A tidally induced global corrugation pattern in an external disc galaxy similar to the Milky Way
Facundo A. Gómez, Sergio Torres-Flores, Catalina Mora-Urrejola, Antonela Monachesi, Simon D. M. White, Nicolas P. Maffione, Robert J. J. Grand, Federico Marinac… 展开作者
Facundo A. Gómez, Sergio Torres-Flores, Catalina Mora-Urrejola, Antonela Monachesi, Simon D. M. White, Nicolas P. Maffione, Robert J. J. Grand, Federico Marinacci, Rüdiger Pakmor, Volker Springel, Carlos S. Frenk, Philippe Amram, Benoît Epinat, Claudia Mendes de Oliveira
备注：
11 pages,12 figures. Accepted for publication in ApJ
We study the two dimensional (2D) line-of-sight velocity ($V_{\rm los}$) field of the low-inclination, late-type galaxy VV304a. The resulting 2D kinematic map reveals a global, coherent and extended perturbation that is likely associated with a recent interaction with the massive companion VV304b. We use multi-band imaging and a suite of test particle simulations to quantify the plausible strength of in-plane flows due to non-axisymmetric perturbations and show that the observed velocity flows are much too large to be driven either by spiral structure nor by a bar. We use fully cosmological hydrodynamical simulations to characterize the contribution from in- and off-plane velocity flows to the $V_{\rm los}$ field of recently interacting galaxy pairs like the VV304 system. We show that, for recently perturbed low-inclination galactic discs, the structure of the residual velocity field, after subtraction of an axisymmetric rotation model, can be dominated by vertical flows. Our results indicate that the $V_{\rm los}$ perturbations in VV304a are consistent with a corrugation pattern. Its $V_{\rm los}$ map suggests the presence of a structure similar to the Monoceros ring seen in the Milky Way. Our study highlights the possibility of addressing important questions regarding the nature and origin of vertical perturbations by measuring the line-of-sight velocities in low-inclination nearby galaxies.
Evidence for wave-like corrugations are well established in the Milky Way and in nearby disc galaxies. These were originally detected as a displacement of the interstellar medium about the midplane, either in terms of vertical distance or vertical velocity. Over the past decade, similar patterns have emerged in the Milky Way's stellar disc. We investigate how these vertical waves are triggered by a passing satellite. Using high-resolution N-body/hydrodynamical simulations, we systematically study how the corrugations set up and evolve jointly in the stellar and gaseous discs. We find that the …
We infer the past orbit of the Sagittarius (Sgr) dwarf galaxy in the Milky Way halo by integrating backwards from its observed position and proper motions, including the effects of dynamical friction. Given measured proper motions, we show that there is a relation between the eccentricity ($e$) of Sgr's orbit and the mass of the Milky Way ($M_{T}$) in the limit of no dynamical friction. That relation can be fit by a power-law of the form: $e \approx 0.49 \left(M_{T}/10^{12} M_{\odot}\right)^{-0.88}$. At a fixed Milky Way mass, the dynamical friction term increases the mean eccentricity of the …
Using numerical analysis of fundamental frequencies we study the orbital structure of a tidally induced bar formed in a simulated dwarf galaxy orbiting a Milky Way-like host. We find that only about 10% of stars have frequencies compatible with x1 orbits, the classical periodic orbits in a barred potential. The rest of the stars follows box orbits parallel to the bar, with varying degree of elongation.
