Tree-ring structure of Galactic bar resonance

Galaxy models have long predicted that galactic bars slow down by losing angular momentum to their postulated dark haloes. When the bar slows down, resonance sweeps radially outwards through the galactic disc while growing in volume, thereby sequentially capturing new stars at its surface/separatrix. Since trapped stars conserve their action of libration, which measures the relative distance to the resonance centre, the order of capturing is preserved: the surface of the resonance is dominated by stars captured recently at large radius, while the core of the resonance is occupied by stars trapped early at small radius. The slow down of the bar thus results in a rising mean metallicity of trapped stars from the surface towards the centre of the resonance as the Galaxy’s metallicity declines towards large radii. This argument, when applied to Solar neighbourhood stars, allows a novel precision measurement of the bar’s current pattern speed Ωp=35.5±0.8kms−1kpc−1⁠, placing the corotation radius at RCR=6.6±0.2kpc⁠. With this pattern speed, the corotation resonance precisely fits the Hercules stream in agreement with kinematics. Beyond corroborating the slow bar theory, this measurement manifests the deceleration of the bar of more than 24 per cent since its formation and thus the angular momentum transfer to the dark halo by dynamical friction. The measurement therefore supports the existence of a standard dark-matter halo rather than alternative models of gravity

The Tilt of the Local Velocity Ellipsoid as Seen by Gaia

The Gaia Radial Velocity Spectrometer (RVS) provides a sample of 7,224,631 stars with full six-dimensional phase space information. Bayesian distances of these stars are available from the catalogue of Sch\"onrich et al. (2019). We exploit this to map out the behaviour of the velocity ellipsoid within 5 kpc of the Sun. We find that the tilt of the disc-dominated RVS sample is accurately described by the relation $\alpha = (0.952 \pm 0.007)\arctan (|z|/R)$, where ($R,z$) are cylindrical polar coordinates. This corresponds to velocity ellipsoids close to spherical alignment (for which the normalising constant would be unity) and pointing towards the Galactic centre. Flattening of the tilt of the velocity ellipsoids is enhanced close to the plane and Galactic centre, whilst at high elevations far from the Galactic center the population is consistent with exact spherical alignment. Using the LAMOST catalogue cross-matched with Gaia DR2, we construct thin disc and halo samples of reasonable purity based on metallicity. We find that the tilt of thin disc stars straddles $\alpha = (0.909-1.038)\arctan (|z|/R)$, and of halo stars straddles $\alpha = (0.927-1.063)\arctan (|z|/R)$. We caution against the use of reciprocal parallax for distances in studies of the tilt, as this can lead to serious artefacts.Comment: MNRAS, revised version contains additional checks on the integrity of the distance

Metallicity-suppressed collapsars cannot be the dominant r-process source in the milky way

We develop a high-performance analytical model of Galactic Chemical Evolution, which accounts for delay time distributions and lock-up of stellar yields in a thermal-phased ISM. The model is capable of searching, for the first time, through the high-dimensional parameter space associated with the r-process enrichment of the Milky Way by its possible sources: Neutron Star Mergers and Collapsar events. Their differing formation mechanisms give these two processes different time dependencies, a property which has frequently been used to argue in favour of collapsars as the dominant r-process source. However, we show that even with large degrees of freedom in the allowed thermal, structural, and chemical properties of the galaxy, large regions of parameter space are in strong tension with the data. In particular, whilst we are able to find models in which neutron star mergers produce the majority of r-process material, the data rule out all models with dominant collapsar yields. With no other identified source, we conclude that Neutron Star Mergers must be the dominant contributors to the modern Milky Way r-process budget

More than just a wrinkle: a wave-like pattern in Ug versus Lz from Gaia data

We present a newly found wave-like pattern in mean Galactocentric radial velocity U¯¯¯¯g versus guiding centre radius Rg or angular momentum Lz of stars in the RV subsample of Gaia DR2. The short-wave pattern has a wavelength of order 1.2kpc in Rg or 285kpckms−1 in Lz. The pattern shows only weak changes with Galactocentric radius R and little change in strength in particular with the vertical energy Ez of the stars or the distance to the Galactic plane |z|. The pattern is to first order symmetric around the plane, i.e. has no significant odd terms in z. There is a weak phase shift with the pattern moving towards slightly lower Lz (i.e. trailing) with |z| and Ez. However, we observe a highly significant phase shift in Galactic azimuth ϕ, which is different for different peaks. The peak around Lz∼2100kpckms−1 only shows a weak change with ϕ, while the rest of the pattern shows a clearly detectable shift of dLz/dϕ=(200±22)kpckms−1rad−1⁠. If we consider all peaks to belong to the same pattern, this would suggest a wavenumber m = 4. We further find that the wave-like pattern in Ug appears to be related to the W¯¯¯¯¯ versus Lz pattern detected in Gaia DR1. A comparison of the U¯¯¯¯g−Lz wave pattern with changes of U¯¯¯¯g versus R, which have been previously discussed, suggests that the latter can be understood as just the U¯¯¯¯g−Lz pattern washed out by blurring (i.e. orbital excursions around their guiding centre) of disc stars

Metallicity-suppressed collapsars cannot be the dominant r-process source in the milky way

We develop a high-performance analytical model of Galactic Chemical Evolution, which accounts for delay time distributions and lock-up of stellar yields in a thermal-phased ISM. The model is capable of searching, for the first time, through the high-dimensional parameter space associated with the r-process enrichment of the Milky Way by its possible sources: Neutron Star Mergers and Collapsar events. Their differing formation mechanisms give these two processes different time dependencies, a property which has frequently been used to argue in favour of collapsars as the dominant r-process source. However, we show that even with large degrees of freedom in the allowed thermal, structural, and chemical properties of the galaxy, large regions of parameter space are in strong tension with the data. In particular, whilst we are able to find models in which neutron star mergers produce the majority of r-process material, the data rule out all models with dominant collapsar yields. With no other identified source, we conclude that Neutron Star Mergers must be the dominant contributors to the modern Milky Way r-process budget

