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Register a new userThe tilt of the velocity ellipsoid in the Milky Way with Gaia DR2
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The velocity distribution of stars is a sensitive probe of the gravitational potential of the Galaxy, and hence of its dark matter distribution. In particular, the shape of the dark halo (e.g. spherical, oblate, or prolate) determines velocity correlations, and different halo geometries are expected to result in measurable differences. Here we explore and interpret the correlations in the $(v_R, v_z)$-velocity distribution as a function of position in the Milky Way. We selected a high-quality sample of stars from the Gaia DR2 catalogue and characterised the orientation of the velocity distribution or tilt angle over a radial distance range of $[4-13]~$kpc and up to $3.5~$kpc away from the Galactic plane while taking into account the effects of the measurement errors. We find that the tilt angles change from spherical alignment in the inner Galaxy ($Rsim4~$kpc) towards more cylindrical alignments in the outer Galaxy ($Rsim11~$kpc) when using distances that take a global zero-point offset in the parallax of $-29~mu$as. However, if the amplitude of this offset is underestimated, then the inferred tilt angles in the outer Galaxy only appear shallower and are intrinsically more consistent with spherical alignment for an offset as large as $-54~mu$as. We further find that the tilt angles do not seem to strongly vary with Galactic azimuth and that different stellar populations depict similar tilt angles. Therefore we introduce a simple analytic function that describes the trends found over the full radial range. Since the systematic parallax errors in Gaia DR2 depend on celestial position, magnitude, and colour in complex ways, it is not possible to fully correct for them. Therefore it will be particularly important for dynamical modelling of the Milky Way to thoroughly characterise the systematics in astrometry in future Gaia data releases.
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Until the recent advent of $Gaia$ Data Release 2 (DR2) and deep multi-object spectroscopy, it has been difficult to obtain 6-D phase space information for large numbers of stars beyond 4 kpc, in particular towards the Galactic centre, where dust and crowding effects are significant. In this study we combine line-of-sight velocities from the Abundances and Radial velocity Galactic Origins Survey (ARGOS) spectroscopic survey with proper motions from $Gaia$ DR2, to obtain a sample of $sim$ 7,000 red clump stars with 3-D velocities. We perform a large scale stellar kinematics study of the Milky Way (MW) bulge to characterize the bulge velocity ellipsoids. We measure the tilt $l_{v}$ of the major-axis of the velocity ellipsoid in the radial-longitudinal velocity plane in 20 fields across the bulge. The tilt or vertex deviation, is characteristic of non-axisymmetric systems and a significant tilt is a robust indicator of non-axisymmetry or bar presence. We compare the observations to the predicted kinematics of an N-body boxy-bulge model formed from dynamical instabilities. In the model, the $l_{v}$ values are strongly correlated with the angle ($alpha$) between the bulge major-axis and the Sun-Galactic centre line-of-sight. We use a maximum likelihood method to obtain an independent measurement of $alpha$, from bulge stellar kinematics alone. The most likely value of $alpha$ given our model is $alpha = (29 pm 3)^{circ}$. In the Baades window, the metal-rich stars display a larger vertex deviation ($l_{v} = -40^{circ}$) than the metal-poor stars ($l_{v} = 10^{circ}$) but we do not detect significant $l_{v}-$metallicity trends in the other fields.
Measuring the escape velocity of the Milky Way is critical in obtaining the mass of the Milky Way, understanding the dark matter velocity distribution, and building the dark matter density profile. In Necib $&$ Lin (2021), we introduced a strategy to robustly measure the escape velocity. Our approach takes into account the presence of kinematic substructures by modeling the tail of the stellar distribution with multiple components, including the stellar halo and the debris flow called the Gaia Sausage (Enceladus). In doing so, we can test the robustness of the escape velocity measurement for different definitions of the tail of the velocity distribution, and the consistency of the data with different underlying models. In this paper, we apply this method to the second data release of Gaia and find that a model with at least two components is preferred. Based on a fit with three bound components to account for the disk, relaxed halo, and the Gaia Sausage, we find the escape velocity of the Milky Way at the solar position to be $v_{rm{esc}}= 484.6^{+17.8}_{-7.4}$ km/s. Assuming a Navarro-Frenck-White dark matter profile, and taken in conjunction with a recent measurement of the circular velocity at the solar position of $v_c = 230 pm 10$ km/s, we find a Milky Way concentration of $c_{200} = 13.8^{+6.0}_{-4.3}$ and a mass of $M_{200} = 7.0^{+1.9}_{-1.2} times 10^{11} M_{odot}$, which is considerably lighter than previous measurements.