Bars in galaxies may form not only through instability, but also as a result of an interaction with another galaxy. In particular, they may appear in disky dwarf galaxies interacting with Milky Way-like galaxies. Here we report the results of N-body/SPH simulations of such dwarfs orbiting in the static potential of a larger galaxy. We used several models of the dwarf galaxy, all of the same mass, but covering a large range of gas fractions: 0, 30 and 70%. We also tested the impact of subgrid star formation processes. In all cases bars of similar length formed in the stellar disk of the dwarfs …
Bars in galaxies may develop through a global instability or due to an interaction with another system. We study bar formation in disky dwarf galaxies orbiting a Milky Way-like galaxy. We employ $N$-body simulations to study the impact of initial orbital parameters: the size of the dwarf galaxy orbit and the inclination of its disc with respect to the orbital plane. In all cases a bar develops in the center of the dwarf during the first pericenter on its orbit around the host. Between subsequent pericenter passages the bars are stable, but at the pericenters they are usually weakened and short…
We present a kinematic study of six dwarf spheroidal galaxies (dSph) satellites of the Milky Way (MW), namely Carina, Draco, Fornax, Sculptor, Sextans, and Ursa Minor. We combine proper motions (PMs) from the $Gaia$ Data Release 3 (DR3) and line-of-sight velocities ($v_{\mathrm{los}}$) from the literature to derive their 3D internal kinematics and to study the presence of internal velocity gradients. We find velocity gradients along the line-of-sight for Carina, Draco, Fornax, and Ursa Minor, at $\geq 1σ$ level of significance . The value of such gradients appears to be related to the orbital …
The Eastern Banded Structure (EBS) and Hydra~I halo overdensity are very nearby (d $\sim$ 10 kpc) objects discovered in SDSS data. Previous studies of the region have shown that EBS and Hydra I are spatially coincident, cold structures at the same distance, suggesting that Hydra I may be the EBS's progenitor. We combine new wide-field DECam imaging and MMT/Hectochelle spectroscopic observations of Hydra I with SDSS archival spectroscopic observations to quantify Hydra I's present-day chemodynamical properties, and to infer whether it originated as a star cluster or dwarf galaxy. While previous…
The multiple corrugations in the Galactic disk derived from the LAMOST and Gaia survey data
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Shandong Key Laboratory of Space Environment and Exploration Technology, Institute of Space Sciences, School of Space Science and Technology, Shandong University（山东大学空间科学与空间技术学院空间科学研究院（山东省空间环境与探空技术重点实验室））；School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（南京大学现代天文与天体物理教育部重点实验室）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）；School of Physics and Optoelectronic Engineering, Hainan University（海南大学物理与电子工程学院）
The origin of grand design spiral structure in galaxies is still under debate but one of promising scenarios involves tidal interactions. We use N-body simulations to study the evolution of a Milky Way-size galaxy in a Virgo-like cluster. The galaxy is placed on a typical eccentric orbit and evolved for 10 Gyr. We find that grand design spiral arms are triggered by pericenter passages and later on they wind up and dissipate. The arms formed in the simulations are approximately logarithmic, but are also dynamic, transient and recurrent.
Signatures of a Tidally Induced Spiral Arm at the Anticenter of the Milky Way and a Kinematically Extended Anticenter Stream Using DESI DR2
University of California, Santa Cruz（加州大学圣克鲁兹分校）；University of Edinburgh（爱丁堡大学）；University of Toronto（多伦多大学）；University of Michigan（密歇根大学）；Observatório Nacional（国家天文台）；Shanghai Jiao Tong University（上海交通大学）
We present an upgraded version of the \MOCCA code for the study of dynamical evolution of globular clusters (GCs) and its first application to the study of evolution of multiple stellar populations. We explore initial conditions spanning different structural parameters for the first (FG) and second generation of stars (SG) and we analyze their effect on the binary dynamics and survival. Here, we focus on the number ratio of FG and SG binaries, its spatial variation, and the way their abundances are affected by various cluster initial properties. We find that present-day SG stars are more abund…
Approximately once every $10^4$ years, a star passes close enough to the supermassive black hole Sgr A* at the center of the Milky Way to be pulled apart by the black hole's tidal forces. The star is then "spaghettified" into a long stream of matter, with approximately one half being bound to Sgr A* and the other half unbound. Within this stream, the local self-gravity dominates the tidal field of Sgr A*, which at minimum restricts the stream to a small finite width. As the stream cools from adiabatic expansion and begins to recombine, the residual self-gravity allows for planetary-mass fragme…