Warp, waves, and wrinkles in the Milky Way

We derive unbiased distance estimates for the Gaia-TGAS data set by correcting for the bias due to the distance dependence of the selection function, which we measure directly from the data. From these distances and proper motions, we estimate the vertical and azimuthal velocities, W and Vϕ, and angular momentum Lz for stars in the Galactic centre and anticentre directions. The resulting mean vertical motion W shows a linear increase with both Vϕ and Lz at 10σ significance. Such a trend is expected from and consistent with the known Galactic warp. This signal extends to stars with guiding centre radii Rg < R0, placing the onset of the warp at R ≲ 7 kpc. At equally high significance, we detect a previously unknown wave-like pattern of W over guiding centre Rg with an amplitude ~1 kms-1 and a wavelength ~2.5 kpc. This pattern is present in both the centre and anticentre directions, consistent with a winding (corrugated) warp or bending wave, likely related to known features in the outer disc (TriAnd and Monoceros overdensities), and may be caused by the interaction with the Sgr dwarf galaxy ~1Gyr ago. The only significant deviation from this simple fit is a stream-like feature near Rg ~9 kpc (|Lz| ~2150 kpc km s-1)

Resonance sweeping by a decelerating Galactic bar

We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disc to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disc affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed Ωp and amplitude. When the bar slows down, its resonances sweep through phase space, trapping, and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local vR-vφ plane and the double-peaked structure of mean vR in the Lz–φ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar Ω˙p=(−4.5±1.4)kms−1kpc−1Gyr−1 with additional systematic uncertainty arising from unmodelled impacts of e.g. spiral arms

The tilt of the local velocity ellipsoid as seen by Gaia

The Gaia Radial Velocity Spectrometer (RVS) provides a sample of 7224 631 stars with full six-dimensional phase space information. Bayesian distances of these stars are available from the catalogue of Schönrich, McMillan &amp; Eyer. We exploit this to map out the behaviour of the velocity ellipsoid within 5 kpc of the Sun. We find that the tilt of the disc-dominated RVS sample is accurately described by the relation $\alpha = (0.952 \pm 0.007)\arctan (|z|/R)$, where (R, z) are cylindrical polar coordinates. This corresponds to velocity ellipsoids close to spherical alignment (for which the normalizing constant would be unity) and pointing towards the Galactic Centre. Flattening of the tilt of the velocity ellipsoids is enhanced close to the plane and Galactic Centre, whilst at high elevations far from the Galactic Centre the population is consistent with exact spherical alignment. Using the LAMOST catalogue cross-matched with Gaia DR2, we construct thin disc and halo samples of reasonable purity based on metallicity. We find that the tilt of thin disc stars straddles $\alpha = (0.909{\!-\!}1.038)\arctan (|z|/R)$, and of halo stars straddles $\alpha = (0.927{\!-\!}1.063)\arctan (|z|/R)$. We caution against the use of reciprocal parallax for distances in studies of the tilt, as this can lead to serious artefacts

Galactic Bar Resonances Inferred from Kinematically Hot Stars in Gaia EDR3

Using a numerical simulation of an isolated barred disc galaxy, we first demonstrate that the resonances of the inner bar structure induce more prominent features in the action space distribution for the kinematically hotter stars, which are less sensitive to the local perturbation, such as the transient spiral arms. Then, we analyse the action distribution for the kinematically hotter stars selected from the Gaia EDR3 data as the stars with higher values of radial and vertical actions. We find several resonance features, including two new features, in the angular momentum distribution similar to what are seen in our numerical simulations. We show that the bar pattern speeds of about Ωbar ∼ 34 and 42 km s−1 kpc−1 explain all these features equally well. The resonance features we find correspond to the inner 4:1, co-rotation (CR), outer 4:1, outer Lindblad, and outer 4:3 (CR, outer 4:1, outer Lindblad, outer 4:3, and outer 1:1) resonances, when Ωbar ∼ 34 (42) km s−1 kpc−1 is assumed

On the Kinematic Signature of the Galactic Warp As Revealed by the LAMOST-TGAS Data

Using a sample of about 123,000 stars with accurate 3D velocity measurements from the LAMOST-TGAS data, we confirm the kinematic signature of the Galactic warp recently found by Schonrich & Dehnen. The data reveal a clear trend of increasing mean vertical velocity Vz as a function of absolute vertical angular momentum Lz and azimuthal velocity Vφ for guiding center radius Rg between 6.0 and 10.5 kpc. The trend is consistent with a largescale Galactic warp. Similar to Schonrich & Dehnen, we also find a wave-like pattern of Vz versus Lz with an amplitude of ∼0.9 km s-1 on a scale of ∼2.0 kpc, which could arise from bending waves or a winding warp. Finally, we confirm a prominent, localized peak in Vz near Lz ∼ 2150 kpc km s-1 (corresponding to Rg ∼ 9 kpc and Vφ ∼ 255 km s-1). The additional line-of-sight velocity information from LAMOST reveals that stars in this feature have a large, inward radial velocity of VR ∼ -13.33 ± 0.59 km s-1 and a small radial velocity dispersion of σR ∼ 25.27 ± 0.89 km s-1, suggesting that a stellar stream gives rise to this feature