Galactic seismology: the evolving "phase spiral" after the Sagittarius dwarf impact
Joss Bland-Hawthorn, Thor Tepper-Garcia
备注：
21 pages, 16 figures, MNRASaccepted (Feb 2021). We have left modifications in bold print. A brief history of Galactic seismology is given in the Appendix
In 2018, the ESA \Gaia\ satellite discovered a remarkable spiral pattern ("phase spiral") in the $z-V_z$ phase plane throughout the solar neighbourhood, where $z$ and $V_z$ are the displacement and velocity of a star perpendicular to the Galactic disc. In response to Binney \& Schönrich's analytic model of a disc-crossing satellite to explain the \Gaia\ data, we carry out a high-resolution, N-body simulation (N$\:\approx 10^8$ particles) of an impulsive mass ($2\times 10^{10}$ \Msun) that interacts with a cold stellar disc at a single transit point. The disc response is complex since the impulse triggers a superposition of two distinct bisymmetric ($m=2$) modes $-$ a density wave and a corrugated bending wave $-$ that wrap up at different rates. Stars in the {\it faster} density wave wrap up with time $T$ according to $ϕ_D(R,T)=(Ω_D(R) + Ω_{\rm o})\:T$ where $ϕ_D$ describes the spiral pattern and $Ω_D =Ω(R) -κ(R)/2$, where $κ$ is the epicyclic frequency. While the pattern speed $Ω_{\rm o}$ is small, it is non-zero. The {\it slower} bending wave wraps up according to $Ω_B\approxΩ_D/2$ producing a corrugated wave. The bunching effect of the density wave triggers the phase spiral as it rolls up and down on the bending wave ("rollercoaster" model). The phase spiral emerges slowly about $ΔT \approx 400$ Myr after impact. It appears to be a long-lived, disc-wide phenomenon that continues to evolve over most of the 2~Gyr simulation. Thus, given Sagittarius' (Sgr) low total mass today ($M_{\rm tot}\sim 3\times 10^8$ \Msun\ within 10 kpc diameter), we believe the phase spiral was excited by the disc-crossing dwarf some $1-2$ Gyr {\it before} the recent transit. For this to be true, Sgr must be losing mass at 0.5-1 dex per orbit loop.
We report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way as part of the BeSSeL Survey with the VLBA. Combining these results with eight others from the literature, we investigated the structure and kinematics of the arm between Galactocentric azimuth around -2 and 65 deg. We found that the spiral pitch angle is 7.3 +- 1.5 deg; the arm's half-width, defined as the rms deviation from the fitted spiral, is around 0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 +- 0.2 kpc from the Sun. Unlike for adj…
Here we fitted a 4-arm spiral structure to the more accurate data on global arm pitch angle and arm longitude tangents, to get the start of each spiral arm near the Galactic nucleus. We find that the tangent to the 'start of the Sagittarius' spiral arm (arm middle) is at l= -17 degrees +/- 0.5 degree, while the tangent to the 'start of the Norma' spiral arm (arm middle) is at l= +20 degrees +/- 0.5 degree. Earlier, we published a compilation of observations and analysis of the tangent to each spiral arm tracer, from longitudes +23 degrees to +340 degrees; here we cover the arm tracers in the r…
As part of the BeSSeL Survey, we report trigonometric parallaxes and proper motions of molecular maser sources associated with 13 distant high mass star forming regions in the Sagittarius spiral arm of the Milky Way. In particular, we obtain improved parallax distance estimates for three well studied regions: 1.9 +0.1/-0.1 kpc for M17, 5.3 +1.3/-0.9 kpc for W51, and 7.9 +0.9/-0.7 kpc for GAL 045.5+00.0. Peculiar motions for all but one source are less than 20 km/s. We fit a log-periodic spiral to the locations and estimate an average pitch angle of 7.2+-1.9 deg. We find that the section of the…
We present the pilot results of the `MAGMO' project, targeted observations of ground-state hydroxyl masers towards sites of 6.7-GHz methanol maser emission in the Carina-Sagittarius spiral arm tangent, Galactic longitudes 280 degrees to 295 degrees. The `MAGMO' project aims to determine if Galactic magnetic fields can be traced with Zeeman splitting of masers associated with star formation. Pilot observations of 23 sites of methanol maser emission were made, with the detection of ground-state hydroxyl masers towards 11 of these and six additional offset sites. Of these 17 sites, nine are new d…
We report on trigonometric parallax measurements for the Galactic star forming region G14.33-0.64 toward the Sagittarius spiral arm. We conducted multi-epoch phase-referencing observations of an H2O maser source in G14.33-0.64 with the Japanese VLBI array VERA. We successfully detected a parallax of 0.893+/-0.101 mas, corresponding to a source distance of 1.12+/-0.13 kpc, which is less than half of the kinematic distance for G14.33-0.64. Our new distance measurement demonstrates that the Sagittarius arm lies at a closer distance of ~1 kpc, instead of previously assumed ~2-3 kpc from kinematic …
