ALBERT

Description: We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, Rb, measurements fluctuate on a dynamical time-scale by up to 100 per cent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 per cent, correlating with Rb. The Tremaine–Weinberg method estimates of the bars’ instantaneous pattern speeds show variations around the mean of up to $sim !20{{ m per cent}}$, typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range ∼60–200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 per cent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by $sim !15{{ m per cent}}$ and those of SBbc types by $sim !55{{ m per cent}}$. Consequently, bars longer than their corotation radius reported in the literature, dubbed ‘ultrafast bars’, may simply correspond to the largest biases. Given that the Scutum–Centaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1–1.5 kpc, while its present pattern speed may be 5–10 $ m km s^{-1} kpc^{-1}$ smaller than its time-averaged value.

Description: We have observed an N -body/smoothed particle hydrodynamics simulation of a Milky Way-like barred spiral galaxy. We present a simple method that samples N -body model particles into mock Gaia stellar observations and takes into account stellar populations, dust extinction and Gaia ’ s science performance estimates. We examine the kinematics of stars with V  ≤ 16 mag around a nearby spiral arm at a similar position to the Perseus arm at three lines of sight in the disc plane; ( l , b ) = (90, 0), (120, 0) and (150, 0) deg. We find that the structure of the peculiar kinematics around the corotating spiral arm, which is found in Kawata et al. ( 2014b ), is still visible in the observational data expected to be produced by Gaia , despite the dust extinction and expected observational errors of Gaia . These observable kinematic signatures will enable testing whether the Perseus arm of the Milky Way is similar to the corotating spiral arms commonly seen in N -body simulations.

Description: We estimate the size and distribution of the parent populations for the six largest (at least 20 stars in the solar neighbourhood) chemical groups identified in the chemical tagging experiment by Mitschang et al. 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 neighbourhood and where orbits have apocentre 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 disc 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 disc groups exhibit both wide angular momentum and eccentricity distributions implying that both heating and radial migration has taken place.

Description: Using the RAdial Velocity Experiment fourth data release (RAVE DR4), and a new metallicity calibration that will be also taken into account in the future RAVE DR5, we investigate the existence and the properties of supersolar metallicity stars ([ M /H]  +0.1 dex) in the sample, and in particular in the solar neighbourhood. We find that RAVE is rich in supersolar metallicity stars, and that the local metallicity distribution function declines remarkably slowly up to +0.4 dex. Our results show that the kinematics and height distributions of the supersolar metallicity stars are identical to those of the [ M /H]  0 thin-disc giants that we presume were locally manufactured. The eccentricities of the supersolar metallicity stars indicate that half of them are on a roughly circular orbit ( e  ≤ 0.15), so under the assumption that the metallicity of the interstellar medium at a given radius never decreases with time, they must have increased their angular momenta by scattering at corotation resonances of spiral arms from regions far inside the solar annulus. The likelihood that a star will migrate radially does not seem to decrease significantly with increasing amplitude of vertical oscillations within range of oscillation amplitudes encountered in the disc.

Description: 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 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 New Catalogue of Optically Visible Open Clusters and Candidates , and a sample of 500 giant stars observed by Apache Point Observatory Galactic Evolution Experiment, we find p  = 23.0 ± 0.5 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 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 disc stars with spectroscopic information will become available thanks to future surveys such as 4MOST.

Description: By selecting in the Radial Velocity Experiment-fourth data release (RAVE-DR4) survey the stars located between 1 and 2 kpc above the Galactic plane, we question the consistency of the simplest three-component model (thin disc, thick disc and halo) for the Milky Way. We confirm that the metallicity and azimuthal velocity distribution functions of the thick disc are not Gaussian. In particular, we find that the thick disc has an extended metallicity tail going at least down to [ M/H ] = -2 dex, contributing roughly 3 per cent of the entire thick disc population and having a shorter scalelength compared to the canonical thick disc. The mean azimuthal velocity of these metal-poor stars allows us to estimate the correlation between the metallicity ([ M/H ]) and the orbital velocity ( V ), which is an important constraint on the formation mechanisms of the Galactic thick disc. Given our simple approach, we find V /[ M/H ] 50 km s –1 dex –1 , which is in very good agreement with previous literature values. We complete the study with a brief discussion on the implications of the formation scenarios for the thick disc and suggest that given the above-mentioned characteristics, a thick disc mainly formed by radial migration mechanisms seems unlikely.

