Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to ...each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE, and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with Fe/H < −1 have more retrograde motions than those with Fe/H > −1. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot yet be fully settled. In the low-energy part of the halo, we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low-inclination, mildly eccentric retrograde orbits, moving at vϕ ≈ −150 km s−1, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data, piecing together all the fragments found in the retrograde halo remains challenging. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.
The assembly of our Galaxy can be reconstructed using the motions and chemistry of individual stars
. Chemo-dynamical studies of the stellar halo near the Sun have indicated the presence of multiple ...components
, such as streams
and clumps
, as well as correlations between the stars' chemical abundances and orbital parameters
. Recently, analyses of two large stellar surveys
revealed the presence of a well populated elemental abundance sequence
, two distinct sequences in the colour-magnitude diagram
and a prominent, slightly retrograde kinematic structure
in the halo near the Sun, which may trace an important accretion event experienced by the Galaxy
. However, the link between these observations and their implications for Galactic history is not well understood. Here we report an analysis of the kinematics, chemistry, age and spatial distribution of stars that are mainly linked to two major Galactic components: the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object that at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia-Enceladus. The stars that originate in Gaia-Enceladus cover nearly the full sky, and their motions reveal the presence of streams and slightly retrograde and elongated trajectories. With an estimated mass ratio of four to one, the merger of the Milky Way with Gaia-Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk, thus contributing to the formation of this component approximately ten billion years ago. These findings are in line with the results of galaxy formation simulations, which predict that the inner stellar halo should be dominated by debris from only a few massive progenitors
.
Aims.
About 10 billion years ago the Milky Way merged with a massive satellite,
Gaia
-Enceladus. To gain insight into the properties of its debris we analyse in detail a suite of simulations that ...includes an experiment that produces a good match to the kinematics of nearby halo stars inferred from
Gaia
data.
Methods.
We compare the kinematic distributions of stellar particles in the simulations and study the distribution of debris in orbital angular momentum, eccentricity, and energy, and its relation to the mass loss history of the simulated satellite.
Results.
We confirm that
Gaia
-Enceladus probably fell in on a retrograde, 30° inclination orbit. We find that while 75% of the debris in our preferred simulation has high eccentricity (> 0.8), roughly 9% has eccentricity lower than 0.6. Star particles lost early have large retrograde motions, and a subset of these have low eccentricity. Such stars would be expected to have lower metallicities as they stem from the outskirts of the satellite, and hence naively they could be confused with debris associated with a separate system. These considerations seem to apply to some of the stars from the postulated Sequoia galaxy.
Conclusions.
When a massive disc galaxy undergoes a merger event, it leaves behind debris with a complex phase-space structure, a wide range of orbital properties, and a range of chemical abundances. Observationally, this results in substructures with very different properties, which can be misinterpreted as implying independent progeny. Detailed chemical abundances of large samples of stars and tailored hydrodynamical simulations are critical to resolving such conundrums.
Context.
The
Gaia
mission has provided the largest catalogue ever of sources with tangential velocity information. However, it is difficult to use this catalogue for dynamical studies because most of ...the stars lack line-of-sight velocity measurements. Recently, we presented a selection of ∼10
7
halo stars with accurate distances that were selected based on their photometry and proper motions.
Aims.
Using this sample, we model the tail of the velocity distribution in the stellar halo locally and as a function of distance. Our goal is to measure the escape velocity, and based on this, to constrain the mass of our Galaxy.
Methods.
We fitted the tail of the velocity distribution with a power-law distribution, a commonly used approach that has long been established. For the first time, we used tangential velocities that were accurately measured for an unprecedented number of halo stars to estimate the escape velocity.
Results.
