The Comparative Chemical Evolution of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM

Abstract

Building on our previous spectroscopic and photometric analysis of the isolated Local Group dwarf irregular (dIrr) galaxy WLM, we present a comparison of the metallicities of its RGB stars with respect to the well studied Local Group dwarf spheroidal galaxies (dSphs) and Magellanic Clouds. We calculate a mean metallicity of [Fe/H]= −1.28 ± 0.02, and intrinsic spread in metallicity of σ = 0.38 ± 0.04 dex, similar to the mean and spread observed in the massive dSph Fornax and the Small Magellanic Cloud. Thus, despite its isolated environment the global metallicity still follows expectations for WLM's mass and its global chemical evolution is similar to other nearby luminous dwarf galaxies (gas-rich or gas-poor). The data also show a radial gradient in [Fe/H] of d[Fe/H]/dr c = −0.04 ± 0.04 dex r −1 c , which is flatter than that seen in the unbiased and spatially extended surveys of dSphs. Comparison of the spatial distribution of [Fe/H] in WLM, the Magellanic Clouds, and a sample of Local Group dSphs, shows an apparent dichotomy in the sense that the dIrrs have statistically flatter radial [Fe/H] gradients than the low angular momentum dSphs. The correlation between angular momentum and radial metallicity gradient is further supported when considering the Local Group dEs. This chemodynamic relationship offers a new and useful constraint for environment driven dwarf galaxy evolution models in the Local Group.

The difference in metallicity distribution functions of halo stars and globular clusters as a function of galaxy type

“…The Fornax dSph, for example, contains four GCs with [M/H] ∼ −2.5 to -1.9 and one more metalrich GC at [M/H] ∼ −1.5 dex, whereas its stellar MDF peaks at [M/H] ∼ −1.0 with a long tail towards lower metallicities (Strader et al 2003;Helmi et al 2006;Larsen et al 2012b). A similar situation is found in the dwarf galaxy WLM, where the GCs with a metallicity of [M/H] = −2.0 are significantly more metal-poor than the average of -1.3 of the field stars in this galaxy (Leaman et al 2013;Larsen et al 2012a). The dwarf galaxy IKN has five GCs but only the metallicity [M/H] = −2.1 of IKN-5 is known with some confidence.…”

“…For dwarf galaxies in the Local Group (LG), where metallicities can be measured spectroscopically, the age-metallicity degeneracy problem in the interpretation of color-color distributions can be avoided. Leaman et al (2013) derived the metallicity gradient in the LG dwarf irregular WLM and compared it with those in the LG dSphs and the Magellanic Clouds. The metallicity gradient is small or absent in dIrrs,…”

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

“…Since effects of this size are frequently less than the random error in the abundance determinations via the Ca ii triplet method for red giants in relatively distant dwarf galaxies (e.g., Leaman et al 2013), where the chemical evolution history is generally unknown, the results here reaffirm that the Ca ii triplet approach can be used in the knowledge that the outcomes are not significantly affected by ignorance of the actual [Ca/Fe] abundance ratios.…”

“…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). …”

Are the globular clusters with significant internal [Fe/H] spreads all former dwarf galaxy nuclei?

“…For example, Leaman et al (2013) give MDFs for six dSphs. One common feature of the dSph MDFs (see also Kirby et al 2011a,b) is that they rise relatively slowly on the metal-poor side of the metallicity peak.…”