The Metallicity Gradient of the Thick Disk Based on Red Horizontal Branch Stars From SDSS DR8

TL;DR

This study uses SDSS DR8 data to analyze the metallicity gradient in the Milky Way's thick disk, revealing a significant vertical gradient and minor radial variation, challenging previous assumptions of negligible gradients.

Contribution

It provides the first detailed measurement of the thick disk's metallicity gradient using a large sample of red horizontal branch stars from SDSS DR8.

Findings

Vertical metallicity gradient of -0.12 dex/kpc in the thick disk.

Detection of a minor radial metallicity gradient in the thick disk.

Evidence of a vertical gradient variation across different galactic directions.

Abstract

Based on SDSS DR8, we have selected a sample of 1728 red horizontal branch stars with |Z|<3 kpc by using a color-metallicity relation and stellar parameters. The sample stars clearly trace a typical thick disk population with peaks at |Z|=1.26 kpc and [Fe/H]=-0.54. The vertical metallicity gradient of the thick disk is estimated in two ways. One is a fit to the Gaussian peaks of the metallicity histograms of the thick disk by subtracting minor contributions from the thin disk and the inner halo based on the Besancon Galaxy model. The resulting gradient is -0.12+-0.01 dex/kpc for 0.5 <|Z| <3 kpc. The other method is to linearly fit the data based on stars with 1 <|Z| <3 kpc being the main component of the thick disk. Five subgroups are then selected in different directions in the X-|Z| plane to investigate the difference in the vertical metallicity gradient between the Galactocenter and anti-Galactocenter directions. We found that a vertical gradient o-0.22+-0.07 dex/kpc is detected for five directions except for one involving the pollution of stars from the bulge. The results indicate that the vertical gradient is dominant, but a radial gradient has a minor contribution for the thick disk population represented by RHB stars with 1 <|Z| <3 kpc. The present work strongly suggests the existence of a metallicity gradient in the thick disk, which is thought to be negligible in most previous works in the literature.