We present first results for Faraday rotation of compact polarized sources (1 to 2 GHz continuum) in The HI/OH/Recombination line (THOR) survey of the inner Galaxy. In the Galactic longitude range 39 degr < l < 52 degr, we find rotation measures in the range -310 rad/m2 < RM < +4219 rad/m2, with the highest values concentrated within a degree of l = 48 degrees at the Sagittarius arm tangent. Most of the high RMs arise in diffuse plasma, along lines of sight that do not intersect HII regions. For l > 49 degr, RM drops off rapidly, while at l < 47 degr, the mean RM is higher with a larger standa…
We bring the two color maps for the plane component magnetic field of our Galaxy in coordinates of (R;l) and (DM;l). It was shown that magnetic field has reversals of the direction in neighbor spiral arms, in agreement with known models for the Galactic magnetic field. For the Sagittarius spiral arm region there is, however, some disagreement with standard magnetic field models. The major discrepancy is the fact that in the Sagittarius arm region the magnetic field in Southern hemisphere of the Galaxy, have opposite direction to the field of the Northern hemisphere. We think that the Sagittari…
It is increasingly apparent that common merger events play a large role in the evolution of disk galaxies at all cosmic times, from the wet accretion of gas-filled dwarf galaxies during the era of peak star formation, to the collisions between large, dynamically-advanced spiral galaxies and their dry companion satellites, a type of interaction that continues to influence disk structure into the present day. We also live in a large spiral galaxy currently undergoing a series of impacts from an infalling, disrupting dwarf galaxy. As next-generation astrometry proposes to place our understanding …
We have performed the observation of the "Galactic Center Mini-spiral(GCMS)" in H42alpha recombination line as a part of the first large-scale mosaic observation in the Sagittarius A complex using Atacama Millimeter/sub-millimeter Array (ALMA). We revealed the kinematics of the ionized gas streamers of the GCMS. We found that the ionized gas streamers of the Northern Arm(NA) and Eastern Arm(EA) in their outer regions somewhat deviate from the Keplerian orbits which were derived previously from the trajectories in the inner regions. In addition, we found that the streamer corresponding to the B…
We have carried out survey observations toward the Galactic plane at l~38 deg in the 12CO and 13CO J=1-0 lines using the Nobeyama Radio Observatory 45-m telescope. A wide area (0.8 x 0.8 deg) was mapped with high spatial resolution (17"). The line of sight samples the gas in both the Sagittarius arm and the inter-arm regions. The present observations reveal how the structure and physical conditions vary across a spiral arm. We classify the molecular gas in the line of sight into two distinct components based on its appearance: the bright and compact B component and the fainter and diffuse (i.e…
The study of dynamical properties of Galactic open clusters is a fundamental prerequisite for the comprehension of their dissolution processes. In this work, we characterized 12 open clusters, namely: Collinder 258, NGC 6756, Czernik 37, NGC 5381, Ruprecht 111, Ruprecht 102, NGC 6249, Basel 5, Ruprecht 97, Trumpler 25, ESO 129-SC32 and BH 150, projected against dense stellar fields. In order to do that, we employed Washington $CT_{1}$ photometry and GAIA DR2 astrometry, combined with a decontamination algorithm applied to the three-dimensional astrometric space of proper motions and parallaxes…
Model of the Galaxy with the ring R1R2' can explain some large-scale morphological features of the Galactic spiral structure. The Carina-Sagittarius arm can consist of two ascending segments of the outer rings R1 and R2 which almost touch each other near the Carina region. The Perseus and Crux arms can be partially identified with the descending segments of the ring R2. Model of the two-component outer ring can also explain the existence of some maxima in diagrams (l, VLSR) which are supposed to correspond to the directions tangential to the spiral arms. On the basis of numerical simulations w…
Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that ar…
We consider a sample of Galactic classical Cepheids with highly accurate estimates of their distances taken from Skowron et al., where they were determined based on the period-luminosity relation. We have refined the geometric characteristics of two spiral arm segments-the Carina-Sagittarius and Outer ones. For this purpose, we have selected 269 Cepheids belonging to the Carina-Sagittarius arm with ages in the range 80-120 Myr. From them we have estimated the pitch angle of the spiral pattern $i=-11.9\pm0.2^\circ$ and the position of this arm $a_0=7.32\pm0.05$ kpc for the adopted $R_0=8.1\pm0.…