Description: We use a high-resolution cosmological zoom simulation of a Milky Way-sized halo to study the observable features in velocity and metallicity space associated with the dynamical influence of spiral arms. For the first time, we demonstrate that spiral arms, that form in a disc in a fully cosmological environment with realistic galaxy formation physics, drive large-scale systematic streaming motions. In particular, on the trailing edge of the spiral arms the peculiar galactocentric radial and azimuthal velocity field is directed radially outward and azimuthally backward, whereas it is radially inward and azimuthally forward on the leading edge. Owing to the negative radial metallicity gradient, this systematic motion drives, at a given radius, an azimuthal variation in the residual metallicity that is characterized by a metal-rich trailing edge and a metal-poor leading edge. We show that these signatures are theoretically observable in external galaxies with integral field unit instruments such as VLT/MUSE, and if detected, would provide evidence for large-scale systematic radial migration driven by spiral arms.

Description: We study the relation between stellar ages and vertical velocity dispersion (the age–velocity relation, or AVR) in a sample of seven simulated disc galaxies. In our simulations, the shape of the AVR for stars younger than 9 Gyr depends strongly on the merger history at low redshift, with even 1:10–1:15 mergers being able to create jumps in the AVR (although these jumps might not be detectable if the errors on stellar ages are of the order of 30 per cent). For galaxies with a quiescent history at low redshift, we find that the vertical velocity dispersion rises smoothly for ages up to 8–9 Gyr, following a power law with a slope of ~0.5, similar to what is observed in the solar neighbourhood by the Geneva-Copenhagen Survey. For these galaxies, we show that the slope of the AVR is not imprinted at birth, but is the result of subsequent heating. By contrast, in all our simulations, the oldest stars form a significantly different population, with a high velocity dispersion. These stars are usually born kinematically hot in a turbulent phase of intense mergers at high redshift, and also include some stars accreted from satellites. This maximum in z is strongly decreased when age errors are included, suggesting that observations can easily miss such a jump with the current accuracy of age measurements.

Description: We study seven simulated disc galaxies, three with a quiescent merger history, and four with mergers in their last 9 Gyr of evolution. We compare their structure at z  = 0 by decomposing them into ‘mono-age populations’ (MAPs) of stars within 500 Myr age bins. All studied galaxies undergo a phase of merging activity at high redshift, so that stars older than 9 Gyr are found in a centrally concentrated component, while younger stars are mostly found in discs. We find that most MAPs have simple exponential radial and vertical density profiles, with a scaleheight that typically increases with age. Because a large range of merger histories can create populations with simple structures, this suggests that the simplicity of the structure of mono-abundance populations observed in the Milky Way by Bovy et al. is not necessarily a direct indicator of a quiescent history for the Milky Way. Similarly, the anticorrelation between scalelength and scaleheight does not necessarily imply a merger-free history. However, mergers produce discontinuities between thin and thick disc components, and jumps in the age–velocity relation. The absence of a structural discontinuity between thin and thick disc observed in the Milky Way would seem to be a good indicator that no merger with a mass ratio larger than 1:15–1:10 occurred in the last 9 Gyr. Mergers at higher redshift might nevertheless be necessary to produce the thickest, hottest components of the Milky Way's disc.

Description: The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using 72 365 red-clump stars, we examine the mean velocity components in 3D between 6 〈 R  〈 10 kpc and –2 〈 Z  〈 2 kpc, concentrating on north–south differences. Simple parametric fits to the ( R , Z ) trends for V and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V R , finding that the gradient is marked below the plane (〈 V R 〉/ R  = –8 km s –1 kpc –1 for Z  〈 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, V Z , also shows clear, large-amplitude (| V Z | = 17 km s –1 ) structure, with indications of a rarefaction–compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north–south differences in V R and V Z along the line of sight, with the V R estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.