Flat rotation curves in disk galaxies represent the main evidence for large amounts of surrounding dark matter. Despite of the difficulty in identifying the dark matter contribution to the total mass density in our Galaxy, stellar kinematics, as tracer of gravitational potential, is the most reliable observable for gauging different matter components. This work tests the flatness of the MW rotation curve with a simple general relativistic model suitable to represent the geometry of a disk as a stationary axisymmetric dust metric at a sufficiently large distance from a central body. Circular velocities of unprecedented accuracy were derived from the Gaia DR2 data for a carefully selected sample of disk stars. We then fit these velocities to both the classical, i.e. including a dark matter halo, rotation curve model and a relativistic analogue, as derived form the solution of Einsteins equation. The GR-compliant MW rotational curve model results statistically indistinguishable from its state-of-the-art DM analogue. This supports our ansatz that a stationary and axisymmetric galaxy-scale metric could fill the gap in a baryons-only Milky Way, suggestive of star orbits dragged along the background geometry. We confirmed that geometry is a manifestation of gravity according to the Einstein theory, in particular the weak gravitational effect due to the off-diagonal term of the metric could mimic for a DM-like effect in the observed flatness of the MW rotation curve. In the context of Local Cosmology, our findings are suggestive of a Galaxy phase-space as the exterior gravitational field of a Kerr-like source (inner rotating bulge) without the need of extra-matter.
We measure the escape speed curve of the Milky Way based on the analysis of the velocity distribution of $sim 2850$ counter-rotating halo stars from the Gaia DR2. The distances were estimated through the StarHorse code, and only stars with distance errors smaller than 10 per cent were used in the study. The escape speed curve is measured at Galactocentric radii ranging from $sim 5$ kpc to $sim 10.5$ kpc. The local Galactic escape at the Suns position is estimated to be $v_mathrm{e}(r_odot)=580 pm 63~mathrm{km~s^{-1}}$, and it rises towards the Galactic center. Defined as the minimum speed required to reach three virial radii, our estimate of the escape speed as a function of radius implies, for a Navarro-Frenk-White profile and local circular velocity of $240~mathrm{km~s^{-1}}$, a dark matter mass $M_{200}=1.28^{+0.68}_{-0.50} times 10^{12}~M_odot$ and a high concentration $c_{200}=11.09^{+2.94}_{-1.79}$. Assuming the mass-concentration relation of $Lambda$CDM, we get $M_{200}=1.55_{-0.51}^{+0.64}times 10^{12}~M_odot$, $c_{200}=7.93_{-0.27}^{+0.33}$, for a local circular velocity of $228~mathrm{km~s^{-1}}$.
Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates $(R,phi,z)$, ridge-like structures can be seen in the vphiR{} plane and asymmetric arch-like structures in the vphivR{} plane. We show that the ridges are also clearly present when the third dimension of the vphiR{} plane is represented by $langle z rangle$, $langle V_z rangle$, $langle V_R rangle$, $langle$[Fe/H]$rangle$ and $langle[alpha/{rm Fe}]rangle$. The maps suggest that stars along the ridges lie preferentially close to the Galactic midplane ($|z|<0.2$ kpc), and have metallicity and $alpha$ elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy $-$ the ridges and arches are simply surfaces of constant energy. We identify 8 distinct ridges in the gaia{} data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in $langle z rangle$ and $langle V_z rangle$ suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.
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