On the relation between metallicity and RGB color in HST/ACS data^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam Germany
e-mail: dstreich@aip.de
² Department of Physics & Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA

Received: 19 December 2012
Accepted: 10 January 2014

Abstract

Context. The determination of stellar metallicity and its gradient in external galaxies is a difficult task, but crucial for the understanding of galaxy formation and evolution.

Aims. The color of the Red Giant Branch (RGB) can be used to determine metallicities of stellar populations that have only shallow photometry. We will quantify the relation between metallicity and color in the widely used HST/ACS filters F606W and F814W.

Methods. We use a sample of globular clusters from the ACS Globular Cluster Survey and measure their RGB color at given absolute magnitudes to derive the color–metallicity relation. We especially investigate the scatter and the uncertainties in this relation and show its limitations.

Results. There is a clear relation between metallicity and RGB color. A comparison with isochrones shows reasonably good agreement with BaSTI models, a small offset to Dartmouth models, and a larger offset to Padua models.

Conclusions. Even for the best globular cluster data available, the metallicity of a simple stellar population can be determined from the RGB alone only with an accuracy of 0.3 dex for [M/H]  ≲ −1, and 0.15 dex for [M/H]  ≳ −1. For mixed populations, as they are observed in external galaxies, the uncertainties will be even larger due to uncertainties in extinction, age, etc. Therefore caution is necessary when interpreting photometric metallicities.