Radial Mixing due to Spiral-Bar Resonance Overlap: Implications to the Milky Way

We have recently identified a previously unknown radial migration mechanism resulting from the overlap of spiral and bar resonances in galactic discs (Minchev & Famaey 2010, Minchev et al. 2010). This new mechanism is much more efficient than mixing by transient spirals and its presence is unavoidable in all barred galaxies, such as our own Milky Way. The consequences of this are a strong flattening in the metallicity gradient in the disc, an extended disc profile, and the formation of a thick disc component, all taking place in only a couple of Gyr. This timescale is drastically shorter than previously expected and thus can put strong constraints on the longevity, strength and pattern speeds of the Galactic bar and Spiral Structure.Comment: 4 pages, 2 figures, presented at ELSA 2010: Gaia, at the frontiers of astrometry, 7-11 June 2010, S\`evres, Paris; To published in EAS Series; Proceedings editors: C. Turon, F. Arenou & F. Meynadie

Radial migration in galactic disks caused by resonance overlap of multiple patterns: Self-consistent simulations

We have recently identified a new radial migration mechanism resulting from the overlap of spiral and bar resonances in galactic disks. Here we confirm the efficiency of this mechanism in fully self-consistent, Tree-SPH simulations, as well as high-resolution pure N-body simulations. In all barred cases we clearly identify the effect of spiral-bar resonance overlap by measuring a bimodality in the changes of angular momentum in the disk, dL, whose maxima are near the bar's corotation and outer Lindblad resonance. This contrasts with the smooth distribution of dL for a simulation with no stable bar present, where strong radial migration is induced by multiple spirals. The presence of a disk gaseous component appears to increase the rate of angular momentum exchange by about 20%. The efficiency of this mechanism is such that galactic stellar disks can extend to over 10 scale-lengths within 1-3 Gyr in both Milky Way size and low-mass galaxies (circular velocity ~100 km/s). We also show that metallicity gradients can flatten in less than 1 Gyr rendering mixing in barred galaxies an order of magnitude more efficient than previously thought.Comment: replaced with accepted version: 5 pages, 5 figures (one new figure added), minor change

A New Mechanism for Radial Migration in Galactic Disks: Spiral-Bar Resonance Overlap

While it has long been known that a large number of short-lived transient spirals can cause stellar migration, here we report that another mechanism is also effective at mixing disks of barred galaxies. The resonance overlap of the bar and spiral structure induces a nonlinear response leading to a strong redistribution of angular momentum in the disk. We find that, depending on the amplitudes of the perturbers, the changes in angular momentum, dL, could occur up to an order of magnitude faster than in the case of recurrent spirals. The signature of this mechanism is a bimodality in dL with maxima near the bar's corotation and its outer Lindblad resonance; this is independent of the properties of the spiral structure. For parameters consistent with the Milky Way the disk mixes in about 3 Gyr and the stellar velocity dispersion increases with time in a manner roughly consistent with observations. This new mechanism could account for both the observed age-velocity relation and the absence of age-metallicity relation in the solar neighborhood. Spiral-bar interaction could also explain observations showing that strongly barred galaxies have weaker metallicity gradients than weakly barred or non-barred galaxies.Comment: 10 pages, 7 figures. Substantially expanded. Main results remain the same. Accepted for publication in Ap

A new method for estimating the pattern speed of spiral structure in the Milky Way

In the last few decades many efforts have been made to understand the effect of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of the fundamental parameters of the spiral structure is its angular velocity, or pattern speed $\Omega_p$, which determines the location of resonances in the disc and the spirals' radial extent. The most direct method for estimating the pattern speed relies on backward integration techniques, trying to locate the stellar birthplace of open clusters. Here we propose a new method based on the interaction between the spiral arms and the stars in the disc. Using a sample of around 500 open clusters from the {\it New Catalogue of Optically Visible Open Clusters and Candidates}, and a sample of 500 giant stars observed by APOGEE, we find $\Omega_p = 23.0\pm0.5$ km s$^{-1}$ kpc$^{-1}$, for a local standard of rest rotation $V_0=220$~km s$^{-1}$ and solar radius $R_0=8.0$~kpc. Exploring a range in $V_0$ and $R_0$ within the acceptable values, 200-240 km s$^{-1}$ and 7.5-8.5 kpc, respectively, results only in a small change in our estimate of $\Omega_p$, that is within the error. Our result is in close agreement with a number of studies which suggest values in the range 20-25 km s$^{-1}$ kpc$^{-1}$. An advantage of our method is that we do not need knowledge of the stellar age, unlike in the case of the birthplace method, which allows us to use data from large Galactic surveys. The precision of our method will be improved once larger samples of disk stars with spectroscopic information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA

