F Turnoff Distribution in the Galactic Halo Using Globular Clusters as Proxies

TL;DR

This study characterizes the absolute magnitude distribution of F turnoff stars in the Milky Way halo using SDSS data from eleven globular clusters, providing correction functions and insights into their ages and metallicities.

Contribution

The paper presents a universal Gaussian model for F turnoff star magnitudes and a correction function for observational effects, enhancing stellar population analysis in the Galactic halo.

Findings

F turnoff star magnitude distribution is consistent across clusters.

A correction function accounts for magnitude-dependent contamination.

Cluster populations align with the Milky Way Age-Metallicity Relationship.

Abstract

F turnoff stars are important tools for studying Galactic halo substructure because they are plentiful, luminous, and can be easily selected by their photometric colors from large surveys such as the Sloan Digital Sky Survey (SDSS). We describe the absolute magnitude distribution of color-selected F turnoff stars, as measured from SDSS data, for eleven globular clusters in the Milky Way halo. We find that the M_g distribution of turnoff stars is intrinsically the same for all clusters studied, and is well fit by two half Gaussian functions, centered at mu=4.18, with a bright-side sigma=0.36, and with a faint-side sigma=0.76. However, the color errors and detection efficiencies cause the observed sigma of the faint-side Gaussian to change with magnitude due to contamination from redder main sequence stars (40% at 21st magnitude). We present a function that will correct for this magnitude-dependent change in selected stellar populations, when calculating stellar density from color-selected turnoff stars. We also present a consistent set of distances, ages and metallicities for eleven clusters in the SDSS Data Release 7. We calculate a linear correction function to Padova isochrones so that they are consistent with SDSS globular cluster data from previous papers. We show that our cluster population falls along the Milky Way Age-Metallicity Relationship (AMR), and further find that isochrones for stellar populations on the AMR have very similar turnoffs; increasing metallicity and decreasing age conspire to produce similar turnoff magnitudes and colors for all old clusters that lie on the AMR.