Pericenter Passage Research Articles

On the Corotation of Milky Way Satellites: LMC-mass Satellites Induce Apparent Motions in Outer Halo Tracers

Abstract Understanding the physical mechanism behind the formation of a corotating thin plane of satellite galaxies, like the one observed around the Milky Way (MW), has been challenging. The perturbations induced by a massive satellite galaxy, like the Large Magellanic Cloud (LMC), provide valuable insight into this problem. The LMC induces an apparent corotating motion in the outer halo by displacing the inner regions of the halo with respect to the outer halo. Using the Latte suite of Feedback In Realistic Environments cosmological simulations of MW-mass galaxies, we confirm that the apparent motion of the outer halo induced by the infall of a massive satellite changes the observed distribution of orbital poles of outer-halo tracers, including satellites. We quantify the changes in the distribution of orbital poles using the two-point angular correlation function and find that all satellites induce changes. However, the most massive satellites with pericentric passages between ≈30 and 100 kpc induce the largest changes. The best LMC-like satellite analog shows the largest change in orbital pole distribution. The dispersion of orbital poles decreases by 20° during the first two pericentric passages. Even when excluding the satellites brought in with the LMC-like satellite, there is clustering of orbital poles. These results suggest that in the MW, the recent pericentric passage of the LMC should have changed the observed distribution of orbital poles of all other satellites. Therefore, studies of kinematically coherent planes of satellites that seek to place the MW in a cosmological context should account for the existence of a massive satellite like the LMC.

Impact of dynamical friction on the tidal formation of NGC 1052-DF2

ABSTRACT The formation of dark matter-deficient galaxies (DMDGs) through tidal interactions has been a subject of growing interest, particularly with the discovery of galaxies such as NGC 1052-DF2. Previous studies suggested that strong tidal forces could strip dark matter (DM) from satellite galaxies, but the role of dynamical friction in this process has been largely overlooked. In this paper, we present self-consistent N-body simulations that incorporate the effects of dynamical friction on the tidal formation of NGC 1052-DF2, and compare them with the one without dynamical friction. We find that dynamical friction significantly accelerates the decay of the satellite galaxy’s orbit, causing it to experience more frequent tidal stripping and leading to the earlier formation of a DM-deficient state, approximately $7\!-\!8$ Gyr after infall. This is a few Gyr earlier than simulations without dynamical friction. Our results suggest that DMDGs can form in a wider range of orbital configurations, particularly on more circular orbits, than previously thought. Furthermore, we find that globular clusters in the DM-deficient phase exhibit elevated velocity dispersion, providing an observational signature of this evolutionary stage. We also examine the evolution of satellite in the phase space of total energy versus angular momentum, and show that a vertically narrow feature in this phase space is a clear signature of pericentre passage. These findings broaden the understanding of how DMDGs form and highlight the critical role of dynamical friction in shaping the evolutionary history of satellite galaxies in massive haloes.

A Multiwavelength Approach to Constraining the Merger Properties of ACT-CL J0034.4+0225

ACT-CL J0034.4+0225 is a previously unrecognized merging galaxy cluster at z = 0.38588 ± 0.00068. Our primary evidence is provided by a 21 ks Chandra image that shows two surface brightness peaks separated by ∼49″ (259 kpc) surrounded by an extended cluster gas distribution. Each gas peak contains a brightest cluster galaxy, offset from the gas peak. We collect new South African Large Telescope optical spectra that, when augmented by archival data, yield redshifts for the two BGCs and 58 other cluster members. Archival Giant Metrewave Radio Telescope and MeerKAT data reveal a radio halo that encompasses the X-ray peaks. We provide and compare three X-ray-based mass estimates (5.0 × 1014 M ⊙, 6.4 × 1014 M ⊙, and 8.6 × 1014 M ⊙). The Planck and ACT Sunyaev–Zel’dovich masses are ≈5.8 × 1014 M ⊙. We constrain the merger state and properties by comparing them to an existing suite of N-body/hydrodynamical models using the measured gas peak separation (259 kpc, projected) and radial velocity difference (0–1000 km s−1). This constrains the epoch of the merger to be within ∼100 Myr of first pericenter passage. A strong lensing analysis constrains the mass ratio to be in the range 1:1–1:20, while the cluster morphology prefers values near the equal-mass range.

