The WAGGS project – II. The reliability of the calcium triplet as a metallicity indicator in integrated stellar light

Abstract

Using data from the WiFeS Atlas of Galactic Globular cluster Spectra we study the behaviour of the calcium triplet (CaT), a popular metallicity indicator in extragalactic stellar population studies. A major caveat of these studies is that the potential sensitivity to other stellar population parameters such as age, calcium abundance and the initial mass function has not yet been empirically evaluated. Here we present measurements of the strength of the CaT feature for 113 globular clusters in the Milky Way and its satellite galaxies. We derive empirical calibrations between the CaT index and both the iron abundance ([Fe/H]) and calcium abundance ([Ca/H]), finding a tighter relationship for [Ca/H] than for [Fe/H]. For stellar populations 3 Gyr and older the CaT can be used to reliably measure [Ca/H] at the 0.1 dex level but becomes less reliable for ages of ∼ 2 Gyr and younger. We find that the CaT is relatively insensitive to the horizontal branch morphology. The stellar mass function however affects the CaT strengths significantly only at low metallicities. Using our newly derived empirical calibration, we convert our measured CaT indices into [Ca/H] values for the globular clusters in our sample.

The SLUGGS survey: measuring globular cluster ages using both photometry and spectroscopy

“…We test our method on observations of GCs in the MW and its satellite galaxies. We use [Fe/H] measurements calculated from the CaT measurements given in table 5 of Usher et al (2019) and equation 1 of Usher et al (2019). We note that the Usher et al (2019) measurements were made using the same code and similar resolution spectra to our SLUGGS DEIMOS spectra.…”

“…We use [Fe/H] measurements calculated from the CaT measurements given in table 5 of Usher et al (2019) and equation 1 of Usher et al (2019). We note that the Usher et al (2019) measurements were made using the same code and similar resolution spectra to our SLUGGS DEIMOS spectra. For the MW GCs we use the BV RI photometry from the 2010 edition of the Harris (1996Harris ( , 2010 catalogue, for the GCs in the Large and Small Magellanic Clouds we use the BV photometry from van den Bergh (1981) and for the Fornax dwarf spheroidal GCs we use the BV photometry from van den Bergh (1969).…”

Integrated Spectra of Milky Way Globular Clusters

“…The references and comments given here are by no means exhaustive, since there is more extensive literature addressing the clusters. Usher et al (2019) derived Ca abundances from EW measurements following prescriptions given in Usher et al (2012), similarly to other studies in the literature such as e.g. Vásquez et al (2018) applied to individual stars, or Foster et al (2010) applied to integrated light.…”

“…Vásquez et al (2018) applied to individual stars, or Foster et al (2010) applied to integrated light. Usher et al (2019) derive Ca abundances for all sample clusters in the present work and their values are given in Table 6. Colucci et al (2017), following earlier work by Sakari et al (2013), measured EWs of individual lines of Mg, Ca, Ti, Cr, Ba, and Eu, from the same WAGGS spectra used here, and they report the corresponding ele- ment abundances.…”

“…Ca Lines - Figure 6 shows the fits to the Ca I 6162.167 A, as well as to the nearby Na I 6160.753Å line, with the adopted abundances indicated in the figure and in Table 3. Usher et al (2019) derived the Ca abundances for their full sample clusters, based on measurements of equivalent widths with well-defined continua and wavelength range of Ca II triplet lines (CaT). The values from Usher et al (2019) and the present results are compatible (See Section 4.3).…”