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

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.

The GHOSTS survey – II. The diversity of halo colour and metallicity profiles of massive disc galaxies

“…However the metallicities obtained from RGB colours have large uncertainties. Streich et al (2014) estimate a lower uncertainty of 0.3 dex for metallicities derived for colours F 606W − F 814W < 1.2 whereas metallicities derived for colours redder than 1. …”

“…However, because the colours of the RGB stars are more sensitive to metallicity than to age and because there is a direct relation between RGB colours and metallicities (see e.g. Streich et al 2014, and references therein), one can assume that the colour profiles presented in the previous section reflect metallicity profiles. This assumption will allow us to compare our results with other work in which metallicities of individual fields and/or metallicity profiles of stellar haloes are constructed.…”

“…To obtain metallicities from the median RGB colours, we use the observational relation between the HST colours F 606W − F 814W and metallicities derived by Streich et al (2014). They use a sample of globular clusters observed as part of the ACS Globular Cluster Survey (Sarajedini et al 2007;Dotter, Sarajedini & Anderson 2011) and relate their RGB colours at the same absolute magnitude as we do (i.e.…”

“…the six GHOSTS galaxies presented in this work in addition to the MW and M31. The median colour values at 30 kpc for the MW and M31 were estimated using the Streich et al (2014) relationship, assuming [α/Fe] = 0.3. The metallicity at 30 kpc of M31's halo is from Gilbert et al (2014) 10 and the metallicity at 30 kpc of the MW's halo is the mean metallicity between the value reported in Sesar, Jurić & Ivezić (2011) and Xue et al (2015), i.e.…”

Diverse stellar haloes in nearby Milky Way mass disc galaxies

“…While both haloes are richly substructured and qualitatively agree with the ΛCDM paradigm of galaxy formation, they display significant differences. The Milky Way halo has a weak to no metallicity gradient (Sesar et al 2011;Xue et al 2015) and its stellar density distribution can be described by a broken power-law -within 25 − 30 kpc, it follows an oblate, ρ ∝ r −γ powerlaw distribution with index γ ∼ 2.5 − 3 (Yanny et al 2000;Bell et al 2008;Jurić et al 2008;van Vledder et al 2016) whereas a more rapidly declining stellar density is detected beyond ∼ 30 kpc, with γ ∼ 3.5 (Deason et al 2011;Sesar et al 2011;Deason et al 2014;Cohen et al 2015;Slater et al 2016). M31, on the other hand, has a clear metallicity gradient with a 1 dex variation in [Fe/H] from 10 to ∼100 kpc (Gilbert et al 2014;Ibata et al 2014) and its stellar density distribution can be described by a single power-law with γ ∼ 3.3 (Guhathakurta et al 2005;Gilbert et al 2012).…”