ABSTRACT
We study the orbits of dwarf galaxies in the combined presence of the Milky Way and Large Magellanic Cloud (LMC) and find six dwarfs that were likely accreted with the LMC (Car 2, Car 3, Hor ...1, Hyi 1, Phe 2, and Ret 2), in addition to the Small Magellanic Cloud (SMC), representing strong evidence of dwarf galaxy group infall. This procedure depends on the gravitational pull of the LMC, allowing us to place a lower bound on the Cloud’s mass of $M_{\rm LMC} \gt 1.24\times 10^{11} \, \mathrm{M}_\odot$ if we assume that these are LMC satellites. This mass estimate is validated by applying the technique to a cosmological zoom-in simulation of a Milky Way-like galaxy with an LMC analogue where we find that while this lower bound may be overestimated, it will improve in the future with smaller observational errors. We apply this technique to dwarf galaxies lacking radial velocities and find that Eri 3 has a broad range of radial velocities for which it has a significant chance (>0.4) of having been bound to the Cloud. We study the non-Magellanic classical satellites and find that Fornax has an appreciable probability of being an LMC satellite if the LMC is sufficiently massive ($\text{$\sim$} 2.5\times 10^{11} \, \mathrm{M}_\odot$). In addition, we explore how the orbits of Milky Way satellites change in the presence of the LMC and find a significant change for several objects. Finally, we find that the dwarf galaxies likely to be LMC satellites are slightly smaller than Milky Way satellites at a fixed luminosity, possibly due to the different tidal environments they have experienced.
The last breath of the Sagittarius dSph Vasiliev, Eugene; Belokurov, Vasily
Monthly notices of the Royal Astronomical Society,
10/2020, Letnik:
497, Številka:
4
Journal Article
Recenzirano
ABSTRACT
We use the astrometric and photometric data from Gaia Data Release 2 and line-of-sight velocities from various other surveys to study the 3D structure and kinematics of the Sagittarius dwarf ...galaxy. The combination of photometric and astrometric data makes it possible to obtain a very clean separation of Sgr member stars from the Milky Way foreground; our final catalogue contains 2.6 × 105 candidate members with magnitudes G < 18, more than half of them being red clump stars. We construct and analyse maps of the mean proper motion and its dispersion over the region ∼30 × 12 deg, which show a number of interesting features. The intrinsic 3D density distribution (orientation, thickness) is strongly constrained by kinematics; we find that the remnant is a prolate structure with the major axis pointing at ∼45° from the orbital velocity and extending up to ∼5 kpc, where it transitions into the stream. We perform a large suite of N-body simulations of a disrupting Sgr galaxy as it orbits the Milky Way over the past 2.5 Gyr, which are tailored to reproduce the observed properties of the remnant (not the stream). The richness of available constraints means that only a narrow range of parameters produce a final state consistent with observations. The total mass of the remnant is $\sim \!4\times 10^8\, \mathrm{M}_\odot$, of which roughly a quarter resides in stars. The galaxy is significantly out of equilibrium, and even its central density is below the limit required to withstand tidal forces. We conclude that the Sgr galaxy will likely be disrupted over the next Gyr.