In the solar neighbourhood, we obtain a very precise estimate of the escape velocity, which is 497
−8
+8
km s
−1
. This estimate is most likely biased low, our best guess is by 10%. As a result, the true escape velocity is most likely closer to 550 km s
−1
. The escape velocity directly constrains the total mass of the Milky Way. To find the best-fitting halo mass and concentration parameter, we adjusted the dark (spherical Navarro-Frenk-White) halo of a realistic Milky Way potential while keeping the circular velocity at the solar radius fixed at
v
c
(
R
⊙
) = 232.8 km s
−1
. The resulting halo parameters are
M
200
+10%
= 1.11
−0.07
+0.08
· 10
12
M
⊙
, and the concentration parameter is
c
+10%
= 11.8
−0.3
+0.3
, where we use the explicit notation to indicate that they are corrected for the 10% bias. The slope of the escape velocity with galactocentric distance is as expected in the inner Galaxy based on Milky Way models. Curiously, we find a disagreement beyond the solar radius where the estimated escape velocity is higher than at the solar radius. This result is likely an effect of a change in the shape of the velocity distribution and could be related to the presence of velocity clumps. A tentative analysis of the escape velocity as a function of (
R
,
z
) shows that the slope is shallower than expected for a spherical halo when standard values are used for the characteristic parameters describing the galactic disc.
Context.
The
Gaia
mission has provided the largest ever astrometric chart of the Milky Way. Using it to map the Galactic halo is helpful for disentangling its merger history.
Aims.
The identification ...of halo stars in
Gaia
DR2 with reliable distance estimates requires special methods because such stars are typically farther away and scarce.
Methods.
We applied the reduced proper motion method to identify halo main sequence stars on the basis of
Gaia
photometry and proper motions. Using the colour-absolute-magnitude relation for this type of star, we calculated photometric distances. Our selection results in a set of ∼10
7
tentative main sequence halo stars with typical distance uncertainties of 7% and with median velocity errors of 20 km s
−1
. The median distance of our sample is ∼4.4 kpc, with the faintest stars located at ∼16 kpc.
Results.
The spatial distribution of the stars in our sample is centrally concentrated. A visual inspection of the mean velocities of stars on the sky reveals large-scale patterns as well as clear imprints of the GD-1 stream and tentative hints of the Jhelum and Leiptr streams. Incompleteness and selection effects limit our ability to interpret the patterns reliably as well as to identify new substructures. We define a pseudo-velocity space by setting the line-of-sight velocities of our sample stars to zero. In this space, we recover several known structures such as the footprint of
Gaia
-Enceladus (i.e., the
Gaia
-Sausage) as well as the Helmi Streams and some other retrograde substructures (Sequoia, Thamnos). We show that the two-point velocity correlation function reveals significant clustering on scales smaller than 100 km s
−1
of a similar amplitude as found for the 6D
Gaia
halo sample. This clumping of stars in velocity space might hint at the presence of nearby streams that are predominantly phase-mixed.
Conclusions.
A spectroscopic follow-up of our halo main sequence sample is bound to yield unprecedented views of Galactic history and dynamics. In future
Gaia
data releases, the level of systematics will be reduced and the astrometry will be more precise, which will allow for the identification of more substructures at larger distances.
Context.
When a subhalo interacts with a cold stellar stream, the otherwise nearly smooth distribution of stars is disturbed, and this creates a gap. The properties of these gaps depend on the ...interaction parameters. Their characterisation could thus lead to a determination of the mass spectrum of the perturbers and might reveal the existence of dark subhalos orbiting the Milky Way.
Aims.
Our goal is to construct a fully analytical model of the formation and evolution of gaps embedded in streams orbiting in a realistic Milky Way potential.
Methods.
To this end, we extended our previous model for spherical potentials and predict the properties of gaps in streams evolving in axisymmetric Stäckel potentials. We used action-angles and their simple behaviour to calculate the divergence of initially nearby orbits that are slightly perturbed by the interaction with a subhalo.
Results.
Our model, corroborated by
N
-body experiments, predicts that the size of a gap grows linearly with time. We obtain analytical expressions for the dependences of the growth rate on the orbit of the stream, the properties of the subhalo (mass and scale radius), and the geometry of the encounter (relative velocity and impact parameter). We find that the density at the centre of the gap decreases with time as a power law in the same way as the density of a stream. This causes the density contrast between a pristine and a perturbed stream on the same orbit to asymptotically reach a constant value that only depends on the encounter parameters.
Conclusions.
We find that at a fixed age, smallish gaps are sensitive mostly to the mass of the subhalo, while gaps formed by subhalo flybys with a low relative velocity, or when the stream and subhalo move in parallel, are degenerate to the encounter parameters.