The Three Hundred: The existence of massive dark matter-deficient satellite galaxies in cosmological simulations

The observation of a massive galaxy with an extremely low dark matter content (i.e. NGC 1277) has posed questions about how such objects form and evolve in a hierarchical universe. We here report on the finding of several massive, dark matter-deficient galaxies in a set of 324 galaxy clusters theoretically modelled by means of full-physics hydrodynamical simulations. We first focus on two example galaxies selected amongst the most massive and dark matter-deficient ones. By tracing the evolution of these galaxies, we find that their lack of dark matter is a result of multiple pericentre passages. While orbiting their host halo, tidal interactions gradually strip away dark matter while preserving the stellar component. A statistical analysis of all massive satellite galaxies in the simulated clusters shows that the stellar-to-total mass ratio today is strongly influenced by the number of orbits and the distance at pericentres. Galaxies with more orbits and closer pericentres are more dark matter-deficient. Additionally, we find that massive, dark matter-deficient galaxies at the present day are either the remnants of very massive galaxies at infall or former central galaxies of infalling groups. We conclude that such massive yet dark matter-deficient galaxies exist and are natural by-products of typical cluster galaxy evolution, with no specific requirement for an exotic formation scenario.

Testing General Relativity with Juno at Jupiter

The Juno spacecraft has been orbiting Jupiter since 2016 July to deepen our comprehension of the solar system by studying the gas giant. The radio science experiment enables the determination of Jupiter’s gravitational field, thus shedding light on its interior structure. The experiment relies on determining the orbit of the spacecraft during its pericenter passages. Previous gravity data analyses assumed the correctness of the general theory of relativity, which was used for trajectory integration and radio signal propagation modeling. In this work, we aim to test general relativity within the unique context of a spacecraft orbiting Jupiter, by employing the parameterized post-Newtonian (PPN) formalism, an established framework for comparing various gravitational theories. Within this framework, we focus our attention toward the PPN parameters γ and β, which offer insights into the curvature of spacetime and the nonlinearity of gravitational effects, respectively. Additionally, we extend our investigation to the Lense–Thirring effect, which models the dragging of spacetime induced by a rotating mass. By measuring the relativistic frequency shift on Doppler observables caused by Jupiter during Juno’s perijove passes, we estimate γ = 1 + (1.5 ± 4.9) × 10−3, consistent with the general theory of relativity. Our estimated γ is primarily influenced by its effect on light-time computation, with a negligible contribution from spacecraft dynamics. Furthermore, we also present a modest level of accuracy for the β parameter, reflecting the minimal dynamical perturbation on Juno from general relativity. This also applies to the Lense–Thirring effect, whose signal is too small to be confidently resolved.