The kinematics of the outer rings and pseudorings is determined by two processes: the resonance tuning and the gas outflow. The resonance kinematics is clearly observed in the pure rings, while the kinematics of the gas outflow is manifested itself in the pseudorings. The direction of systematical motions in the pure rings depends on the position angle of a point with respect to the bar major axis and on the class of the outer ring. The direction of the radial and azimuthal components of the residual velocities of young stars in the Perseus, Carina, and Sagittarius regions can be explained by …
We study the dynamics of tidally induced spiral arms in our Galaxy in the context of its encounters with Sagittarius. We built toy models of the interaction between a host and a satellite galaxy using orbital integrations after a tidal encounter. We derived analytically the shape of the structures in phase space as a function of time for simple power-law potential models. We compared these models to a more realistic N-body simulation of the MW Sagittarius-like interaction and to data from Gaia DR3. As found before, an impulsive distant tidal approach of a galactic satellite leads to a two-arme…
We analyse an N-body simulation of the interaction of the Milky Way (MW) with a Sagittarius-like dSph (Sgr), looking for signatures which may be attributed to its orbital history in the phase space volume around the Sun in light of $Gaia$ DR2 discoveries. The repeated impacts of Sgr excite coupled vertical and radial oscillations in the disc which qualitatively, and to a large degree quantitatively are able to reproduce many features in the 6D $Gaia$ DR2 samples, from the median $V_{R},V_ϕ, V_{z} $ velocity maps to the local $δρ(v_{z},z)$ phase-space spiral which is a manifestation of the glob…
Stars born on near-circular orbits in spiral galaxies can subsequently migrate to different orbits due to interactions with non-axisymmetric disturbances within the disc such as bars or spiral arms. This paper extends the study of migration to examine the role of external influences using the example of the interaction of the Sagittarius dwarf galaxy (Sgr) with the Milky Way (MW). We first make impulse approximation estimates to characterize the influence of Sgr disc passages. The tidal forcing from Sgr can produce changes in both guiding radius ($ΔR_g$) and orbital eccentricity, as quantified…
An updated model for the Perseus Spiral Arm from Trigonometric Parallax and 3-dimensional kinematic distances of distant massive stars
University of Tasmania（塔斯马尼亚大学）；International Centre for Radio Astronomy Research, The University of Western Australia（西澳大利亚大学国际射电天文研究中心）；Nanjing University（南京大学）
To pinpoint the peak location of the synchrotron total intensity emission in a spiral arm, we use a map of the spiralarm locations (from the observed arm tangent). Thus In a typical spiral arm in Galactic Quadrant I, we find the peak of the synchrotron radiation to be located about 220 +/-40 pc away from the inner arm edge (hot dust lane) inside the spiral arm. While most of the galactic disk has a clockwise largescale magnetic field, we make a statistical analysis to delimitate more precisely the smaller reverse annulus wiith a counterclockwise galactic magnetic field. We find an annulus widt…
Satellites orbiting disc galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their discs. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disc has only recently been fully mapped thanks to Gaia DR2 data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disc. Here, we model Gaia DR2 observed colour-magnitude diagrams to obtain the first detailed star …
The Milky Way is one of the very few spiral galaxies known to host large-scale magnetic field reversals. The existence of the field reversal in the first Galactic quadrant near the Sagittarius spiral arm has been well established, yet poorly characterised due to the insufficient number of reliable Faraday depths (FDs) from extragalactic radio sources (EGSs) through this reversal region. We have therefore performed broadband (1-2 GHz) spectro-polarimetric observations with the Karl G. Jansky Very Large Array (VLA) to determine the FD values of 194 EGSs in the Galactic longitude range of $20^\ci…
Context: In spiral galaxies, star formation tends to trace features of the spiral pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging to connect individual star-forming regions to their larger Galactic environment owing to our perspective from within the disk. One feature in nearly all modern models of the Milky Way is the Sagittarius Arm, located inward of the Sun with a pitch angle of ~12 deg. Aims: We map the 3D locations and velocities of star-forming regions in a segment of the Sagittarius Arm using young stellar objects (YSOs) fr…
