Spectroscopic versus photometric metallicities: Milky Way dwarf spheroidal companions as a test case

Abstract

Aims. The method of deriving photometric metallicities using red giant branch stars is applied to resolved stellar populations under the common assumption that they mainly consist of single-age old stellar populations. We explore the effect of the presence of mixed-age stellar populations on deriving photometric metallicities. Methods. We use photometric data sets for the five Galactic dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to derive their photometric metallicity distribution functions from their resolved red giant branches using isochrones of the Dartmouth Stellar Evolutionary Database. We compare the photometric metallicities with published spectroscopic metallicities based on the analysis of the near-infrared Ca triplet (Ca T), both on the metallicity scale of Carretta & Gratton and on the scale defined by the Dartmouth isochrones. In addition, we compare the photometric metallicities with published spectroscopic metallicities based on spectral synthesis and medium-resolution spectroscopy, and on high resolution spectra where available. Results. The mean properties of the spectroscopic and photometric metallicity samples are comparable within the intrinsic scatter of each method although the mean metallicities of dSphs with pronounced intermediate-age population fractions may be underestimated by the photometric method by up to a few tenths of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the photometric metallicities show a wide range of values along the fiducial spectroscopic metallicity range, with the tendency to have systematically lower photometric metallicities for those dwarf spheroidals with a higher fraction of intermediate-age populations. Such discrepancies persist even in the case of the purely old Sculptor dSph, where one would naïvely expect a very good match when comparing with medium or low resolution metallicity measurements. Overall, the agreement between Ca T metallicities and photometric metallicities is very good in the metallicity range from ∼−2 dex to ∼−1.5 dex. We find that the photometric method is reliable in galaxies that contain small (less than 15%) intermediate-age stellar fractions. Therefore, in the presence of mixed-age stellar populations, one needs to quantify the fraction of the intermediate-age stars in order to assess their effect on determining metallicities from photometry alone. Finally, we note that the comparison of spectroscopic metallicities of the same stars obtained with different methods reveals similarly large discrepancies as the comparison with photometric metallicities.

HST-ACS photometry of the isolated dwarf galaxy VV124=UGC 4879

“…6, to check the findings of Pap-I and to verify whether the distribution and mean color of RGB stars is compatible with the gradient found with the HB stars. It must be stressed that the following analysis is not intended to provide accurate metallicity estimates, since it is well known that photometric determinations of the metallicity can be affected by serious degeneracies when the underlying age distribution is not well constrained (see Cole et al 2005;Lianou, Grebel & Koch 2011, and references therein). On the other hand we look for differences in the inner and outer samples of RGB stars that may or may not be in agreement with the observed HB gradient.…”

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group

“…(7) and (8) of Table 2. We note here that we use Dartmouth isochrones (Dotter et al 2007(Dotter et al , 2008 to define the luminous AGB stars in order to remain consistent with our previous investigations (Lianou et al 2010(Lianou et al , 2011.…”

“…We use for our analysis Dartmouth isochrones (Dotter et al 2007(Dotter et al , 2008, in order to remain consistent with our previous investigations on photometric metallicities (Lianou et al 2010(Lianou et al , 2011. Using Padova isochrones instead to derive the RGB photometric metallicities introduces an offset in metallicity on the order of ∼0.3 dex, making the stars appear more metalrich than when using Dartmouth isochrones (see Appendix B).…”

“…We further note that the presence of an age spread points to the need for either deeper photometric observations reaching the old MS turn-offs or spectroscopic observations in order to break the age-metallicity degeneracy and to estimate the true metallicities. Other than age and the age-metallicity degeneracy, the systematics that may affect the photometric metallicities are the adopted distance modulus, the reddening, and the α-elements enhancement assumption (e.g., Lianou et al 2011). Varying the distance modulus by 1% leads to a difference in the [Fe/H] photometric metallicities of 10%, where an increase of the distance modulus leads to more metal-poor values.…”

“…We use the photometric metallicities derived in Lianou et al (2011), and the spectroscopic metallicities from Koch et al (2006), with the aim to compare their cumulative metallicity distributions. We briefly mention that the photometric metallicities of Carina are derived using the same interpolation scheme as that presented in this work, while the spectroscopic metallicities are based on low-resolution spectra.…”