Jupiter's Hotspots as observed by JIRAM-Juno: limb darkening in thermal infrared

ABSTRACT The Jupiter InfraRed Auroral Mapper (JIRAM) instrument onboard the Juno spacecraft performed repeated observations of Jupiter's North Equatorial Belt (NEB) around the time of 12th Juno pericenter passage on 2018 April 1. The data consist of thermal infrared images and show, among other atmospheric features, two bright Hotspots on the boundary between the NEB and the Equatorial Zone. Night-time images of the same areas at different emission angles were used to constrain the trend of the limb-darkening function. Comparison with simulated observations, computed for different emission angles, total opacities, single scattering albedo ω0, and asymmetry parameter g suggests that ω0 ∼ 0.90 ± 0.05 and g ∼ 0.37 ± 0.15 provide best match with data. Subsequently, we computed the ω0 and g resulting from different size distributions, taking into account the complex refractive indices of ammonium hydrosulfide (NH4SH) by Howett et al. [2007] and Ferraro et al. [1980]. Only the former data set is marginally consistent with JIRAM observations. Similarly, ammonia and hydrazine barely reproduce the experimental data. Tholin, although not usually considered a realistic component for Jupiter's aerosols, provides a better match for particle radii between 0.7 and 1 μm, both as a pure material as well as a thick coating over NH4SH cores. Notably, this radius range is consistent with the mean radius of aerosols as estimated by Ragent et al. [1998] on the basis of Galileo entry probe data. Comparison with literature suggests that similar results can be achieved by a large variety of contaminants bearing C–N bounds.

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 undermines 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; thus, their velocity dispersion may not reflect the EFE at their instantaneous distance from the cluster centre. In this work, we simulated UDGs within the Coma cluster in MOND, using the Phantom of Ramses ( por ) code. We 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 in the context of MOND, and without alterations (e.g. a screening of the EFE in galaxy clusters or much higher baryonic masses than currently estimated), we find that UDGs must be out-of-equilibrium objects on their first infall onto the cluster, .

Effect of the Large Magellanic Cloud on the kinematics of Milky Way satellites and virial mass estimate

We present a study illustrating the effects of the passage of a Large Magellanic Cloud (LMC) mass satellite on the distance and velocity distributions of satellites in Λ+Cold Dark Matter simulations of Milky Way (MW) sized halos. In agreement with previous studies, we find that during such a passage the velocity distribution develops a high-velocity tail, which can bias velocity-based virial halo mass estimates. When the velocity distribution of MW satellites is corrected for effects of the LMC passage, it is consistent with the distributions in halos of masses as low as and as high as 1.5×1012M⊙. We present a new halo mass estimator , where c is the coefficient calibrated using satellite systems in the simulated MW-sized halos, is the variance of 3D velocities taken with the sign of the radial velocity of each satellite, and is the median halocentric distance of the satellites. We show that the estimator has only s=8% scatter around the median relation of the estimated and true halo masses and deviates by <2s from the median during the pericentric passage of an LMC-like subhalo. This is because and deviate in the opposite directions during such passages. We apply the estimator to the MW satellite system and estimate the virial mass of the Milky Way of , in good agreement with several recent estimates using other methods.

UVIT Study of the MAgellanic Clouds (U-SMAC) – I. Recent star formation history and kinematics of the Shell region in the north-eastern Small Magellanic Cloud

ABSTRACT The interactions between the Magellanic Clouds significantly affect the shape and distribution of the young stellar population, particularly in the periphery of the Small Magellanic Cloud (SMC). We present the first far-UV (FUV) map of the north-east SMC-Shell region using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. The detected FUV stars are combined with Gaia Early Data Release 3 data to create a FUV–optical catalogue of ∼14 400 stars. FUV–optical colour-magnitude diagrams are used along with isochrones to estimate the stellar ages. The detected stars are formed in multiple episodes. We identified two episodes of star formation (∼60 and ∼260 Myr ago), where the episode at ∼260 Myr is linked to the recent interaction with the Large Magellanic Cloud (LMC) and the episode at ∼60 Myr is linked to the pericentric passage of the SMC around our Galaxy. The median proper motion (PM) and velocity dispersion are found to be similar to the SMC main body, indicating that this region has not experienced significant tidal effects. The FUV stellar surface density and the dispersion in PM suggest that the extent of the inner SMC in the north-east direction to be around 2.2°. We detect arm-like and arc-like structures in the FUV stellar density map, and their kinematics appear to be similar to the SMC main body. These extended outer features are the spatial stellar overdensities formed over multiple episodes of star formation, but without apparent kinematic distinction.

