2 research outputs found
Resonance sweeping by a decelerating Galactic bar
We provide the first quantitative evidence for the deceleration of the
Galactic bar from local stellar kinematics in agreement with dynamical friction
by a typical dark matter halo. The kinematic response of the stellar disk to a
decelerating bar is studied using secular perturbation theory and test particle
simulations. We show that the velocity distribution at any point in the disk
affected by a naturally slowing bar is qualitatively different from that
perturbed by a steadily rotating bar with the same current pattern speed
and amplitude. When the bar slows down, its resonances sweep
through phase space, trapping and dragging along a portion of previously free
orbits. This enhances occupation on resonances, but also changes the
distribution of stars within the resonance. Due to the accumulation of orbits
near the boundary of the resonance, the decelerating bar model reproduces with
its corotation resonance the offset and strength of the Hercules stream in the
local - plane and the double-peaked structure of mean in
the - plane. At resonances other than the corotation, resonant
dragging by a slowing bar is associated with a continuing increase in radial
action, leading to multiple resonance ridges in the action plane as identified
in the Gaia data. This work shows models using a constant bar pattern speed
likely lead to qualitatively wrong conclusions. Most importantly we provide a
quantitative estimate of the current slowing rate of the bar
with additional systematic uncertainty arising from unmodeled impacts of e.g.
spiral arms.Comment: 20 pages, 23 figures. Accepted for publication in MNRAS after 1st
revision. Improved quantification of bar slowing rate. Conclusions unchange
The Prince and the Pauper: Evidence for the early high-redshift formation of the Galactic -poor disc population
Context. The presence of [/Fe]-[Fe/H] bi-modality in the Milky Way
disc has animated the Galactic archaeology community since more than two
decades. Aims. Our goal is to investigate the chemical, temporal, and
kinematical structure of the Galactic discs using abundances, kinematics, and
ages derived self-consistently with the new Bayesian framework SAPP. Methods.
We employ the public Gaia-ESO spectra, as well as Gaia EDR3 astrometry and
photometry. Stellar parameters and chemical abundances are determined for 13
426 stars using NLTE models of synthetic spectra. Ages are derived for a
sub-sample of 2 898 stars, including subgiants and main-sequence stars. The
sample probes a large range of Galactocentric radii, 3 to 12 kpc, and
extends out of the disc plane to 2 kpc. Results. Our new data confirm the
known bi-modality in the [Fe/H] - [/Fe] space, which is often viewed as
the manifestation of the chemical thin and thick discs. The over-densities
significantly overlap in metallicity, age, and kinematics, and none of these is
a sufficient criterion for distinguishing between the two disc populations.
Different from previous studies, we find that the -poor disc population
has a very extended [Fe/H] distribution and contains 20 old stars
with ages of up to 11 Gyr. Conclusions. Our results suggest that the
Galactic thin disc was in place early, at look-back times corresponding to
redshifts z 2 or more. At ages 9 to 11 Gyr, the two disc
structures shared a period of co-evolution. Our data can be understood within
the clumpy disc formation scenario that does not require a pre-existing thick
disc to initiate a formation of the thin disc. We anticipate that a similar
evolution can be realised in cosmological simulations of galaxy formation.Comment: 14 pages, 10 figures, re-submitted to A&