The time evolution of gaps in tidal streams

We model the time evolution of gaps in tidal streams caused by the impact of a dark matter subhalo, while both orbit a spherical gravitational potential. To this end, we make use of the simple behaviour of orbits in action-angle space. A gap effectively results from the divergence of two nearby orbits whose initial phase-space separation is, for very cold thin streams, largely given by the impulse induced by the subhalo. We find that in a spherical potential the size of a gap increases linearly with time for sufficiently long timescales. We have derived an analytic expression that shows how the growth rate depends on the mass of the perturbing subhalo, its scale and its relative velocity with respect to the stream. We have verified these scalings using N-body simulations and find excellent agreement. For example, a subhalo of mass 10^8 Msun directly impacting a very cold thin stream on an inclined orbit can induce a gap that may reach a size of several tens of kpc after a few Gyr. The gap size fluctuates importantly with phase on the orbit, and it is largest close to pericentre. This indicates that it may not be fully straightforward to invert the spectrum of gaps present in a stream to recover the mass spectrum of the subhalos.Comment: 4 pages, 4 figures, ApJ Letters in pres

One large blob and many streams frosting the nearby stellar halo in Gaia DR2

We explore the phase-space structure of nearby halo stars identified kinematically from Gaia DR2 data. We focus on their distribution in velocity and in "integrals of motion" space as well as on their photometric properties. Our sample of stars selected to be moving at a relative velocity of at least 210 km/s with respect to the Local Standard of Rest, contains an important contribution from the low rotational velocity tail of the disk(s). The $V_R$-distribution of these stars depicts a small asymmetry similar to that seen for the faster rotating thin disk stars near the Sun. We also identify a prominent, slightly retrograde "blob", which traces the metal-poor halo main sequence reported by Gaia Collaboration et al. (2018d). We also find many small clumps especially noticeable in the tails of the velocity distribution of the stars in our sample. Their HR diagrams disclose narrow sequences characteristic of simple stellar populations. This stream-frosting confirms predictions from cosmological simulations, namely that substructure is most apparent amongst the fastest moving stars, typically reflecting more recent accretion events.Comment: 5 pages, 5 figures, accepted for publication in ApJ

Characterization and history of the Helmi streams with Gaia DR2

The halo of the Milky Way has long been hypothesized to harbour significant amounts of merger debris. This view has been supported over more than a decade by wide-field photometric surveys which have revealed the outer halo to be lumpy. The recent release of Gaia DR2 is allowing us to establish that mergers also have been important and possibly built up the majority of the inner halo. In this work we focus on the Helmi streams, a group of streams crossing the Solar vicinity and known for almost two decades. We characterize their properties and relevance for the build-up of the Milky Way's halo. We identify new members of the Helmi streams in an unprecedented dataset with full phase-space information combining Gaia DR2, and the APOGEE DR2, RAVE DR5 and LAMOST DR4 spectroscopic surveys. Based on the orbital properties of the stars, we find new stream members up to a distance of 5 kpc from the Sun, which we characterize using photometry and metallicity information. We also perform N-body experiments to constrain the time of accretion and properties of the progenitor of the streams. We find nearly 600 new members of the Helmi streams. Their HR diagram reveals a broad age range, from approximately 11 to 13 Gyr, while their metallicity distribution goes from $\sim$ 2.3 to $\sim$1.0, and peaks at [Fe/H] $\sim$1.5. These findings confirm that the streams originate in a dwarf galaxy. Furthermore, we find 7 globular clusters to be likely associated, and which follow a well-defined age-metallicity sequence whose properties suggest a relatively massive progenitor object. Our N-body simulations favour a system with a stellar mass of $\sim 10^8\,\mathrm{M}_\odot$ accreted $5 - 8$ Gyr ago. The debris from the Helmi streams is an important donor to the MilkyWay halo, contributing approximately 15\% of its mass in field stars and 10\% of its globular clusters.Comment: 14 pages, 17 figures, submitte

Determination of the escape velocity of the Milky Way using a proper motion selected halo sample