One-sided H α excess before the first pericentre passage in galaxy pairs

ABSTRACT We present novel insights into the interplay between tidal forces and star formation in interacting galaxies before their first pericentre passage. We investigate seven close pair galaxies devoid of visible tidal disturbances, such as tails, bridges, and shells. Using integral field spectroscopy data of extended Calar Alto Legacy Integral Field Area, we unveil a previously unreported phenomenon: H $\alpha$ emission, a proxy for recent star formation, exhibits a significant enhancement in regions facing the companion galaxy, reaching up to 1.9 times higher flux compared to opposite directions. Notably, fainter companions within pairs display a more pronounced one-sided H $\alpha$ excess, exceeding the typical range observed in isolated galaxies with 2$\sigma$ confidence level. Furthermore, the observed H $\alpha$ excess in fainter companion galaxies exhibits a heightened prominence at the outer galactic regions. These findings suggest that tidal forces generated before the first pericentre passage exert a stronger influence on fainter galaxies due to their shallower potential wells by their brighter companions. This unveils a more intricate interplay between gravitational interactions and star formation history within interacting galaxies than previously understood, highlighting the need further to explore the early stages of interaction in galaxy evolution.

Ram pressure stripping in clusters: gravity can bind the ISM but not the CGM

ABSTRACT We explore the survival of a galaxy’s circumgalactic medium (CGM) as it experiences ram pressure stripping (RPS) moving through the intracluster medium (ICM). For a satellite galaxy, the CGM is often assumed to be entirely stripped/evaporated, an assumption that may not always be justified. We carry out 3D-hydrodynamic simulations of the interstellar and circumgalactic media (ISM + CGM) of a galaxy like JO201 moving through the ICM. The CGM can survive long at cluster outskirts ($\gtrsim 2 \rm \ Gyr$) but at smaller clustercentric distances, 90 per cent of the CGM mass is lost within ∼500 Myr. The gravitational restoring force on the CGM is mostly negligible and the CGM–ICM interaction is analogous to ‘cloud–wind interaction’. The CGM stripping time-scale does not depend on the ram pressure but on the CGM to ICM density contrast χ. Two distinct regimes emerge for CGM stripping: the χ > 1 regime, which is the well-known ‘cloud crushing’ problem, and the χ < 1 regime, which we refer to as the (relatively unexplored) ‘bubble drag’ problem. The first pericentric passage near the cluster core can rapidly – over a crossing time tdrag ∼ R/vrel – strip the CGM in the bubble drag regime. The ISM stripping criterion unlike the CGM criterion, still depends on the ram pressure $\rho _{\rm ICM} v_{\rm rel}^2$. The stripped tails of satellites contain contributions from both the disc and the CGM. The X-ray plume in M89 in the Virgo cluster and a lack of it in the nearby M90 might be attributed to their orbital histories. M90 has likely undergone stripping in the bubble drag regime due to a pericentric passage close to the cluster centre.

A Sagittarius-like simulated dwarf spheroidal galaxy from TNG50

The Sagittarius dwarf spheroidal (Sgr dSph) galaxy provides one of the most convincing examples of tidal interaction between satellite galaxies and the Milky Way (MW). The main body of the dwarf was recently demonstrated to have an elongated, prolate, bar-like shape and to possess some internal rotation. Whether these features are temporary results of the strong tidal interaction at the recent pericenter passage or are due to a disky progenitor is a matter of debate. I present an analog of Sgr selected among bar-like galaxies from the TNG50 simulation of the IllustrisTNG project. The simulated dwarf is initially a disky galaxy with mass exceeding $10^ $ M$_ and evolves around a MW-like host on a tight orbit with seven pericenter passages and a period of about 1 Gyr. At the second pericenter passage, the disk transforms into a bar and the bar-like shape of the stellar component is preserved until the end of the evolution. The morphological transformation is accompanied by strong mass loss, leaving a dwarf with a final mass of below $ M$_ The gas is lost completely and the star formation ceases at the third pericenter passage. At the last pericenters, the dwarf possesses a bar-like shape, a little remnant rotation, and the metallicity gradient, which are consistent with observations. The more concentrated metal-rich stellar population rotates faster and has a lower velocity dispersion than the more extended metal-poor one. The metallicity distribution evolves so that the most metal-poor stars are stripped first, which explains the metallicity gradient detected in the Sgr stream. This study demonstrates that a dSph galaxy with properties akin to the Sgr dwarf can form from a disky progenitor with a mass of above $ M$_ by tidal evolution around the MW in the cosmological context.

