that disk galaxies with exponential profiles in both gas and stars are a generic outcome of collapsing gas clouds in MOND. These models have a slight lack of stellar angular momentum because of their somewhat compact stellar bulge, which is connected to the simple initial conditions and the negligible later gas accretion. We also analyse how the addition of more complex baryonic physics changes the main resulting properties of the models and find this to be negligibly so in the Milgromian framework.

astro-ph.GA↗

arXiv2020

Baryon-Interacting Dark Matter: heating dark matter and the emergence of galaxy scaling relations

The empirical scaling relations observed in disk galaxies remain challenging for models of galaxy formation. The most striking among these is the Mass Discrepancy-Acceleration Relation (MDAR), which encodes both a tight baryonic Tully-Fisher relation (BTFR) and the observed diversity of galaxy rotation curves through the central surface density relation (CSDR). Building on our earlier work, we propose here that the MDAR is the result of interactions between baryons and 'Baryon-Interacting Dark Matter' (BIDM), which heat up the dark matter. Following a bottom-up, hydrodynamical approach, we find that the MDAR follows if: $i)$ the BIDM equation of state approximates that of an ideal gas; $ii)$ the BIDM relaxation time is order the Jeans time; $iii)$ the heating rate is inversely proportional to the BIDM density. Remarkably, under these assumptions the set of hydrodynamical equations together with Poisson's equation enjoy an anisotropic scaling symmetry. In the BIDM-dominated regime, this gives rise to an enhanced symmetry which fully captures the low-acceleration limit of the MDAR. We then show that, assuming a cored pseudo-isothermal profile at equilibrium, this set of equations gives rise to parameters reproducing the MDAR. Specifically, in the flat part of the rotation curve the asymptotic rotational velocity matches the parametric dependence of the BTFR. Moreover, in the central region of high-surface brightness galaxies, the profile reproduces the CSDR. Finally, by studying the time-dependent approach to equilibrium, we derive a global combination of the BTFR and CSDR, which matches the expectations in low surface-brightness galaxies. The form of the heating rate also makes model-independent predictions for various cosmological observables. We argue that our scenario satisfies existing observational constraints, and, intriguingly, offers a possible explanation to the EDGES anomaly.

astro-ph.CO↗

arXiv2019

Phase-space correlation in stellar streams of the Milky Way halo: The clash of Kshir and GD-1

We report the discovery of a 70-degree long stellar stream in the Milky Way halo, which criss-crosses the well known "GD-1" stream. We show that this new stellar structure ("Kshir") and GD-1 lie at similar distance, and are remarkably correlated in kinematics. We propose several explanations for the nature of this new structure and its possible association with GD-1. However, a scenario in which these two streams were accreted onto the Milky Way within the same dark matter sub-halo seems to provide a natural explanation for their phase-space entanglement, and other complexities of this coupled-system.

astro-ph.GA↗

arXiv2019

Dwarfs or giants? Stellar metallicities and distances in the Canada-France-Imaging-Survey from $ugrizG$ multi-band photometry

We present a new fully data-driven algorithm that uses photometric data from the Canada-France-Imaging-Survey (CFIS; $u$), Pan-STARRS 1 (PS1; $griz$), and Gaia ($G$) to discriminate between dwarf and giant stars and to estimate their distances and metallicities. The algorithm is trained and tested using the SDSS/SEGUE spectroscopic dataset and Gaia photometric/astrometric dataset. At [Fe/H]$<-1.2$, the algorithm succeeds in identifying more than 70% of the giants in the training/test set, with a dwarf contamination fraction below 30% (with respect to the SDSS/SEGUE dataset). The photometric metallicity estimates have uncertainties better than 0.2 dex when compared with the spectroscopic measurements. The distances estimated by the algorithm are valid out to a distance of at least $\sim 80$ kpc without requiring any prior on the stellar distribution, and have fully independent uncertainities that take into account both random and systematic errors. These advances allow us to estimate these stellar parameters for approximately 12 million stars in the photometric dataset. This will enable studies involving the chemical mapping of the distant outer disc and the stellar halo, including their kinematics using the Gaia proper motions. This type of algorithm can be applied in the Southern hemisphere to the first release of LSST data, thus providing an almost complete view of the external components of our Galaxy out to at least $\sim 80$ kpc. Critical to the success of these efforts will be ensuring well-defined spectroscopic training sets that sample a broad range of stellar parameters with minimal biases. A catalogue containing the training/test set and all relevant parameters within the public footprint of CFIS is available online.

astro-ph.GA↗