Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy

By A. Koch, E. Grebel, R. Wyse, J. Kleyna, M. Wilkinson, Daniel Harbeck, G. Gilmore, N. Evans II. 2005

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The Carina dwarf spheroidal galaxy is the only galaxy of this type that shows clearly episodic star formation separated by long pauses. Here we present metallicities for 437 radial velocity members of this Galactic satellite. The metallicities and radial velocities were measured as part of a Large Programme with the Very Large Telescope at the European Southern Observatory, Chile. We obtained medium-resolution spectroscopy with the multiobject spectrograph FLAMES. Our target red giants cover the entire projected surface area of Carina. Our spectra are centered at the near-infrared Ca II triplet, which is a well-established metallicity indicator for old and intermediate-age red giants. The resulting data sample provides the largest collection of spectroscopically derived metallicities for a Local Group dwarf spheroidal galaxy to date. Four of our likely radial velocity members of Carina lie outside this galaxy's nominal tidal radius, supporting earlier claims of the possible existence of such stars beyond the main body of Carina. We find a mean metallicity of [Fe/H] ∼ -1.7 dex on the 1997 metallicity scale of Carretta and Gratton for Carina. The formal FWHM of the metallicity distribution function is 0.92 dex, while the full range of metallicities is found to span approximately -3.0 dex < [Fe/H] < 0.0 dex. The metallicity distribution function might be indicative of several subpopulations distinct in metallicity. There appears to be a mild radial gradient such that more metal-rich populations are more centrally concentrated, matching a similar trend for an increasing fraction of intermediate-age stars (see the 2001 work of Harbeck and coworkers). This, as well as the photometric colors of the more metal-rich red giants, suggests that Carina exhibits an age-metallicity relation. Indeed, the age-metallicity degeneracy seems to conspire to form a narrow red giant branch despite the considerable spread in metallicity and wide range of ages. The metallicity distribution function is not well matched by a simple closed-box model of chemical evolution. Qualitatively better matches are obtained by chemical models that also take into account infall and outflows. A G dwarf problem remains for all these models.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/5f01c5fca81db9614b72bdd044bc6d910abaddcd

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A METHOD FOR MEASURING (SLOPES OF) THE MASS PROFILES OF DWARF SPHEROIDAL GALAXIES (2011, methodology): “Following Rutledge et al. (1997) and Koch et al. (2006), W09 simultaneously fit the data for each cluster with straight lines sharing a common slope, obtaining ΣMg = −(0.079±0.002)(V −VHB) +ΣMg′, (3) whereV −VHB is the offset inV−band luminosity from the horizontal branch.”
Formation of isolated dwarf galaxies with feedback (2009, background): “…et al. 2006) have revealed surprisingly complex star formation histories, sometimes over several Gyrs, and at least in one case in multiple bursts (Koch et al. 2006, 2008; Orban et al. 2008) About two dozen dwarf spheroidal galaxies have so far been discovered as satellites of the Milky Way,…”
The stellar halo of the Galaxy (2008, background): “First, in the absence of spectroscopic measurements, the chemical properties of the stars cannot be determined reliably because of the well-known age-metallicity degeneracy (a clear example of which is the dwarf galaxy Carina, Koch et al. (2006)).”
Breathing in Low-Mass Galaxies: A Study of Episodic Star Formation (2007, background): “However, there is growing evidence from dwarf spheroidal age-metallicity relations that gas accretion and outflows may have played a role, leading to G- and K-dwarf problems comparable to those seen in the Milky Way (Koch et al. 2006b,a).”
HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES (2009, background): “…stars in the brighter dSphs is not currently known, since no stars with [Fe/H] ≤ −3 have been detected in those galaxies, but the large number of stars observed without finding any EMP stars (e.g., Helmi et al. 2006; Koch et al. 2006) indicates that ∼ 0.1% would be a reasonable assumption.”
Exploring Halo Substructure with Giant Stars. XI. The Tidal Tails of the Carina Dwarf Spheroidal Galaxy and the Discovery of Magellanic Cloud Stars in the Carina Foreground (2006, methodology): “Despite having a complex and episodic star formation history, Carina has a relatively low mean metallicity of [Fe/H]∼ −1.9 (Monelli et al. 2003; Koch et al. 2006).”
A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES: Where Are the Very Metal-poor Stars? (2006, methodology): “OBSERVATIONS AND ANALYSIS For each galaxy we derive metallicity estimates as well as accurate radial velocity measurements for several hundredred giant branch (RGB) stars located over a large area extending out to the tidal radius (Tolstoy et al. 2004, Koch et al. 2006, Battaglia et al. 2006).”
A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters (2018, background): “We note that, as has been argued in the past, King model parameters do not have obvious physical interpretation for darkmatter dominated systems (e.g., Koch et al. 2006; Gilmore et al. 2007).”
STELLAR VELOCITIES IN THE CARINA, FORNAX, SCULPTOR, AND SEXTANS dSph GALAXIES: DATA FROM THE MAGELLAN/MMFS SURVEY (2008, methodology): “…strengths from these low S/N spectra provides an extra dimension of information that can be used to study stellar metallicity distributions (e.g., Koch et al. 2006, 2007b) to identify correlations between kinematics and metallicity (Tolstoy et al. 2004; Battaglia et al. 2006), and to help clean…”