ABSTRACT
The Milky Way halo is one of the few galactic haloes that provides a unique insight into galaxy formation by resolved stellar populations. Here, we present a catalogue of ∼47 million halo ...stars selected independent of parallax and line-of-sight velocities, using a combination of Gaia DR3 proper motion and photometry by means of their reduced proper motion. We select high tangential velocity (halo) main sequence stars and fit distances to them using their simple colour-absolute-magnitude relation. This sample reaches out to ∼21 kpc with a median distance of 6.6 kpc thereby probing much further out than would be possible using reliable Gaia parallaxes. The typical uncertainty in their distances is $0.57_{-0.26}^{+0.56}$ kpc. Using the colour range 0.45 < (G0 − GRP, 0) < 0.715, where the main sequence is narrower, gives an even better accuracy down to $0.39_{-0.12}^{+0.18}$ kpc in distance. The median velocity uncertainty for stars within this colour range is 15.5 km s−1. The distribution of these sources in the sky, together with their tangential component velocities, are very well-suited to study retrograde substructures. We explore the selection of two complex retrograde streams: GD-1 and Jhelum. For these streams, we resolve the gaps, wiggles and density breaks reported in the literature more clearly. We also illustrate the effect of the kinematic selection bias towards high proper motion stars and incompleteness at larger distances due to Gaia’s scanning law. These examples showcase how the full RPM catalogue made available here can help us paint a more detailed picture of the build-up of the Milky Way halo.
ABSTRACT
Stream stars removed by tides from their progenitor satellite galaxy or globular cluster act as a group of test particles on neighbouring orbits, probing the gravitational field of the Milky ...Way. While constraints from individual streams have been shown to be susceptible to biases, combining several streams from orbits with various distances reduces these biases. We fit a common gravitational potential to multiple stellar streams simultaneously by maximizing the clustering of the stream stars in action space. We apply this technique to members of the GD-1, Palomar 5 (Pal 5), Orphan, and Helmi streams, exploiting both the individual and combined data sets. We describe the Galactic potential with a Stäckel model, and vary up to five parameters simultaneously. We find that we can only constrain the enclosed mass, and that the strongest constraints come from the GD-1, Pal 5, and Orphan streams whose combined data set yields $M(\lt 20\, \mathrm{kpc}) = 2.96^{+0.25}_{-0.26} \times 10^{11} \, \mathrm{ M}_{\odot}$. When including the Helmi stream in the data set, the mass uncertainty increases to $M(\lt 20\, \mathrm{kpc}) = 3.12^{+3.21}_{-0.46} \times 10^{11} \, \mathrm{M}_{\odot}$.
The early Universe presented a star formation environment that was almost devoid of heavy elements. The lowest metallicity stars thus provide a unique window into the earliest Galactic stages, but ...are exceedingly rare and difficult to find. Here, we present the discovery of an ultra-metal-poor star, Pristine_221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine survey. Follow-up medium- and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine_221.8781+9.7844 (Fe/H = −4.66 ± 0.13 in 1D LTE) with an enhancement of 0.3–0.4 dex in α-elements relative to Fe, and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6, well below typical A(C) values for such ultra-metal-poor stars. This makes Pristine_221.8781+9.7844 one of the most metal-poor stars; in fact, it is very similar to the most metal-poor star known (SDSS J102915+172927). The existence of a class of ultra-metal-poor stars with low(er) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived, low-mass stars were formed.
Aims.
We study the distribution of nearby thick-disc and halo stars in subspaces defined by their characteristic orbital parameters. Our aim is to establish the origin of the structure reported in ...particular in the
R
max
−
z
max
space.
Methods.
To this end, we computed the orbital parameters and frequencies of stars for a generic and for a Stäckel Milky Way potential.
Results.
We find that for both the thick-disc and halo populations, very similar prominent substructures are apparent for the generic Galactic potential, while no substructure is seen for the Stäckel model. This indicates that the origin of these features is not merger-related, but due to the non-integrability of the generic potential. This conclusion is strengthened by our frequency analysis of the orbits of stars, which reveals the presence of prominent resonances, with ∼30% of the halo stars associated with resonance families. In fact, the stars in resonances define the substructures seen in the spaces of characteristic orbital parameters. Intriguingly, we find that some stars in our sample and in debris streams are on the same resonance as the Sagittarius dwarf.
Conclusions.
Our study constitutes a step towards disentangling the imprint of merger debris from substructures driven by internal dynamics. Given their prominence, these resonant-driven overdensities could potentially be useful in constraining the exact form of the Galactic potential.
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