ABSTRACT
We assemble a catalogue of candidate Sagittarius stream members with 5D and 6D phase-space information, using astrometric data from Gaia DR2, distances estimated from RR Lyrae stars, and ...line-of-sight velocities from various spectroscopic surveys. We find a clear misalignment between the stream track and the direction of the reflex-corrected proper motions in the leading arm of the stream, which we interpret as a signature of a time-dependent perturbation of the gravitational potential. A likely cause of this perturbation is the recent passage of the most massive Milky Way satellite – the Large Magellanic Cloud (LMC). We develop novel methods for simulating the Sagittarius stream in the presence of the LMC, using specially tailored N-body simulations and a flexible parametrization of the Milky Way halo density profile. We find that while models without the LMC can fit most stream features rather well, they fail to reproduce the misalignment and overestimate the distance to the leading arm apocentre. On the other hand, models with an LMC mass in the range $(1.3\pm 0.3)\times 10^{11}\, \mathrm{M}_\odot$ rectify these deficiencies. We demonstrate that the stream can not be modelled adequately in a static Milky Way. Instead, our Galaxy is required to lurch toward the massive in-falling Cloud, giving the Sgr stream its peculiar shape and kinematics. By exploring the parameter space of Milky Way potentials, we determine the enclosed mass within 100 kpc to be $(5.6\pm 0.4)\times 10^{11}\, \mathrm{M}_\odot$, and the virial mass to be $(9.0\pm 1.3)\times 10^{11}\, \mathrm{M}_\odot$, and find tentative evidence for a radially-varying shape and orientation of the Galactic halo.
Clouds in arms Belokurov, Vasily A; Erkal, Denis
Monthly notices of the Royal Astronomical Society. Letters,
01/2019, Letnik:
482, Številka:
1
Journal Article
Recenzirano
ABSTRACT We use astrometry and broad-band photometry from Data Release 2 of the ESA’s Gaia mission to map out low surface-brightness features in the stellar density distribution around the Large and ...Small Magellanic Clouds. The LMC appears to have grown two thin and long stellar streams in its Northern and Southern regions, highly reminiscent of spiral arms. We use computer simulations of the Magellanic Clouds’ in-fall to demonstrate that these arms were likely pulled out of the LMC’s disc due to the combined influence of the SMC’s most recent fly-by and the tidal field of the Milky Way.
There is hope to discover dark matter subhaloes free of stars (predicted by the current theory of structure formation) by observing gaps they produce in tidal streams. In fact, this is the most ...promising technique for dark substructure detection and characterization as such gaps grow with time, magnifying small perturbations into clear signatures observable by ongoing and planned Galaxy surveys. To facilitate such future inference, we develop a comprehensive framework for studies of the growth of the stream density perturbations. Starting with simple assumptions and restricting to streams on circular orbits, we derive analytic formulae that describe the evolution of all gap properties (size, density contrast, etc.) at all times. We uncover complex, previously unnoticed behaviour, with the stream initially forming a density enhancement near the subhalo impact point. Shortly after, a gap forms due to the relative change in period induced by the subhalo's passage. There is an intermediate regime where the gap grows linearly in time. At late times, the particles in the stream overtake each other, forming caustics, and the gap grows like
$\sqrt{t}$
. In addition to the secular growth, we find that the gap oscillates as it grows due to epicyclic motion. We compare this analytic model to N-body simulations and find an impressive level of agreement. Importantly, when analysing the observation of a single gap we find a large degeneracy between the subhalo mass, the impact geometry and kinematics, the host potential, and the time since flyby.
ABSTRACT
Recent measurements suggest that the Large Magellanic Cloud (LMC) may weigh as much as 25 per cent of the Milky Way (MW). In this work, we explore how such a large satellite affects mass ...estimates of the MW based on equilibrium modelling of the stellar halo or other tracers. In particular, we show that if the LMC is ignored, the MW mass within 200 kpc is overestimated by as much as 50 per cent. This bias is due to the bulk motion in the outskirts of the Galaxy’s halo and can be, at least in part, accounted for with a simple modification to the equilibrium modelling. Finally, we show that the LMC has a substantial effect on the orbit Leo I which acts to increase its present-day speed relative to the MW. We estimate that accounting for a $1.5\times 10^{11} \, \mathrm{M}_\odot$ LMC would lower the inferred MW mass to $\sim 10^{12} \, \mathrm{M}_\odot$.