Dissipative Dark Substructure: The Consequences of Atomic Dark Matter on Milky Way Analog Subhalos

Abstract Using cosmological hydrodynamical zoom-in simulations, we explore the properties of subhalos in Milky Way analogs that contain a subcomponent of atomic dark matter (ADM). ADM differs from cold dark matter (CDM) due to the presence of self-interactions that lead to energy dissipation, analogous to standard model baryons. This model can arise in dark sectors that are natural and theoretically motivated extensions to the standard model. The simulations used in this work were carried out using GIZMO and utilize the FIRE-2 galaxy formation physics in the standard model baryonic sector. For the parameter points we consider, the ADM gas cools efficiently, allowing it to collapse to the center of subhalos. This increases a subhalo’s central density and affects its orbit, with more subhalos surviving small pericentric passages. The subset of subhalos that host satellite galaxies have cuspier density profiles and smaller stellar half-mass radii relative to CDM. The entire population of dwarf galaxies produced in the ADM simulations is more compact than those seen in CDM simulations, unable to reproduce the entire diversity of observed dwarf galaxy structures. Additionally, we also identify a population of highly compact subhalos that consist nearly entirely of ADM and form in the central region of the host, where they can leave distinctive imprints in the baryonic disk. This work presents the first detailed exploration of subhalo properties in a strongly dissipative dark matter scenario, providing intuition for how other regions of ADM parameter space, as well as other dark sector models, would impact galactic-scale observables.

Study of the $ 50$ kpc circumgalactic environment around the merger system J2057-0030 at $ z $ sim 4.6 using ALMA

We present ALMA band-7 observations of J2057-0030, a multi-component merger system at $z$ sim 4.68 spanning at least 50 kpc in size, using the C$ ii lambda 157.74 mu m line and underlying far-infrared (FIR) continuum. We find two main components, the quasar (QSO) and a dusty star-forming galaxy (DSFG), both detected in C$ ii and continuum emission as well as multiple neighboring clumps detected only in C$ ii . Three of these clumps form a (tidal) tail that extends from the QSO in a straight direction to the west, covering a projected distance of sim 10 kpc. This perturbed morphology, added to a spatial distance of sim 20 kpc and a velocity offset of $ v $ = 68 km s$^ $ between the QSO and the DSFG, strongly supports a merging scenario. By fitting a spectral energy distribution model to the continuum data, we estimate star formation rates of approx 402 $M_ odot $ yr$^ $ for the QSO host and approx 244 $M_ odot $ yr$^ $ for the DSFG, which locate them on or close to the main sequence of star-forming galaxies. The J2057-0030 QSO was selected for being one of the brightest unobscured quasars at its redshift while presenting a rather modest star formation rate. Based on a commonly accepted paradigm regarding the formation of quasars, this result is expected for a quasar that has already passed an obscured phase of rapid star formation during a major merger. However, we see that the merger event in this system is far from being finished, and it is rather likely somewhere between the first pericenter and subsequent close passages. This is presumably another case of a high-$z$ quasar residing in a high-density environment with a companion obscured galaxy.