We use time-resolved spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to examine the distribution of radial velocity (RV) variations in 249 stars identified as members of the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy by Hayes et al (2020). We select Milky Way (MW) stars that have stellar parameters ($log(g)$, $T_{eff}$, and $[Fe/H]$) similar to those of the Sagittarius members by means of a k-d tree of dimension 3. We find that the shape of the distribution of RV shifts in Sgr dSph stars is similar to that measured in their MW analogs, but the total fract…
The morphology and kinematics of the spiral structure of the Milky Way is a long-standing problem in astrophysics. In this review we firstly summarize various methods with different tracers used to solve this puzzle. The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes (a few $μ$as) and proper motions ($\approx$ 1 km~s$^{-1}$) by using Very Long Baseline Interferometry (VLBI). On the other hand, Gaia mission is providing the largest, uniform sample of parallaxes for O-t…
The Milky Way spiral arms are well established from star counts as well as from the locus of molecular clouds and other young objects, however, they have only recently started to be observed from a kinematics point of view. Using the kinematics of thin disc stars in Gaia EDR3 around the extended solar neighbourhood, we create x-y projections coloured by the radial, residual rotational, and vertical Galactocentric velocities ($U,ΔV,W$). The maps are rich in substructures and reveal the perturbed state of the Galactic disc. We find that local differences between rotational velocity and the azimu…
We have investigated the distributions of stellar azimuthal and radial velocity components $V_Φ$ and $V_{R}$ in the vertical position-velocity plane $Z$-$V_{Z}$ across the Galactic disc of $6.34 \lesssim R \lesssim 12.34$\,kpc and $|Φ| \lesssim 7.5^{\circ}$ using a Gaia and Gaia-LAMOST sample of stars. As found in previous works, the distributions exhibit significant spiral patterns. The $V_{R}$ distributions also show clear quadrupole patterns, which are the consequence of the well-known tilt of the velocity ellipsoid. The observed spiral and quadrupole patterns in the phase space plane vary …
Context. The physical processes driving the formation of Galactic spiral arms are still under debate. Studies using open clusters favour the description of the Milky Way spiral arms as long-lived structures following the classical density wave theory. Current studies comparing the Gaia DR2 field stars kinematic information of the Solar neighbourhood to simulations, find a better agreement with short-lived arms with a transient behaviour. Aims. Our aim is to provide an observational, data-driven view of the Milky Way spiral structure and its dynamics using open clusters as the main tracers, and…
We aim to explore the actions of the new Gaia DR3 astrometry to find structures in the Galactic disc. We compute the actions and the orbital parameters of the Gaia DR3 stars with full astrometry and velocities assuming an axisymmetric model for the Milky Way. Using Gaia DR3 photometric data, we select a subset of giants stars with better astrometry as control sample. The maps of the percentiles of the radial action J_R reveal spiral-like shape structures. We find a high J_R region centered at R~10.5 kpc of 1 kpc width, as well as three arc-shape regions dominated by circular orbits at inner ra…
From the sun's location in the Galactic disk, one can use different arm tracers (CO, HII, thermal or ionized or relativistic electrons, masers, cold or hot dust, etc) to locate a tangent to each spiral arm in the disk of the Milky Way galaxy. We present a Master catalog of the astronomically observed tangents to the Galaxy's spiral arms, using different arm tracers from the literature. Some arm tracers can have slightly divergent results from several papers, so a mean is taken - see Appendix for CO, HII, and masers. The Master catalog of means currently consists of 63 mean tracer entries, spre…
We have measured trigonometric parallaxes for four water masers associated with distant massive young stars in the inner regions of the Galaxy using the VLBA as part of the BeSSeL Survey. G026.50$+$0.28. is located at the near end of the Galactic bar, perhaps at the origin of the Norma spiral arm. G020.77$-$0.05 is in the Galactic Center region and is likely associated with a far-side extension of the Scutum arm. G019.60$-$0.23 and G020.08$-$0.13 are likely associated and lie well past the Galactic Center. These sources appear to be in the Sagittarius spiral arm, but an association with the Pe…