Using N-body simulations, we study the properties of a bar induced in a dwarf galaxy as a result of tidal interaction with the Milky Way. The dwarf is initially composed of a disk embedded in a dark matter halo and we follow its evolution on a typical orbit for 10 Gyr. It undergoes an evolution typical of tidally stirred dwarf galaxies: it loses mass, the stellar component transforms from a disk to a spheroid and the rotation of the stars is partially replaced by random motions. A tidally induced bar forms at the first pericentre passage and survives until the end of the evolution. Fourier dec…
Using six-dimesional phase-space information from the Fourth Data release of the Radial Velocity Experiment (RAVE) over the range of Galactic longitude 240$^{\circ}< l <$ 360$^{\circ}$ and $V_{LSR} < -239$ kms$^{-1}$, we have computed orbits for 329 RAVE stars that were originally selected as chemically and kinematically related to $ω$ Centauri. The orbits were integrated in a Milky-Way-like axisymmetric Galactic potential, ignoring the effects of the dynamical evolution of $ω$ Centauri due to the tidal effects of the Galaxy disk on the cluster along time. We also ignored secular changes in th…
We use a hybrid test particle/N-body simulation to integrate 4 million massless test particle trajectories within a fully self-consistent 10^5 particle N-body simulation. The number of massless particles allows us to resolve fine structure in the spatial distribution and phase space of a dwarf galaxy as it is disrupted in the tidal field of a Milky Way type galaxy. The tidal tails exhibit nearly periodic clumping or a smoke-like appearance. By running simulations with different satellite particle mass, halo particle mass, number of massive and massless particles and with and without a galaxy d…
Bars in disky galaxies can be formed by interactions with other systems, including those of comparable mass. It has long been established that the effect of such interactions on galaxy morphology depends strongly on the orbital configuration, in particular the orientation of the intrinsic spin of the galactic disk with respect to its orbital angular momentum. Prograde encounters modify the morphology strongly, including the formation of tidally induced bars, while retrograde flybys should have little effect on morphology. Recent works on the subject reached conflicting conclusions, one using t…
It is now clear that the stars in the Solar neighbourhood display large-scale coherent vertical breathing motions. At the same time, Milky Way-like galaxies experience tidal interactions with satellites/companions during their evolution. While these tidal interactions can excite vertical oscillations, it is still not clear whether vertical breathing motions are excited \textit{directly} by the tidal encounters or are driven by the tidally-induced spirals. We test whether excitation of breathing motions are directly linked to tidal interactions by constructing a set of $N$-body models (with mas…
Orbits are the key building blocks of any density distribution and their study helps us understand the kinematical structure and the evolution of galaxies. Here we investigate orbits in a tidally induced bar of a dwarf galaxy, using an $N$-body simulation of an initially disky dwarf galaxy orbiting a Milky Way-like host. After the first pericenter passage, a tidally induced bar forms in the stellar component of the dwarf. The bar evolution is different than in isolated galaxies and our analysis focuses on the period before it buckles. We study the orbits in terms of their dominant frequencies,…
We have performed N-body simulations of tidally filling star clusters with a range of orbits in a Milky Way-like potential to study the effects of orbital inclination and eccentricity on their structure and evolution. At small galactocentric distances Rgc, a non-zero inclination results in increased mass loss rates. Tidal heating and disk shocking, the latter sometimes consisting of two shocking events as the cluster moves towards and away from the disk, help remove stars from the cluster. Clusters with inclined orbits at large Rgc have decreased mass loss rates than the non-inclined case, sin…
Tidal interactions between disc galaxies and low mass companions are an established method for generating galactic spiral features. In this work we present a study of the structure and dynamics of spiral arms driven in interactions between disc galaxies and perturbing companions in 3-D N-body/smoothed hydrodynamical numerical simulations. Our specific aims are to characterize any differences between structures formed in the gas and stars from a purely hydrodynamical and gravitational perspective, and to find a limiting case for spiral structure generation. Through analysis of a number of diffe…
The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Way's thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we dev…
