[Paper Review] Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF

T. Bensby, Jennifer C. Yee+21 more|CaltechAUTHORS (California Institute of Technology)|Nov 29, 2012

Stellar, planetary, and galactic studiesPhysics and Astronomy95 references206 citations

TL;DR

This study analyzes high-resolution spectra of 58 microlensed dwarf and subgiant stars in the Galactic bulge to investigate its chemical evolution. It reveals a wide metallicity distribution from [Fe/H] = −1.9 to +0.6, a complex MDF with multiple components, and a bimodal age distribution: metal-poor stars are old (10–12 Gyr), while metal-rich stars span ages from 2 to 12 Gyr, indicating a complex formation history influenced by the Galactic bar and multiple stellar populations.

ABSTRACT

Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. [ABRIDGED], we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) The metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H]=-1.9 to +0.6; (ii) The dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. [ABRIDGED]; (iii) The stars with [Fe/H]-0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4-5 Gyr and a tail towards higher ages; (v) There are indications in the [alpha/Fe] - [Fe/H] that the "knee" occurs around [Fe/H] = -0.3 to -0.2, which is a slightly higher metallicity as compared to the "knee" for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.

Motivation & Objective

• To determine the chemical and age distribution of stars in the Galactic bulge using microlensed dwarfs and subgiants.
• To resolve the long-standing ambiguity in the metallicity distribution function (MDF) of the bulge, particularly the apparent paucity of solar-metallicity stars in earlier studies.
• To investigate the formation history of the bulge by analyzing elemental abundances and age distributions in a large, homogeneous sample.
• To test whether the observed abundance patterns can be explained by standard stellar population models or require additional components such as a bar or multiple populations.
• To compare the bulge's chemical evolution with that of the local thin and thick disks, especially the [α/Fe]–[Fe/H] trends and the location of the 'knee'.

Proposed method

• High-resolution spectroscopy was obtained using the MIKE and HIRES instruments on the Magellan Clay and Keck I telescopes.
• Stellar parameters (Teff, log g, [Fe/H], [α/Fe]) were derived using synthetic spectrum fitting with the MARCS model atmosphere grid.
• Abundance analysis was performed using spectral synthesis techniques to measure individual element abundances, including α-elements (O, Mg, Si, Ca, Ti).
• Stellar ages were estimated using the Yonsei-Yale (Yi et al. 2001) isochrones, calibrated with the observed [Fe/H] and [α/Fe] values.
• The metallicity distribution function (MDF) was constructed from the full sample of 58 stars, with careful attention to selection and sampling biases.
• The [α/Fe]–[Fe/H] relation was analyzed to infer the star formation timescale and chemical enrichment history of the bulge.

Experimental results

Research questions

• RQ1 What is the true shape of the metallicity distribution function (MDF) in the Galactic bulge, and does it show evidence of multiple components?
• RQ2 What is the age distribution of bulge stars across different metallicities, and how does it vary with [Fe/H]?
• RQ3 How does the [α/Fe]–[Fe/H] trend in the bulge compare to that of the local thin and thick disks, and what does this imply about its chemical enrichment history?
• RQ4 Can the observed abundance patterns be explained by standard stellar population models, or do they require additional components such as a bar or multiple formation episodes?
• RQ5 To what extent are the results affected by observational biases, such as overrepresentation of young, metal-rich stars in the microlensing sample?

Key findings

• The metallicity distribution spans from [Fe/H] = −1.9 to +0.6, with a wide and complex shape that no longer shows a clear dip at solar metallicity, indicating a multi-component MDF.
• Stars with [Fe/H] ≤ −0.1 are predominantly old, with ages between 10 and 12 Gyr, consistent with a metal-poor, early-formed population.
• Metal-rich stars ([Fe/H] ≥ −0.1) exhibit a broad age distribution from 2 to 12 Gyr, peaking around 4–5 Gyr, suggesting ongoing or episodic star formation.
• The [α/Fe]–[Fe/H] trend shows the 'knee' at [Fe/H] ≈ −0.3 to −0.2, indicating a faster chemical enrichment timescale in the metal-poor bulge compared to the local thick disk.
• The abundance patterns of metal-rich bulge stars are similar to those in the local thin and thick disks, supporting the idea that the bulge may be a conglomeration of disk populations influenced by the bar.
• Sampling biases and helium enrichment models cannot fully explain the observed fraction of young, metal-rich stars, suggesting intrinsic complexity in the bulge's formation history.