Abstract
We consider the orbital evolution of satellites in galaxy mergers, focusing on the evolution of eccentricity. Using a large suite of
N
-body simulations, we study the phenomenon of satellite ...orbital radialization—a profound increase in the eccentricity of its orbit as it decays under dynamical friction. While radialization is detected in a variety of different setups, it is most efficient in cases of high satellite mass, not very steep host density profiles, and high initial eccentricity. To understand the origin of this phenomenon, we run additional simulations with various physical factors selectively turned off: satellite mass loss, reflex motion and distortion of the host, etc. We find that all these factors are important for radialization because it does not occur for point-mass satellites or when the host potential is replaced with an unperturbed initial profile. The analysis of forces and torques acting on both galaxies confirms the major role of self-gravity of both host and satellite in the reduction of orbital angular momentum. The classical Chandrasekhar dynamical friction formula, which accounts only for the forces between the host and the satellite, but not for internal distortions of both galaxies, does not match the evolution of eccentricity observed in
N
-body simulations.
The total stellar halo mass of the Milky Way Deason, Alis J; Belokurov, Vasily; Sanders, Jason L
Monthly notices of the Royal Astronomical Society,
12/2019, Letnik:
490, Številka:
3
Journal Article
Recenzirano
ABSTRACT
We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour, and location on the sky, we decompose ...RGB stars belonging to the disc and halo by fitting two-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of $L_{\rm halo} = 7.9 \pm 2.0 \times 10^8\, \mathrm{L}_\odot$ excluding Sgr, and $L_{\rm halo} = 9.4 \pm 2.4 \times 10^8\, \mathrm{L}_\odot$ including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M⋆/L = 1.5), we estimate a stellar halo mass of $M^\star _{\rm halo} = 1.4 \pm 0.4\times 10^9 \, \mathrm{M}_\odot$. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a ${\sim}10^9\, \mathrm{M}_\odot$ mass and an average metallicity of 〈Fe/H〉 ∼ −1.5 for the Galactic halo points to an ancient (∼10 Gyr) merger event.
Ample observational capabilities exist today to detect the small density perturbations that low-mass dark matter subhaloes impart on stellar streams from disrupting Galactic satellites. In ...anticipation of these observations, we investigate the expected number and size of gaps by combining an analytic prescription for gap evolution on circular orbits with the flux of subhaloes near the stream. We explore the distribution of gap sizes and depths for a typical cold stream around the Milky Way and find that for a given stream age and gap depth, each subhalo mass produces a characteristic gap size. For a stream with an age of a few Gyr, orbiting at a distance of 10–20 kpc from the Galactic centre, even modest subhaloes with a mass of 106–107 M⊙ produce gaps with sizes that are of the order of several degrees. We consider the number and distribution of gap sizes created by subhaloes with masses 105–109 M⊙, accounting for the expected depletion of subhaloes by the Milky Way disc, and present predictions for six cold streams around the Milky Way. For Pal 5, we forecast 0.7 gaps with a density depletion of at least 25 per cent and a typical gap size of 8°. Thus, there appears to be no tension between the recent non-detection of density depletions in the Pal 5 tidal tails and ΛCDM expectations. These predictions can be used to guide the scale of future gap searches.
We measure the orbital properties of halo stars using seven-dimensional information provided by Gaia and the Sloan Digital Sky Survey. A metal-rich population of stars, present in both local main ...sequence stars and more distant blue horizontal branch stars, have very radial orbits (eccentricity ∼0.9) and apocenters that coincide with the stellar halo "break radius" at galactocentric distance r ∼ 20 kpc. Previous work has shown that the stellar halo density falls off much more rapidly beyond this break radius. We argue that the correspondence between the apocenters of high metallicity, high-eccentricity stars, and the broken density profile is caused by the build-up of stars at the apocenter of a common dwarf progenitor. Although the radially biased stars are likely present down to metallicities of Fe/H ∼ −2, the increasing dominance at higher metallicities suggests a massive dwarf progenitor, which is at least as massive as the Fornax and Sagittarius dwarf galaxies, and is likely the dominant progenitor of the inner stellar halo.
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