Detecting Gravitational Wave Bursts from Stellar-mass Binaries in the mHz Band

The dynamical formation channels of gravitational wave (GW) sources typically involve a stage when the compact object binary source interacts with the environment, which may excite its eccentricity, yielding efficient GW emission. For the wide eccentric compact object binaries, the GW emission happens mostly near the pericenter passage, creating a unique, burst-like signature in the waveform. This work examines the possibility of stellar-mass bursting sources in the mHz band for future LISA detections. Because of their long lifetime (∼107 yr) and promising detectability, the number of mHz bursting sources can be large in the local Universe. For example, based on our estimates, there will be ∼3–45 bursting binary black holes in the Milky Way, with ∼102–104 bursts detected during the LISA mission. Moreover, we find that the number of bursting sources strongly depends on their formation history. If certain regions undergo active formation of compact object binaries in the recent few million years, there will be a significantly higher bursting source fraction. Thus, the detection of mHz GW bursts not only serves as a clue for distinguishing different formation channels, but also helps us understand the star formation history in different regions of the Milky Way.

Orbital evolution of eccentric perturbers under dynamical friction: crossing the sound barrier

ABSTRACT In a gaseous medium, dynamical friction (DF) reaches a maximum when the orbital speed of a (point-like) perturber moving on a circular orbit is close to the sound speed. Therefore, in a quasi-steady state, eccentric orbits of perturbers approaching the sound barrier (from below) should rapidly circularize as they experience the strongest drag at pericentre passage. To investigate this effect, we extend the solution for circular DF in a uniform gaseous medium to eccentric Keplerian orbits. We derive an approximation to the steady-state DF force, which is valid for eccentricities as high as e = 0.9 in a limited range of Mach number around the transition to supersonic regime. We validate our analytical result with 3D simulations of the gas density response. Although gaseous DF generally dissipates orbital energy, we find that it can be directed along the motion of the perturber near pericentre passage when the eccentricity is e ≳ 0.9. We apply our results to compute the long-time evolution of the orbital parameters. Most trajectories tend to circularize as the perturber moves into the supersonic regime. However, orbits with eccentricities e ≳ 0.8 below the sound barrier experience a slight increase in eccentricity as they loose orbital energy. Possible extensions to our analytical approach are also discussed.

The density of the Milky Way’s corona atz≈ 1.6 through ram pressure stripping of the Draco dSph galaxy

ABSTRACTSatellite galaxies within the Milky Way’s (MW's) virial radius Rvir are typically devoid of cold gas due to ram pressure stripping by the MW’s corona. The density of this corona is poorly constrained today and essentially unconstrained in the past, but can be estimated using ram pressure stripping. In this paper, we probe the MW's corona at z ≈ 1.6 using the Draco dwarf spheroidal galaxy. We assume that (i) Draco’s orbit is determined by its interaction with the MW, whose dark matter halo we evolve in time following cosmologically motivated prescriptions, (ii) Draco’s star formation was quenched by ram pressure stripping and (iii) the MW’s corona is approximately smooth, spherical, and in hydrostatic equilibrium. We used Gaia proper motions to set the initial conditions and Draco’s star formation history to estimate its past gas content. We found indications that Draco was stripped of its gas during the first pericentric passage. Using 3D hydrodynamical simulations at a resolution that enables us to resolve individual supernovae and assuming no tidal stripping, which we estimate to be a minor effect, we find a density of the MW corona ≥8 × 10−4 cm−3 at a radius ≈0.72Rvir. This provides evidence that the MW’s corona was already in place at z ≈ 1.6 and with a higher density than today. If isothermal, this corona would have contained all the baryons expected by the cosmological baryon fraction. Extrapolating to today shows good agreement with literature constraints if feedback has removed ≲30 per cent of baryons accreted on to the halo.

