The Chemical Enrichment History of the Fornax Dwarf Spheroidal Galaxy from the Infrared Calcium Triplet

Abstract

Near infrared spectra were obtained for 117 red giants in the Fornax dwarf spheroidal galaxy with the FORS1 spectrograph on the VLT, in order to study the metallicity distribution of the stars and to lift the age-metallicity degeneracy of the red giant branch (RGB) in the color-magnitude diagram (CMD). Metallicities are derived from the equivalent widths of the infrared Calcium triplet lines at 8498, 8542, and 8662Å, calibrated with data from globular clusters, the open cluster M67 and the LMC. For a substantial portion of the sample, the strength of the Calcium triplet is unexpect-paring the metallicities of the stars with their positions in the CMD, we have derived the complex age-metallicity relation of Fornax. In the first few Gyr, the metal abundance rose to [Fe/H]∼ −1.0 dex. The enrichment accelerated in the past ∼ 1-4 Gyr to reach [Fe/H]∼ −0.4 dex. More than half the sample is constituted of star younger than ∼ 4 Gyr, thus indicating sustained recent star formation in Fornax. These results are briefly compared to the theoretical predictions on the evolution of dwarf galaxies. They indicate that the capacity of dwarf spheroidal galaxies to retain the heavy elements that they produce is larger than expected.

New constraints on the chemical evolution of the dwarf spheroidal galaxy Leo I from VLT spectroscopy

“…This situation is common among Local Group dwarfs, but the MDF of Leo I is the narrowest observed to date (cf. Pont et al 2004;Koch et al 2006Koch et al , 2007a. Koch et al (2007b); circles with error bars: our data, binned in 1.…”

“…Radial variations in the stellar populations are common in the dwarf spheroidals of the Local Group, where the younger and more metal-rich populations are often concentrated toward the galaxy centre (Harbeck et al 2001;Saviane et al 2001;Pont et al 2004;Tolstoy et al 2004;Koch et al 2006). Leo I remains one of the few dSph's showing little evidence of a population gradient (Held et al 2000;Koch et al 2007b).…”

“…The infrared Ca triplet (CaT) method, originally devised to measure metallicities of stars in Galactic globular clusters (Armandroff & Zinn 1988;Armandroff & Da Costa 1991;Rutledge et al 1997b), has now been applied quite extensively to RGB stars in dwarf galaxies. Using this method, values of [Fe/H] for dwarf galaxies have been obtained for Fornax Pont et al 2004;Battaglia et al 2006), Carina (Koch et al 2006), Sculptor , Leo II (Bosler et al 2007;Koch et al 2007a), NGC 6822 ) and the LMC (Cole et al 2005).…”

The Ca ii triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]

“…In particular, as noted by Carrera et al (2007), in systems like the disk of the LMC where there has been extensive on-going star formation, the magnitude of the old core-helium burning stars, i.e., VHB, is often not well defined as the red clump region of the CMD is dominated by more luminous younger stars. In such situations, however, the distance modulus is often well established, e.g., via Cepheids, and thus the absolute magnitude MV is known more precisely than V − VHB (see also Pont et al 2004).…”

“…These include Galactic globular clusters (e.g., Armandroff & Da Costa 1991;Da Costa & Armandroff 1995;Rutledge et al 1997;Saviane et al 2012), Galactic open clusters (e.g., Cole et al 2004;Warren & Cole 2009;Carretta et al 2009;Carrera et al 2015) and stars in the Galactic Bulge (e.g., Vásquez et al 2015). It has also been used to study both field stars and star clusters in the LMC (e.g., Olszewski et al 1991;Cole et al 2005;Carrera et al 2008a) and SMC (e.g., Da Costa & Hatzidimitriou 1998;Carrera et al 2008b;Parisi et al 2010;Dobbie et al 2014;Parisi et al 2015), as well as red giants in dwarf spheroidal (e.g., Armandroff & Da Costa 1991;Pont et al 2004;Tolstoy et al 2004;Battaglia et al 2008Battaglia et al , 2011 and dwarf irregular galaxies (e.g., Leaman et al 2013). …”