Age Demographics of the Milky Way Disk and Bulge

Abstract

We use the extensive $Gaia$ Data Release 2 set of Long Period Variables to select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disk and bulge as a function of period. We find the morphology of both components evolves as a function of stellar age/chemistry with the stellar disk being stubby at old ages, becoming progressively thinner and more radially extended at younger stellar ages, consistent with the picture of inside-out and upside-down formation of the Milky Way's disk. We see evidence of a perturbed disk, with large-scale stellar over-densities visible both in and away from the stellar plane. We find the bulge is well modelled by a triaxial boxy distribution with an axis ratio of $\sim [1:0.4:0.3]$. The oldest of the Miras ($\sim$ 9-10 Gyr) show little bar-like morphology, whilst the younger stars appear inclined at a viewing angle of $\sim 21^{\circ }$ to the Sun-Galactic Centre line. This suggests that bar formation and buckling took place 8-9 Gyr ago, with the older Miras being hot enough to avoid being trapped by the growing bar. We find the youngest Miras to exhibit a strong peanut morphology, bearing the characteristic X-shape of an inclined bar structure.

MNRAS, submitted

References in corpus (24)

• The Gaia mission
• The Structure of the Milky Way's Bar Outside the Bulge
• Rings and Radial Waves in the Disk of the Milky Way
• On the Formation of Galactic Thick Disks
• Near-Infrared Photometry of Carbon Stars
• Modelling the Galactic bar using OGLE-II Red Clump Giant Stars
• The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields
• The Gaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge
• The Ages of the Thin Disk, Thick Disk, and the Halo from Nearby White Dwarfs
• Tracing the long bar with red-clump giants
• The Long Bar in the Milky Way. Corroboration of an old hypothesis
• Clouds, Streams and Bridges. Redrawing the blueprint of the Magellanic System with Gaia DR1
• The long Galactic bar as seen by UKIDSS Galactic Plane Survey
• A radial age gradient in the geometrically thick disk of the Milky Way
• The Pristine Inner Galaxy Survey (PIGS) I: Tracing the kinematics of metal-poor stars in the Galactic bulge
• A parametric description of the 3D structure of the Galactic Bar/Bulge using the VVV survey
• Carbon-Rich Mira Variables: Kinematics and Absolute Magnitudes
• Transverse kinematics of the Galactic bar-bulge from VVV and Gaia
• Large Magellanic Cloud Near-Infrared Synoptic Survey. V. Period-Luminosity Relations of Miras
• The Clouds are breaking: tracing the Magellanic system with Gaia DR1 Mira variables
• On the 3 dimensional structure of edge-on disk galaxies
• On the Oosterhoff dichotomy in the Galactic bulge: I. spatial distribution
• Distribution of red clump stars does not support the X-shaped Galactic bulge
• The Ages, Masses, Evolution and Kinematics of Mira variable

Cited by in corpus (29)

• From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity
• The Bulge Radial Velocity Assay for RR Lyrae stars (BRAVA-RR) DR2: a Bimodal Bulge?
• The OGLE Collection of Variable Stars. Nearly 66,000 Mira stars in the Milky Way
• 3D hydrodynamic simulations for the formation of the Local Group satellite planes
• The period--luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for $H_0$
• Mira variables in the Milky Way's nuclear stellar disc: discovery and classification
• A three-dimensional map of the Milky Way using 66,000 Mira variable stars
• Simulating the Legacy Survey of Space and Time stellar content with TRILEGAL
• Hunting for C-rich long-period variable stars in the Milky Way's bar-bulge using unsupervised classification of Gaia BP/RP spectra
• The age distribution of stars in the Milky Way bulge
• The period-age relation of long-period variables
• The Galactic neutron star population I - an extragalactic view of the Milky Way and the implications for fast radio bursts
• A kinematic calibration of the O-rich Mira variable period-age relation from Gaia
• The VVV survey: Long-period variable stars I. Photometric catalog of ten VVV/OGLE tiles
• Difference in chemical composition between the bright and faint red clump stars in the Milky Way bulge
• Deceleration of kicked objects due to the Galactic potential
• Age-morphology dependence of the Milky Way boxy/peanut bulge seen in Mira variables
• On the absence of backsplash analogues to NGC 3109 in the $\Lambda$CDM framework
• 3D Parameter Maps of Red Clump Stars in the Milky Way -- Absolute Magnitudes and Intrinsic Colors
• The small boxy/peanut structure of the Milky Way traced by old stars
• Deciphering the Milky Way disc formation time encrypted in the bar chrono-kinematics
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data II. Chrono-chemo-kinematics of the disc
• The Galactic Bulge exploration IV.: RR~Lyrae stars as traces of the Galactic bar -- 3D and 5D analysis, extinction variation
• Radial and azimuthal gradients of the moving groups in Gaia DR3: The slow/fast bar degeneracy problem
• Predicted Trends in Milky Way Bulge Proper Motion Rotation Curves: future Prospects for HST and LSST
• Unveiling VVV/WISE Mira variables on the far side of the Galactic disk: Distances, kinematics and a new extinction law
• An MCMC Approach to the Three-dimensional Structure of the Milky Way Bulge using OGLE-IV $\delta$ Scuti Stars
• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
• Age distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling