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Register a new userGalactoseismology: Discovery of Vertical Waves in the Galactic Disk
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We present evidence for a Galactic North-South asymmetry in the number density and bulk velocity of solar neighborhood stars. The number density profile, which is derived from main-sequence stars in the Sloan Digital Sky Survey, shows a (North - South)/(North + South) deficit at |z| ~ 400 pc and an excess at |z| ~ 800 pc. The bulk velocity profile, which is derived from the Sloan Extension for Galactic Understanding and Exploration, shows a gradual trend across the Galactic midplane as well as smaller-scale features. We speculate that the North-South asymmetry, which has the appearance of a wavelike perturbation, is intrinsic to the disk. We explore the physics of this phenomenon through an analysis of the linearized Boltzmann and Poisson equations and through one-dimensional simulations. The perturbation may be excited by the passage of a satellite galaxy or dark matter subhalo through the Galactic disk, in which case we are witnessing a recent disk-heating event.
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Deep photometric surveys of the Milky Way have revealed diffuse structures encircling our Galaxy far beyond the classical limits of the stellar disk. This paper reviews results from our own and other observational programs, which together suggest that, despite their extreme positions, the stars in these structures were formed in our Galactic disk. Mounting evidence from recent observations and simulations implies kinematic connections between several of these distinct structures. This suggests the existence of collective disk oscillations that can plausibly be traced all the way to asymmetries seen in the stellar velocity distribution around the Sun. There are multiple interesting implications of these findings: they promise new perspectives on the process of disk heating, they provide direct evidence for a stellar halo formation mechanism in addition to the accretion and disruption of satellite galaxies, and, they motivate searches of current and near-future surveys to trace these oscillations across the Galaxy. Such maps could be used as dynamical diagnostics in the emerging field of Galactoseismology, which promises to model the history of interactions between the Milky Way and its entourage of satellites, as well examine the density of our dark matter halo. As sensitivity to very low surface brightness features around external galaxies increases, many more examples of such disk oscillations will likely be identified. Statistical samples of such features not only encode detailed information about interaction rates and mergers, but also about long sought-after dark matter halo densities and shapes. Models for the Milky Ways own Galactoseismic history will therefore serve as a critical foundation for studying the weak dynamical interactions of galaxies across the universe.
We report measurements of parallax and proper motion for four 22 GHz water maser sources as part of VERA Outer Rotation Curve project. All sources show Galactic latitudes of $>$ 2$^{circ}$ and Galactocentric distances of $>$ 11 kpc at the Galactic longitude range of 95$^{circ}$ $< l <$ 126$^{circ}$. The sources trace the Galactic warp reaching to 200$sim$400 pc, and indicate the signature of the warp to 600 pc toward the north Galactic pole. The new results along with previous results in the literature show the maximum height of the Galactic warp is increased with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found an oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of the bending (vertical density) waves, possibly induced by a perturbing satellite (e.g. passage of the Sagittarius dwarf galaxy).
The perturbation mechanism of the Galactic disk has puzzled us for a long time. The imprints from perturbations provide important diagnostics on the disk formation and evolution. Here we try to constrain when the vertical perturbation took place in the disk by tracking the phase mixing history. Firstly, we clearly depict the spiral structures of radial ($v_R$) and azimuthal ($v_{phi}$) velocities in the phase space of the vertical position and velocity ($z$-$v_z$) with 723,871 LAMOST-Gaia combined stars. Then, we investigate the variation of the spirals with stellar age ($tau$) by dividing the sample into seven stellar age bins. Finally, we find that the spirals explicitly exist in all the bins, even in the bin of $tau<0.5$,Gyr, except for the bin of $tau>6.0$,Gyr. This constrains the vertical perturbation probably starting no later than 0.5,Gyr ago. But we can not rule out whether the young stars ($tau<0.5$,Gyr) inherit the oscillations from the perturbed ISM where they born from. This study provides some important observational evidences to understand the disk perturbation mechanisms, even the formation and evolution of our Galaxy.
An oscillating vertical displacement of the Milky Way, with a wavelength of about 8 kpc and and amplitude of about 100 pc (increasing with distance from the Galactic center) is observed towards the Galactic anticenter. These oscillations are thought to be the result of disk perturbations from dwarf satellites of the Milky Way. They explain the Monoceros Ring and could be related to Milky Way spiral structure.
High resolution spectra data of red clump stars towards the NGP have been obtained with the high resolution spectrograph Elodie at OHP for Tycho-2 selected stars. Combined with Hipparcos local analogues, we determine both the gravitational force law perpendicaular to the Galactic plane, and the total surface mass density and thickness of the Galactic disk. The surface mass density of the Galactic disk within 800 pc derived from this analysis is Sigma(|z|<800pc)=76 Msol.pc-2 and, removing the dark halo contribution, the total disk mass density is Sigma0=67 Msol.pc-2 at solar radius. The thickness of the total disk mass distribution is dynamicaly measured for the first time and is found to be 390pc in relative agreement with the old stellar disk scale height. All dynamical evidences concerning the structure of the disk (its local volume density -i.e. the Oort limit-, its surface density and its thickness) are compatible with our knowledge of the corresponding stellar disk properties.
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