We present a new analysis of the observed perturbations of the HI disk of the Milky Way to infer the existence of a dark sub-halo that tidally interacted with the Milky Way disk. We examine tidal interactions between perturbing dark sub-halos and the gas disk of the Milky Way using high resolution Smoothed Particle Hydrodynamics (SPH) simulations. We compare our results to the observed HI map of the Milky Way to find that the Fourier amplitudes of the planar disturbances are best-fit by a perturbing dark sub-halo with a mass that is one-hundredth of the Milky Way with a pericentric distance of…
As part of a series of papers aimed at understanding the evolution of the Milky Way's Central Molecular Zone (CMZ), we present hydrodynamical simulations of turbulent molecular clouds orbiting in an accurate model of the gravitational potential extant there. We consider two sets of model clouds differing in the energy content of their velocity fields. In the first, self--virialised set, the turbulent kinetic energies are chosen to be close in magnitude to the clouds' self--gravitational potential energies. Comparison with isolated clouds evolving without an external potential shows that the se…
There is an increasing number of recent observational results which show that some globular clusters exhibit internal rotation while they travel along their orbital trajectories around the Milky Way center. Based on these findings, we looked for any relationship between the inclination angles of the globular clusters' orbits with respect to the Milky Way plane and those of their rotation. We discovered that the relative inclination, in the sense rotation axis inclination - orbit axis inclination, is a function of the globular cluster's orbit inclination. Rotation and orbit axes are aligned for…
The Magellanic Stream and Leading Arm of HI that stretches from the Large and Small Magellanic Clouds (LMC and SMC) and over 200 degrees of the Southern sky is thought to be formed from multiple encounters between the LMC and SMC. In this scenario, most of the gas in the Stream and Leading Arm is stripped from the SMC, yet recent observations have shown a bifurcation of the Trailing Arm that reveals LMC origins for some of the gas. Absorption measurements in the Stream also reveal an order of magnitude more gas than in current tidal models. We present hydrodynamical simulations of the multiple…
We study the spontaneous generation and evolution of bending waves in $N$-body simulations of two isolated Milky Way-like galaxy models. The models differ by their disc-to-halo mass ratios, and hence by their susceptibility to the formation of a bar and spiral structure. Seeded from shot noise in the particle distribution, bending waves rapidly form in both models and persist for many billions of years. Waves at intermediate radii manifest as corrugated structures in vertical position and velocity that are tightly wound, morphologically leading, and dominated by the $m=1$ azimuthal Fourier com…
During the past 20 years, numerous stellar streams have been discovered in both the Milky Way and the Local Group. These streams have been tidally torn from orbiting systems, which suggests that most of them should roughly trace the orbit of their progenitors around the Galaxy. As a consequence, they play a fundamental role in understanding the formation and evolution of our Galaxy. This project is based on the possibility of applying a technique developed by Binney in 2008 to various tidal streams and overdensities in the Galaxy. The aim is to develop an efficient method to constrain the Gala…
The tidal features of the classical Milky Way satellites: Expected in MOND but inconsistent with cold dark matter models
Helmholtz Institut für Strahlen und Kernphysik, Universität Bonn（波恩大学赫尔姆霍兹射线与核物理研究所）；Institute of Cosmology & Gravitation, University of Portsmouth（朴茨茅斯大学宇宙学与引力研究所）；Astronomical Institute of Charles University, Faculty of Mathematics and Physics（查理大学数学物理学院天文研究所）
We report on the extent of the effects of the Milky Way's gravitational field in shaping the structural parameters and internal dynamics of its globular cluster population. We make use of a homogeneous, up-to-date data set with kinematics, structural properties, current and initial masses of 156 globular clusters. In general, cluster radii increase as the Milky Way potential weakens; with the core and Jacobi radii being those which increase at the slowest and fastest rate respectively. We interpret this result as the innermost regions of globular clusters being less sensitive to changes in the…
Using high-resolution collisionless N-body simulations we study the properties of tidal tails formed in the immediate vicinity of a two-component dwarf galaxy evolving in a static potential of the Milky Way (MW). The stellar component of the dwarf is initially in the form of a disk and the galaxy is placed on an eccentric orbit motivated by CDM-based cosmological simulations. We measure the orientation, density and velocity distribution of the stars in the tails. Due to the geometry of the orbit, in the vicinity of the dwarf, where the tails are densest and therefore most likely to be detectab…
