The Controversial Star-Formation History and Helium Enrichment of the Milky Way Bulge

Abstract

The stellar population of the Milky Way bulge is thoroughly studied, with a plethora of measurements from virtually the full suite of instruments available to astronomers. It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history. Cosmological models predict the bulge to host the Galaxy's oldest stars for [Fe/H]$\lesssim -1$, and this is demonstrated by RR Lyrae stars and globular cluster observations. There is consensus that bulge stars with [Fe/H]$\lesssim 0$ are older than $t\approx 10$ Gyr. However, at super-solar metallicity, there is a substantial unresolved discrepancy. Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch, the abundances of asymptotic giant branch stars, the period distribution of Mira variables, the chemistry and central-star masses of planetary nebulae, all suggest a substantial intermediate-age population ($t\approx 3$ Gyr). This is in conflict with predictions from cosmologically-motivated chemical evolution models and photometric studies of the main-sequence turnoff region, which both suggest virtually no stars younger than $t\approx 8$ Gyr. A possible resolution to this conflict is enhanced helium-enrichment, as this would shift nearly all of the age estimates in the direction of decreasing discrepancy. Enhanced helium-enrichment is also arguably suggested by measurements of the red giant branch bump and the R-parameter.

9 pages, 2 figures, contributed article to PASA special review issue on the Galactic Bulge, edited by Beatriz Barbuy, accepted for publication in PASA

References in corpus (16)

• Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology
• Stellar Proper Motions in the Galactic Bulge from deep HST ACS/WFC Photometry
• Light, Alpha, and Fe-Peak Element Abundances in the Galactic Bulge
• Helium enrichment and Carbon-star Production in Metal-rich Populations
• Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5
• Elemental abundances of Galactic bulge planetary nebulae from optical recombination lines
• The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
• High resolution optical spectroscopy of Praesepe white dwarfs
• Oxygen enrichment in carbon-rich planetary nebulae
• Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae stars from their Lightcurves
• Diagnostics of Stellar Modelling from Spectroscopy and Photometry of Globular Clusters
• A high-velocity bulge RR Lyrae variable on a halo-like orbit
• NGC 6522: A typical globular cluster in the Galactic bulge without signatures of rapidly rotating Population III stars
• On the Correlation Between Metallicity and the X-Shaped Morphology of the Milky Way Bulge
• A new method for deriving the stellar birth function of resolved stellar populations
• Red Giant Branch Bump Star Counts in Data and Stellar Models

Cited by in corpus (27)

• Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way
• Age dissection of the Milky Way discs: red giants in the Kepler field
• PLATO as it is: a legacy mission for Galactic archaeology
• Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes
• The age of the young bulge-like population in the stellar system Terzan5: linking the Galactic bulge to the high-z Universe
• Phylogeny of the Milky Way's inner disk and bulge populations: Implications for gas accretion, (the lack of) inside-out thick disk formation, and quenching
• The large-scale ionization cones in the Galaxy
• The WFC3 Galactic Bulge Treasury Program: Relative Ages of Bulge Stars of High and Low Metallicity
• Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations
• Exploring the stellar age distribution of the Milky Way Bulge using APOGEE
• Hiding its age: the case for a younger bulge
• Millisecond Pulsars from Accretion Induced Collapse as the Origin of the Galactic Centre Gamma-ray Excess Signal
• Quantifying radial migration in the Milky Way: Inefficient over short timescales but essential to the very outer disc beyond ~15 kpc
• Mapping the stellar age of the Milky Way bulge with the VVV. I. The method
• The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
• Weighing the two stellar components of the Galactic Bulge
• Discovery of carbon-rich Miras in the Galactic bulge
• Hunting for C-rich long-period variable stars in the Milky Way's bar-bulge using unsupervised classification of Gaia BP/RP spectra
• HERBS I: Metallicity and alpha enhancement along the Galactic bulge minor axis
• Structural Analogs of the Milky Way Galaxy: Stellar Populations in the Boxy Bulges of NGC 4565 and NGC 5746
• A kinematic calibration of the O-rich Mira variable period-age relation from Gaia
• The Mira-based distance to the Galactic centre
• HERBS II: Detailed chemical compositions of Galactic bulge stars
• A case against an X-shaped structure in the Milky Way young bulge
• Influence of the Galactic bar on the kinematics of the disc stars with Gaia EDR3 data
• Morphologies and Central Stars of Planetary Nebulae in the Galactic bulge from VLT, $\text{HST}$ and Pan-STARRS imaging
• Gaseous abundances in planetary nebulae: What have we learned in the past five years?