[Paper Review] The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo

T. Schoerck, N. Christlieb+23 more|arXiv (Cornell University)|Sep 6, 2008

Stellar, planetary, and galactic studiesPhysics and Astronomy1 references75 citations

TL;DR

This study presents the metallicity distribution function (MDF) of the Galactic halo using 1,638 metal-poor stars from the Hamburg/ESO Survey, correcting for observational biases. It finds a sharp drop in the MDF at [Fe/H] ≈ −3.6, best reproduced only by a model with a critical metallicity for low-mass star formation of Z_cr = 10⁻³.⁴ Z☉, suggesting a fundamental threshold in early star formation.

ABSTRACT

We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". [...] We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant. [ABSTRACT ABRIDGED]

Motivation & Objective

• To determine the metallicity distribution function (MDF) of the Galactic halo using a large, spectroscopically selected sample of metal-poor stars.
• To correct for observational biases introduced by the 'best and brightest stars first' spectroscopic follow-up strategy in the HES survey.
• To compare the observed MDF with theoretical models of Galactic chemical evolution, particularly focusing on the low-metallicity tail.
• To assess the consistency of the halo MDF with those of globular clusters and dSph satellites, accounting for survey selection functions.
• To evaluate the statistical significance of the observed drop in the MDF at [Fe/H] ≈ −3.6 and its implications for early star formation thresholds.

Proposed method

• Selected 1,638 metal-poor stars from the Hamburg/ESO Survey (HES), a wide-area objective-prism survey.
• Applied moderate-resolution spectroscopy (R ≈ 2000) to determine [Fe/H] values, validated against high-resolution (R > 20,000) abundance analyses.
• Corrected for selection biases using a detailed selection function model of the HES, accounting for magnitude and color selection effects.
• Compared the corrected HES MDF with theoretical MDFs from Galactic chemical evolution models, including those with varying critical metallicities (Z_cr).
• Performed Kolmogorov-Smirnov (KS) tests to statistically assess the similarity between observed and predicted MDFs.
• Evaluated consistency with dSph and globular cluster MDFs after applying the same selection function corrections.

Experimental results

Research questions

• RQ1 What is the shape of the Galactic halo metallicity distribution function (MDF) as derived from the HES sample, after correcting for observational biases?
• RQ2 Which model of Galactic chemical evolution best reproduces the observed MDF, particularly the sharp drop at [Fe/H] ≈ −3.6?
• RQ3 How significant is the discrepancy between the observed number of stars at [Fe/H] < −3.6 and model predictions?
• RQ4 To what extent do the MDFs of Galactic globular clusters and dSph satellites agree with the halo MDF when selection effects are accounted for?
• RQ5 What does the current sample imply for the existence of a tail extending to [Fe/H] ≈ −5.5, and what future surveys are needed to confirm it?

Key findings

• The HES MDF shows a sharp drop at [Fe/H] ≈ −3.6, which is not reproduced by most Galactic chemical evolution models.
• Only the model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z_cr = 10⁻³.⁴ Z☉ reproduces the observed drop in the MDF.
• The observed number of stars at [Fe/H] < −3.6 (two stars) is significantly lower than predicted by models (about ten stars), with low KS-test probabilities indicating statistical significance.
• The MDF of the Galactic halo shows qualitative agreement with dSph and globular cluster MDFs after selection function corrections, but statistical tests reveal highly significant differences.
• The current sample contains no stars with [Fe/H] < −4.2, though three additional stars in this range are known from the literature, indicating the existence of such objects.
• Future deeper surveys, such as the Southern Sky Survey and LAMOST, are expected to provide larger, more complete, and unbiased samples for robust comparison with theoretical MDFs.