Abstract
Aims.We present an updated version of the semi-analytic Just-Jahrei (JJ) model of the Galactic disk and constrain its parameters in the Solar neighbourhood.Methods.The new features of the JJ model include a simple two-component gaseous disk, a star-formation rate (SFR) function of the thick disk that has been extended in time, and a correlation between the kinematics of molecular gas and thin-disk populations. Here, we study the vertical number density profiles andW-velocity distributions determined from ∼2 × 106local stars of the secondGaiadata release (DR2). We also investigate an apparent Hess diagram of theGaiaDR2 stars selected in a conic volume towards the Galactic poles. Using a stellar evolution library, we synthesise stellar populations with a four-slope broken power-law initial mass function, the SFR, and an age-metallicity relation. The latter is consistently derived with the observed metallicity distribution of the local Red Clump giants from the Apache Point Observatory Galactic Evolution Experiment (APOGEE). Working within a Bayesian approach, we sample the posterior probability distribution in a multidimensional parameter space using the Markov chain Monte Carlo method.Results.We find that the spatial distribution and motion of theGaiaDR2 stars imply two recent SF bursts centered at ages of ∼0.5 Gyr and ∼3 Gyr and characterised by a ∼30% and ∼55% SF enhancement, respectively, relative to a monotonously declining SFR continuum. The stellar populations associated with this SF excess are found to be dynamically hot for their age: they haveW-velocity dispersions of ∼12.5 km s−1and ∼26 km s−1. The new JJ model is able to reproduce the local star counts with an accuracy of ∼5%.Conclusions.UsingGaiaDR2 data, we self-consistently constrained 22 parameters of the updated JJ model. Our optimised model predicts two SF bursts within the last ∼4 Gyr, which may point to recent episodes of gas infall.
Highlights
Our current understanding of the morphology and kinematics of the Milky Way, and of the physical processes that shape its structure and govern its evolution, is deepening to a greater extent and faster than ever
We check the mean value of the Markov chain Monte Carlo (MCMC) acceptance fraction, af, which is a fraction of the proposed steps accepted by the MCMC routine
They are characterised by the local surface densities and scale heights that are adopted from McKee et al (2015) and Nakanishi & Sofue (2016), respectively
Summary
Our current understanding of the morphology and kinematics of the Milky Way, and of the physical processes that shape its structure and govern its evolution, is deepening to a greater extent and faster than ever. These kinematic signatures of the local stellar streams and moving groups are qualitatively reproduced in N-body simulations with a transient spiral structure in the presence of a bar (Hunt et al 2019; Khanna et al 2019) Another example is a newly discovered ‘phase spiral’ in the (Vz,z) space, whose origin may be related to external perturbations (Bland-Hawthorn et al 2019) or the secular disk evolution (Khoperskov et al 2019). These kinematic features give us strong evidence of ongoing phase mixing processes in the present-day Galaxy, implying that the latter is currently out of an equilibrium state. The authors found a wave-like planeasymmetric pattern both in star counts and stellar kinematics; as they are similar for all stellar populations, these waves may be a disk response to the recent interaction with a satellite since such disk-satellite interactions have been predicted to excite bending and breathing modes in the disk (Widrow et al 2014)
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