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Register a new userThe GALAH survey and Gaia DR2: Linking ridges, arches and vertical waves in the kinematics of the Milky Way
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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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We employ Gaia DR2 proper motions for 151 Milky Way globular clusters from Vasiliev (2019) in tandem with distances and line-of-sight velocities to derive their kinematical properties. To assign clusters to the Milky Way thick disk, bulge, and halo we follow the approach of Posti et al. (2018) who distinguished among different Galactic stellar components using starss orbits. In particular, we use the ratio $L_{z}/e$, the $Z$ projection of the angular momentum to the eccentricity, as population tracer, which we complement with chemical abundances extracted from the literature and Monte-Carlo simulations. We find that 20 globular clusters belong to the bar/bulge of the Milky Way, 35 exhibit disk properties, and 96 are members of the halo. Moreover, we find that halo globular clusters have close to zero rotational velocity with average value $<Theta>$ =1$pm$ 4 km s$^{-1}$. On the other hand, the sample of clusters that belong to the thick disk possesses a significant rotation with average rotational velocity 179 $pm$ 6 km s$^{-1}$. The twenty globular clusters orbiting within the bar/bulge region of the Milky Way galaxy have average rotational velocity of 49 $pm$ 11 km s$^{-1}$.
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.
We present new mass estimates and cumulative mass profiles (CMPs) with Bayesian credible regions for the Milky Way (MW) Galaxy, given the kinematic data of globular clusters as provided by (1) the $textit{Gaia}$ DR2 collaboration and the HSTPROMO team, and (2) the new catalog in Vasiliev (2019). We use globular clusters beyond 15kpc to estimate the CMP of the MW, assuming a total gravitational potential model $Phi(r) = Phi_{circ}r^{-gamma}$, which approximates an NFW-type potential at large distances when $gamma=0.5$. We compare the resulting CMPs given data sets (1) and (2), and find the results to be nearly identical. The median estimate for the total mass is $M_{200}= 0.70 times 10^{12} M_{odot}$ and the $50%$ Bayesian credible interval is $(0.62, 0.81)times10^{12}M_{odot}$. However, because the Vasiliev catalog contains more complete data at large $r$, the MW total mass is slightly more constrained by these data. In this work, we also supply instructions for how to create a CMP for the MW with Bayesian credible regions, given a model for $M(<r)$ and samples drawn from a posterior distribution. With the CMP, we can report median estimates and $50%$ Bayesian credible regions for the MW mass within any distance (e.g., $M(r=25text{kpc})= 0.26~(0.20, 0.36)times10^{12}M_{odot}$, $M(r=50text{kpc})= 0.37~(0.29, 0.51) times10^{12}M_{odot}$, $M(r=100text{kpc}) = 0.53~(0.41, 0.74) times10^{12}M_{odot}$, etc.), making it easy to compare our results directly to other studies.
GALAH and APOGEE are two high resolution multi object spectroscopic surveys that provide fundamental stellar parameters and multiple elemental abundance estimates for $>$ 400,000 stars in the Milky Way. They are complimentary in both sky coverage and wavelength regime. Thus combining the two surveys will provide us a large sample to investigate the disc metallicity and alpha abundance trends. We use the Cannon data-driven approach selecting training sets from among $sim$20,000 stars in common for the two surveys to predict the GALAH scaled stellar parameters from APOGEE spectra as well as APOGEE scaled stellar parameters from GALAH spectra. We provide two combined catalogues with GALAH scaled and APOGEE scaled stellar parameters each having $sim$500,000 stars after quality cuts. With $sim$470,000 stars that are common in both these catalogues, we compare the GALAH scaled and APOGEE scaled metallicity distribution functions (MDF), radial and vertical metallicity gradients as well as the variation of [$alpha$/Fe] vs [Fe/H] trends along and away from the Galactic mid plane. We find mean metallicities of APOGEE scaled sample to be higher compared to that for the GALAH scaled sample. We find similar [$alpha$/Fe] vs [Fe/H] trends using both samples consistent with previous observational as well as simulation based studies. Radial and vertical metallicity gradients derived using the two survey scaled samples are consistent except in the inner and outer Galactocentric radius bins. Our gradient estimates in the solar neighborhood are also consistent with previous studies and are backed by larger sample size compared to previous works.
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.
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