Some galactic theories of spiral arms predict an offset between different tracers of star formation. Our goal here is to find such an offset between the observed location of radio masers and the location of the arms, using a recent 4-arm model fitted to the CO 1-0 gas. Our method here is to compare a recent global 4-arm spiral model (as fitted to the arm tangents in the observed broad CO 1-0 gas) with the recent results for the trigonometric distances of radio masers, for the main arms (Cygnus-Norma, Perseus, Sagittarius-Carina, Scutum, and Norma). Our results indicate that most radio masers a…
Accretion disks are an essential component in the paradigm of the formation of low-mass stars. Recent observations further identify disks surrounding low-mass pre-main-sequence stars perturbed by flybys. Whether disks around more massive stars evolve in a similar manner becomes an urgent question. We report the discovery of a Keplerian disk of a few solar masses surrounding a 32 solar-mass protostar in the Sagittarius C cloud around the Galactic Center. The disk is gravitationally stable with two embedded spirals. A combined analysis of analytical solutions and numerical simulations demonstrat…
Using the astrometry and integrated photometry from the Gaia Early Data Release 3 (EDR3), we map the density variations in the distribution of young Upper Main Sequence (UMS) stars, open clusters and classical Cepheids in the Galactic disk within several kiloparsecs of the Sun. Maps of relative over/under-dense regions for UMS stars in the Galactic disk are derived, using both bivariate kernel density estimators and wavelet transformations. The resulting overdensity maps exhibit large-scale arches, that extend in a clumpy but coherent way over the entire sampled volume, indicating the location…
We investigate the morphology and kinematics of the Galactic spiral structure based on a new sample of O- and early B-type stars. We select 6,858 highly confident OB star candidates from the combined data of the VST Photometric H$α$ Survey Data Release 2 (VPHAS+ DR2) and the Gaia Data Release 2 (Gaia DR2). Together with the O-B2 stars from the literature, we build a sample consisting of 14,880 O- and early B-type stars, all with Gaia parallax uncertainties smaller than 20 per cent. The new sample, hitherto the largest one of O- and early B-type stars with robust distance and proper motion esti…
Small dwarf companions have been long thought to have minimal influence on their host galaxy's evolution without undergoing direct impacts to the host's disc. However, in light of recent discoveries of coplanar, corotating satellite structures around the Milky Way, Andromeda, and Centaurus A, we use an N-body/test particle simulation to show that low-mass dwarf satellites within such structures are able to exert significant influence on their host's disc, driving spiral waves and inducing stellar scattering. This is accomplished through quasi-periodic alignments of multiple small satellites wi…
We have made measurements to determine the parallax and proper motion of the three 6.7-GHz methanol masers G305.200$+$0.019, G305.202$+$0.208 and G305.208$+$0.206. The combined parallax is found to be 0.25$\pm $0.05 mas, corresponding to a distance of 4.1$^{+1.2}_{-0.7}$ kpc. This places the G305.2 star formation region in the Carina-Sagittarius spiral arm. The inclusion of G305.2 increases the Galactic azimuth range of the sources in this arm by 40$^\circ $ from Sato et al., allowing us to determine the pitch angle of this spiral with greater confidence to be $ψ= 19.0 \pm 2.6^\circ $. The fir…
Tree-ring structure of Galactic bar resonance in N-body simulations
The University of Tokyo（东京大学）；University of Toronto（多伦多大学）；Kagoshima University（鹿儿岛大学）；National Astronomical Observatory of Japan（日本国立天文台）；University College London（伦敦大学学院）
In this work we present two new $\sim10^9$ particle self-consistent simulations of the merger of a Sagittarius-like dwarf galaxy with a Milky Way-like disc galaxy. One model is a violent merger creating a thick disc, and a Gaia-Enceladus/Sausage like remnant. The other is a highly stable disc which we use to illustrate how the improved phase space resolution allows us to better examine the formation and evolution of structures that have been observed in small, local volumes in the Milky Way, such as the $z-v_z$ phase spiral and clustering in the $v_{\mathrm{R}}-v_ϕ$ plane when compared to prev…
Signatures of vertical disequilibrium have been observed across the Milky Way's disk. These signatures manifest locally as unmixed phase-spirals in $z$--$v_z$ space ("snails-in-phase") and globally as nonzero mean $z$ and $v_z$ which wraps around as a physical spiral across the $x$--$y$ plane ("snails-in-space"). We explore the connection between these local and global spirals through the example of a satellite perturbing a test-particle Milky Way (MW)-like disk. We anticipate our results to broadly apply to any vertical perturbation. Using a $z$--$v_z$ asymmetry metric we demonstrate that in …
A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. II. Phase Spirals in an Inhomogeneous Disk Galaxy with a Non-responsive Dark Matter Halo