The {\it Gaia} mission has provided the largest catalogue ever of sources with tangential velocity information. However, using this catalogue for dynamical studies is difficult because most of the stars lack line-of-sight velocity measurements. Recently, we presented a selection of $\sim 10^7$ halo stars with accurate distances that have been selected based on their photometry and proper motions. Using this sample, we model the tail of the velocity distribution with a power-law distribution, a commonly used approach first established by \cite{Leonard1990THESPEED}. For the first time ever we use tangential velocities measured accurately for an unprecedented number of halo stars to estimate the escape velocity. In the solar neighbourhood, we obtain a very precise estimate of the escape velocity which is $497^{+8}_{-8}~{\rm km/s}$. This estimate is most likely biased low, our best guess is by 10\%. As a result, the true escape velocity most likely is closer to $550~{\rm km/s}$. 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 NFW) halo of a realistic Milky Way potential while keeping the circular velocity at the solar radius fixed at $v_c(R_\odot) = 232.8~{\rm km/s}$. The resulting halo parameters are $M_{200}^{+10\%} = 1.11^{+0.08}_{-0.07} \cdot10^{12} ~{\rm M}_\odot$ and concentration parameter $c^{+10\%} = 11.8^{+0.3}_{-0.3}$, where we use the explicit notation to indicate that these have been 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 which is likely an effect of a change in the shape of the velocity distribution and could be related to the presence of velocity clumps.Comment: 14 pages, 13 figures, accepted for publication in A&

Origin of the system of globular clusters in the Milky Way

Context. The assembly history experienced by the Milky Way is currently being unveiled thanks to the data provided by the Gaia mission. It is likely that the globular cluster system of our Galaxy has followed a similarly intricate formation path. Aims. To constrain this formation path, we explore the link between the globular clusters and the known merging events that the Milky Way has experienced. Methods. To this end, we combined the kinematic information provided by Gaia for almost all Galactic clusters, with the largest sample of cluster ages available after carefully correcting for systematic errors. To identify clusters with a common origin we analysed their dynamical properties, particularly in the space of integrals of motion. Results. We find that about 40% of the clusters likely formed in situ. A similarly large fraction, 35%, appear to be possibly associated to known merger events, in particular to Gaia-Enceladus (19%), the Sagittarius dwarf galaxy (5%), the progenitor of the Helmi streams (6%), and to the Sequoia galaxy (5%), although some uncertainty remains due to the degree of overlap in their dynamical characteristics. Of the remaining clusters, 16% are tentatively associated to a group with high binding energy, while the rest are all on loosely bound orbits and likely have a more heterogeneous origin. The resulting age–metallicity relations are remarkably tight and differ in their detailed properties depending on the progenitor, providing further confidence on the associations made. Conclusions. We provide a table listing the likely associations. Improved kinematic data by future Gaia data releases and especially a larger, systematic error-free sample of cluster ages would help to further solidify our conclusions

The reduced proper motion selected halo: Methods and description of the catalogue

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. The identification of halo stars in Gaia DR2 with reliable distance estimates requires special methods because such stars are typically farther away and scarce. We apply the reduced proper motion (RPM) 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 stars, we calculate photometric distances. Our selection results in a set of $\sim10^7$ tentative main sequence halo stars with typical distance uncertainties of $7\%$ and with median velocity errors of 20 km/s. The median distance of our sample is $\sim 4.4$ kpc, with the faintest stars located at $\sim 16$ kpc. The spatial distribution of the stars in our sample is centrally concentrated. 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 to zero the line-of-sight velocities of our sample stars. 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, of similar amplitude as found for the 6D Gaia halo sample. This clustering indicates the presence of nearby streams that are predominantly phase-mixed.Comment: 16 pages, 17 figures, accepted; final versio

Time evolution of the properties of gaps in stellar streams in axisymmetric potentials

We present here a model that allows us to predict the properties of gaps in stellar streams, and how these depend on the parameters of the encounters (satellite mass, size and relative velocity). Since the gaps we consider are created by dark matter satellites we hope to use our understanding to constrain the properties of dark matter.<br/

Time evolution of gaps in stellar streams in axisymmetric St\"ackel potentials

When a subhalo interacts with a cold stellar stream it perturbs its otherwise nearly smooth distribution of stars, and this leads to the creation of a gap. The properties of such gaps depend on the parameters of the interaction. Their characterisation could thus lead to the determination of the mass spectrum of the perturbers and possibly reveal the existence of dark subhalos orbiting the Milky Way. 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. To this end, we extend the model of Helmi & Koppelman (2016) for spherical potentials, and predict the properties of gaps in streams evolving in axisymmetric St\"ackel potentials. We make use of action-angles and their simple behaviour to calculate the divergence of initially nearby orbits slightly perturbed by the interaction with a subhalo. Our model, corroborated by N-body experiments, predicts that the size of a gap grows linearly with time. We obtain analytical expressions for the dependencies of the growth rate on the orbit of the stream, the properties of the subhalo (mass, scale-radius), and the geometry of the encounter (relative velocity, 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 results in the density-contrast between a pristine and a perturbed stream on the same orbit asymptotically reaching a constant value dependent only on the encounter parameters. 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 parallel, are degenerate to the encounter parameters.Comment: 15 pages, submitted to A&