The origin and orbit of the old, metal-rich, open cluster NGC 6791: Insights from kinematics

NGC 6791 is a unique stellar system among Galactic open clusters being at the same time one of the oldest open clusters and the most metal rich. Combination of its properties is puzzling and poses question of its origin. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outwards to its current location. In this work we study the cluster's orbit and investigate the possible migration processes which might have displaced NGC 6791 to its present-day position, under the assumption that it actually formed in the inner disk. To this aim we performed integrations of NGC 6791's orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and disk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state-of-the art initial conditions for NGC 6791, we calculate 1000 orbits back in time for about 1 Gyr turning on and off different non-axisymmetric components of the global potential. We then compare statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 10^4 test-particles originating close to the Galactic Center (having initial galocentric radii in the range of 3-5 kpc) and undergoing radial migration during 8 Gyr of forward integration. We find that a model which incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC 6791 from the inner disk (galocentric radii of 3-5 kpc) to its present-day location. Such a model can provide orbital parameters which are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low.Comment: 11 pages, 9 figures, 7 tables, accepted for publication in A&A || v2: minor changes to match the published versio

The Parent Populations of 6 groups identified from Chemical Tagging in the Solar neighborhood

We estimate the size and distribution of the parent populations for the 6 largest (at least 20 stars in the Solar neighborhood) chemical groups identified in the Chemical Tagging experiment by Mitschang et al.~2014. Stars in the abundance groups tend to lie near a boundary in angular momentum versus eccentricity space where the probability is highest for a star to be found in the Solar neighborhood and where orbits have apocenter approximately equal to the Sun's galactocentric radius. Assuming that the parent populations are uniformly distributed at all azimuthal angles in the Galaxy, we estimate that the parent populations of these abundance groups contain at least 200,000 members. The spread in angular momentum of the groups implies that the assumption of a uniform azimuthal distribution only fails for the two youngest groups and only for the highest angular momentum stars in them. The parent populations of three thin disk groups have narrow angular momentum distributions, but tails in the eccentricity and angular momentum distributions suggest that only a small fraction of stars have migrated and increased in eccentricity. In contrast, the parent populations of the thick disk groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.Comment: accepted for publication in MNRA

Evolution of Galactic Discs: Multiple Patterns, Radial Migration and Disc Outskirts

We investigate the evolution of galactic disks in N-body Tree-SPH simulations. We find that disks, initially truncated at three scale-lengths, can triple their radial extent, solely driven by secular evolution. Both Type I (single exponential) and Type II (down-turning) observed disk surface-brightness profiles can be explained by our findings. We relate these results to the strong angular momentum outward transfer, resulting from torques and radial migration associated with multiple patterns, such as central bars and spiral waves of different multiplicity. We show that even for stars ending up on cold orbits, the changes in angular momentum exhibit complex structure as a function of radius, unlike the expected effect of transient spirals alone. Focussing on one of our models, we find evidence for non-linear coupling among m=1, 2, 3 and 4 density waves, where m is the pattern multiplicity. We suggest that the naturally occurring larger resonance widths at galactic radii beyond four scale-lengths may have profound consequences on the formation and location of breaks in disk density profiles, provided spirals are present at such large distances. We also consider the effect of gas inflow and show that when in-plane smooth gas accretion of ~5 M_sun/yr is included, the outer disks become more unstable, leading to a strong increase in the stellar velocity dispersion. This, in turn, causes the formation of a Type III (up-turning) profile in the old stellar population. We propose that observations of Type III surface brightness profiles, combined with an up-turn in the stellar velocity dispersions beyond the disk break, could be a signature of ongoing gas-accretion. The results of this study suggest that disk outskirts comprised of stars migrated from the inner disk would have relatively large radial velocity dispersions, and significant thickness when seen edge-on. [Abridged]Comment: Replaced with accepted version. New Fig. 5 added, Section 10 decreased in size, old Fig. 17 removed. Conclusions remain the same. High-resolution version can be found at http://www.ivanminchev.co