We present N-body simulations of a Sagittarius-like dwarf spheroidal galaxy (Sgr) that follow its orbit about the Milky Way (MW) since its first crossing of the Galaxy's virial radius to the present day. As Sgr orbits around the MW, it excites vertical oscillations, corrugating and flaring the Galactic stellar disc. These responses can be understood by a two-phase picture in which the interaction is first dominated by torques from the wake excited by Sgr in the MW dark halo before transitioning to tides from Sgr's direct impact on the disc at late times. We show for the first time that a massi…
Recently, an ionized cloud of gas was discovered plunging toward the supermassive black hole, SgrA*, at the centre of the Milky Way. The cloud is being tidally disrupted along its path to closest approach at ~3100 Schwarzschild radii from the black hole. Here, we show that the observed properties of this cloud of gas can naturally be produced by a proto-planetary disk surrounding a low-mass star, which was scattered from the observed ring of young stars orbiting SgrA*. As the young star approaches the black hole, its disk experiences both photo-evaporation and tidal disruption, producing a clo…
The infall of the Sagittarius (Sgr) Dwarf Spheroidal Galaxy in the Milky Way halo is an unique opportunity to understand how the different components of a dwarf galaxy could be tidally removed. In this work, we reconstruct the Sgr core morphology and kinematics on the basis of a model that has already successfully reproduced the Sgr stream. Here we use a very high resolution model that almost resolves individual stars in the Sgr core. It reproduces most of the observed morphology and kinematic properties, without specific fine-tuning. We also show that the dark matter may have been almost enti…
A new derivation of systemic proper motions of Milky Way satellites is presented, and applied to 59 confirmed or candidate dwarf galaxy satellites using Gaia Data Release 2. This constitutes all known Milky Way dwarf galaxies (and likely candidates) as of May 2020 except the Magellanic Clouds, the Canis Major and Hydra 1 stellar overdensities, and the tidally disrupting Bootes III and Sagittarius dwarf galaxies. We derive systemic proper motions for the first time for Indus 1, DES J0225+0304, Cetus 2, Pictor 2 and Leo T, but note that the latter three rely on photometry that is of poorer quali…
Using N-body simulations we study the formation and evolution of tidally induced bars in disky galaxies in clusters. Our progenitor is a massive, late-type galaxy similar to the Milky Way, composed of an exponential disk and an NFW dark matter halo. We place the galaxy on four different orbits in a Virgo-like cluster and evolve it for 10 Gyr. As a reference case we also evolve the same model in isolation. Tidally induced bars form on all orbits soon after the first pericenter passage and survive until the end of the evolution. They appear earlier, are stronger, longer and have lower pattern sp…
Globular clusters can form inside their host galaxies at high redshift when gas densities were higher and gas-rich mergers were common. They can also form inside lower-mass galaxies that have since been accreted and tidally disrupted, leaving their globular cluster complement bound to higher-mass halos. We argue that the age-metallicity-specific orbital energy relation in a galaxy's globular cluster system can be used to identify its origin. Gas-rich mergers should produce tightly bound systems in which metal-rich clusters are younger than metal-poor clusters. Globular clusters formed in massi…
Tidally-disrupted globular clusters are transformed into thin, dynamically-cold streams of stars that are extremely valuable tracers of the large- and small-scale distribution of mass in the Galaxy. Using data from the Gaia second data release combined with Pan-STARRS photometry, we present a sample of highly-probable members of the longest cold stream in the Milky Way, GD-1. The resulting map of GD-1: (1) extends the apparent length of the stream by $20^\circ$, (2) reveals plausible locations for the progenitor, (3) detects high-contrast gaps along the stream, and (4) indicates the existence …
We extend our analysis of the observed disturbances on the outskirts of the HI disk of the Milky Way. We employ the additional constraints of the phase of the modes of the observed HI image and asymmetry in the radial velocity field to derive the azimuth of the perturber inferred to be responsible for the disturbances in the HI disk. We carry out a modal analysis of the phase of the disturbances in the HI image and in SPH simulations of a Milky Way-like galaxy tidally interacting with dark perturbers, the relative offset of which we utilize to derive the perturber azimuth. To make a direct con…