The accretion history of the Milky Way: III. Hydrodynamical simulations of Galactic dwarf galaxies at first infall

ABSTRACT Most Milky Way dwarf galaxies are much less bound to their host than are relics of Gaia–Sausage–Enceladus and Sgr. These dwarfs are expected to have fallen into the Galactic halo less than 3 Gyr ago, and will therefore have undergone no more than one full orbit. Here, we have performed hydrodynamical simulations of this process, assuming that their progenitors are gas-rich, rotation-supported dwarfs. We follow their transformation through interactions with the hot corona and gravitational field of the galaxy. Our dedicated simulations reproduce the structural properties of three dwarf galaxies: Sculptor, Antlia II, and with somewhat a lower accuracy, Crater II. This includes reproducing their large velocity dispersions, which are caused by ram-pressure stripping and Galactic tidal shocks. Differences between dwarfs can be interpreted as due to different orbital paths, as well as to different initial conditions for their progenitor gas and stellar contents. However, we failed to suppress in a single orbit the rotational support of our Sculptor analogue if it is fully dark matter dominated. In addition, we have found that classical dwarf galaxies like Sculptor may have stellar cores sufficiently dense to survive the pericentre passage through adiabatic contraction. On the contrary, our Antlia II and Crater II analogues are tidally stripped, explaining their large sizes, extremely low surface brightnesses, and velocity dispersion. This modelling explains differences between dwarf galaxies by reproducing them as being at different stages of out-of-equilibrium stellar systems.

The accretion history of the Milky Way – II. Internal kinematics of globular clusters and of dwarf galaxies

ABSTRACT We study how structural properties of globular clusters and dwarf galaxies are linked to their orbits in the Milky Way halo. From the inner to the outer halo, orbital energy increases and stellar-systems gradually move out of internal equilibrium: in the inner halo, high-surface brightness globular clusters are at pseudo-equilibrium, while further away, low-surface brightness clusters and dwarfs appear more tidally disturbed. Dwarf galaxies are the latest to arrive into the halo as indicated by their large orbital energies and pericentres, and have no time for more than one orbit. Their (gas-rich) progenitors likely lost their gas during their recent arrival in the Galactic halo. If dwarfs are at equilibrium with their dark matter (DM) content, the DM density should anticorrelate with pericentre. However, the transformation of DM dominated dwarfs from gas-rich rotation-supported into gas-poor dispersion-supported systems is unlikely accomplished during a single orbit. We suggest instead that the above anticorrelation is brought by the combination of ram-pressure stripping and of Galactic tidal shocks. Recent gas removal leads to an expansion of their stellar content caused by the associated gravity loss, making them sufficiently fragile to be transformed near pericentre passage. Out of equilibrium dwarfs would explain the observed anticorrelation of kinematics-based DM density with pericentre without invoking DM density itself, questioning its previous estimates. Ram-pressure stripping and tidal shocks may contribute to the dwarf velocity dispersion excess. It predicts the presence of numerous stars in their outskirts and a few young stars in their cores.

The outer low-α disc of the Milky Way – I: evidence for the first pericentric passage of Sagittarius?

ABSTRACT Phase-space data, chemistry, and ages together reveal a complex structure in the outer low-α disc of the Milky Way. The age-vertical velocity dispersion profiles beyond the Solar Neighbourhood show a jump at 6 Gyr for stars beyond the Galactic plane. Stars older than 6 Gyr are significantly hotter than younger stars. The chemistry and age histograms reveal a bump at [Fe/H] = −0.5, [α/Fe] = 0.1, and an age of 7.2 Gyr in the outer disc. Finally, viewing the stars beyond 13.5 kpc in the age-metallicity plane reveals a faint streak just below this bump, towards lower metallicities at the same age. Given the uncertainty in age, we believe these features are linked and suggest a pericentric passage of a massive satellite ∼6 Gyr ago that heated pre-existing stars, and led to a starburst in existing gas. New stars also formed from the metal-poorer infalling gas. The impulse approximation was used to characterize the interaction with a satellite, finding a mass of ∼1011 M⊙, and a pericentric position between 12 and 16 kpc. The evidence points to an interaction with the Sagittarius dwarf galaxy, likely its first pericentric passage.