Galactic Archaeology with CoRoT and APOGEE: Creating mock observations from a chemodynamical model

In a companion paper, we have presented the combined asteroseismic-spectroscopic dataset obtained from CoRoT lightcurves and APOGEE infra-red spectra for 678 solar-like oscillating red giants in two fields of the Galactic disc (CoRoGEE). We have measured chemical abundance patterns, distances, and ages of these field stars which are spread over a large radial range of the Milky Way's disc. Here we show how to simulate this dataset using a chemodynamical Galaxy model. We also demonstrate how the observation procedure influences the accuracy of our estimated ages.Comment: 5 pages, 6 figures. To appear in Astronomische Nachrichten, special issue "Reconstruction the Milky Way's History: Spectroscopic surveys, Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J. Montalb\'an, and M. Steffe

Structure in phase space associated with spiral and bar density waves in an N-body galactic disk

An N-body hybrid simulation, integrating both massive and tracer particles, of a Galactic disk is used to study the stellar phase space distribution or velocity distributions in different local neighborhoods. Pattern speeds identified in Fourier spectrograms suggest that two-armed and three-armed spiral density waves, a bar and a lopsided motion are coupled in this simulation, with resonances of one pattern lying near resonances of other patterns. We construct radial and tangential (uv) velocity distributions from particles in different local neighborhoods. More than one clump is common in these local velocity distributions regardless of the position in the disk. Features in the velocity distribution observed at one galactic radius are also seen in nearby neighborhoods (at larger and smaller radii) but with shifted mean v values. This is expected if the v velocity component of a clump sets the mean orbital galactic radius of its stars. We find that gaps in the velocity distribution are associated with the radii of kinks or discontinuities in the spiral arms. These gaps also seem to be associated with Lindblad resonances with spiral density waves and so denote boundaries between different dominant patterns in the disk. We discuss implications for interpretations of the Milky Way disk based on local velocity distributions. Velocity distributions created from regions just outside the bar's Outer Lindblad resonance and with the bar oriented at 45 degrees from the Sun-Galactic center line more closely resemble that seen in the solar neighborhood (triangular in shape at lower uv and with a Hercules like stream) when there is a strong nearby spiral arm, consistent with the observed Centaurus Arm tangent, just interior to the solar neighborhood.Comment: accepted for publication in MNRA

Empirical derivation of the metallicity evolution with time and radius using TNG50 Milky Way/Andromeda analogues

Recent works have used a linear birth metallicity gradient to estimate the evolution of the [Fe/H] profile in the Galactic disk over time, and infer stellar birth radii (R$_\text{birth}$) from [Fe/H] and age measurements. These estimates rely on the evolution of [Fe/H] at the Galactic center ([Fe/H](0, $\tau$)) and the birth metallicity gradient ($\nabla$[Fe/H]($\tau)$) over time -- quantities that are unknown and inferred under key assumptions. In this work, we use the sample of Milky Way/Andromeda analogues from the TNG50 simulation to investigate the ability to recover [Fe/H](R, $\tau$) and R$_\text{birth}$ in a variety of galaxies. Using stellar disk particles, we test the assumptions required in estimating R$_\text{birth}$, [Fe/H](0, $\tau$), and $\nabla$[Fe/H]($\tau)$ using recently proposed methods to understand when they are valid. We show that $\nabla$[Fe/H]($\tau)$ can be recovered in most galaxies to within 22% from the range in [Fe/H] across age, with better accuracy for more massive and stronger barred galaxies. We also find that the true central metallicity is unrepresentative of the genuine disk [Fe/H] profile; thus we propose to use a projected central metallicity instead. About half of the galaxies in our sample do not have a continuously enriching projected central metallicity, with a dilution in [Fe/H] correlating with mergers. Most importantly, galaxy-specific [Fe/H](R, $\tau$) can be constrained and confirmed by requiring the R$_\text{birth}$ distributions of mono-age, solar neighborhood populations to follow inside-out formation. We conclude that examining trends with R$_\text{birth}$ is valid for the Milky Way disk and similarly structured galaxies, where we expect R$_\text{birth}$ can be recovered to within 16% assuming today's measurement uncertainties in TNG50.Comment: Submitted to A&