We show that a small, but \textit{measurable} shift in the eclipse mid-point time of eclipsing binary (EBs) stars of $\sim$ 0.1 seconds over a decade baseline can be used to directly measure the Galactic acceleration of stars in the Milky Way at $\sim$ kpc distances from the Sun. We consider contributions to the period drift rate from dynamical mechanisms other than the Galaxy's gravitational field, and show that the Galactic acceleration can be reliably measured using a sample of $\textit{Kepler}$ EBs with orbital and stellar parameters from the literature. Given the uncertainties on the form…
The Milky Way Tomography with Subaru Hyper Suprime-Cam. I. Halo substructures
Astronomical Institute, Tohoku University（东北大学天文学研究所）；Department of Advanced Sciences, Faculty of Science and Engineering, Hosei University（法政大学理工学部先进科学科）；National Institute of Technology, Sendai College（仙台工业高等专门学校）；National Astronomical Observatory of Japan（日本国立天文台）；The Graduate University for Advanced Studies（综合研究大学院大学）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体物理学系）；Kavli Institute for the Physics and Mathematics of the Universe（Kavli宇宙物理数学研究所）
Twenty years ago, the mismatch between the observed number of Milky Way satellite galaxies and the predicted number of cold dark matter (CDM) subhalos was dubbed the ``missing satellites problem". Although mostly framed since in terms of satellite counts in luminosity space, the missing satellites problem was originally posed in velocity space. Importantly, the stellar velocity dispersion function encodes information about the density profile of satellites as well as their abundance. In this work, we completeness correct the MW satellite stellar velocity dispersion function down to its ultrafa…
(ABRIDGED) The evolution and structure of the Magellanic Clouds is presently under debate. The classical scenario where both the Large and Small Magellanic Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an alternative where the LMC and SMC are in their first close passage to our Galaxy. Detailed studies of stellar populations in the galaxies should constrain the proposed scenarios. In particular, the west halo of the SMC was recently characterized with radial trends in age and metallicity which indicates tidal disruption. We increase the sample of star clusters in the west…
We combine Gaia EDR3 astrometry with accurate photometry and utilize a probabilistic mixture model to measure the systemic proper motion of 52 dwarf spheroidal (dSph) satellite galaxies of the Milky Way (MW). For the 46 dSphs with literature line-of-sight velocities we compute orbits in both a MW and a combined MW + Large Magellanic Cloud (LMC) potential and identify Car II, Car III, Hor I, Hyi I, Phx II, and Ret II as likely LMC satellites. 40% of our dSph sample has a >25% change in pericenter and/or apocenter with the MW + LMC potential. For these orbits, we Monte Carlo sample over the obse…
Which Milky Way Streams Make the Best Dark Matter Detectors?
MIT Kavli Institute for Astrophysics and Space Research（麻省理工学院卡弗里天体物理学与空间研究研究所）；Kavli Institute for Particle Astrophysics and Cosmology, Stanford University（斯坦福大学卡弗里粒子天体物理学与宇宙学研究所）；Department of Physics, Stanford University（斯坦福大学物理系）；SLAC National Accelerator Laboratory（斯坦福直线加速器国家实验室）；The Observatories of the Carnegie Institution for Science（卡内基科学机构天文台）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体物理科学系）
Recent studies have revealed an oscillating asymmetry in the vertical structure of the Milky Way's disc. Here we analyze 16 high-resolution, fully cosmological simulations of the evolution of individual Milky Way-sized galaxies, carried out with the MHD code AREPO. At redshift zero, about $70\%$ of our galactic discs show strong vertical patterns, with amplitudes that can exceed 2 kpc. Half of these are typical `integral sign' warps. The rest are oscillations similar to those observed in the Milky Way. Such structures are thus expected to be common. The associated mean vertical motions can be …
The Fornax dwarf spheroidal galaxy is the most massive satellites of the Milky Way, claimed to be embedded in a huge dark matter halo, and the only among the Milky Way satellites hosting five globular clusters. Interestingly, their estimated masses, ages and positions seem hardly compatible with the presence of a significant dark matter component, as expected in the $Λ$ CDM scheme. Indeed, if Fornax would have a CDM halo with a standard density profile, all its globular clusters should have sunk to the galactic centre many Gyr ago due to dynamical friction. Due to this, some authors proposed t…
NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs. It is wreathed in an extraordinary neutral hydrogen (H I) complex, which includes rings, shells and a counter-rotating core, spanning 90 kiloparsecs. NGC 4449 is relatively isolated, although an interaction with its nearest known companion-the galaxy DDO 125, some 40 kpc to the south-has been proposed as being responsible for the complexity of its HI structure. Here we report the presence of a dwarf ga…
