A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES: Where Are the Very Metal-poor Stars?

By A. Helmi, M. Irwin, E. Tolstoy, G. Battaglia, V. Hill, P. Jablonka, K. Venn, M. Shetrone, B. Letarte, N. Arimoto and 6 more. 2006

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As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) program, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Sextans, Fornax, and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] ~ -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph's appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/b6517a66a3d5828fdb7da8c5f802fc69a7d53113

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Citations in the corpus

Formation of isolated dwarf galaxies with feedback (2009, background): “Comparing this with observed distributions, e.g. by Helmi et al. (2006), we find an overabundance of both metal-rich and metal-poor stars for a given median metallicity.”
Resolving the Formation of Protogalaxies. III. Feedback from the First Stars (2007, background): “There is a metallicity floor exists of 10−3 and 10−4 of solar metallicity in dSph and halo stars, respectively (Beers & Christlieb 2005; Helmi et al. 2006).”
The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach (2008, background): “…clumpiness in the phase space distribution of halo and disk stars (e.g. Majewski, Munn & Hawley 1996; Helmi et al. 1999; Chiba & Beers 2000; Helmi et al. 2006) to the direct detection of satellite galaxies caught in the act of tidal disruption (e.g. Ibata, Gilmore & Irwin 1994; Martin et…”
The Dynamical and Chemical Evolution of Dwarf Spheroidal Galaxies with GEAR (2009, methodology): “The mean metallicities were calculated from Battaglia et al. (2011) for Sextans, from Battaglia et al. (2006), Tolstoy et al. (2004), and Helmi et al. (2006), for Fornax, Sculptor, and Carina, respectively, with the new CaT calibration of Starkenburg et al. (2010).”
STRUCTURE AND KINEMATICS OF THE STELLAR HALOS AND THICK DISKS OF THE MILKY WAY BASED ON CALIBRATION STARS FROM SLOAN DIGITAL SKY SURVEY DR7 (2009, result): “…(Kirby et al. 2008; Koch et al. 2008b; Norris et al. 2008; Frebel et al. 2009) have clearly demonstrated that the frequently cited result (e.g., Helmi et al 2006) that “the progenitors of nearby dSph’s appear to have been different from the building blocks of the Milky Way,” while true for the…”
METALLICITY EVOLUTION OF DAMPED Lyα SYSTEMS OUT TO z ∼ 5 (2012, background): “While nearby dwarf spheroidal galaxies were originally thought to lack stars with [Fe/H]∼ −3 (Helmi et al. 2006), fainter systems have sufficiently low metallicity that they could be contributors to the Galactic halo (Kirby et al. 2008; Koch et al. 2008; Norris et al. 2008; Frebel et al. 2010; Lai…”
Resolving the Formation of Protogalaxies. II. Central Gravitational Collapse (2007, background): “Desired Improvements Clearly local dwarf spheroidals contain stars with ages consistent with formation at very high redshifts (Ferrara & Tolstoy 2000; Tolstoy et al. 2002, 2003; Helmi et al. 2006).”
Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “These efforts led to the conclusion that such systems contained no stars with [Fe/H] < −3.0 (Helmi et al. 2006).”
A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies (2007, background): “Moreover in Boo and CVnI for which the samples are the more populated and the more reliable (since only giant or sub-giant stars were targeted), there still is a dearth of very metal-poor stars, as in the brighter dwarf galaxies Sculptor, Sextans, Fornax and Carina (Helmi et al. 2006).”
Chemical signatures of formation processes in the stellar populations of simulated galaxies (2011, background): “The apparent lack of such low-metallicity stars in real systems has been suggested as a possible problem for hierarchical galaxy formation models in the past (e.g. Shetrone et al. 2001; Helmi et al. 2006).”
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.”
The stellar halo of the Galaxy (2008, background): “There is, however, a remarkable uniformity in that all dwarf spheroidals studied so far show an absence of stars with metallicities below a value of [Fe/H] ∼ −3 dex (Helmi et al. 2006).”
Analysis and calibration of CaII triplet spectroscopy of red giant branch stars from VLT/FLAMES observations (2007, methodology): “11 shows the CaT [Fe/H] derived from the T01 calibration (equation 14), which we used in Tolstoy et al. (2004), Battaglia et al. (2006) and Helmi et al. (2006).”
THE UNIVERSAL STELLAR MASS–STELLAR METALLICITY RELATION FOR DWARF GALAXIES (2013, methodology): “Since then, the CaT technique has been used to quantify the metallicity distributions of most of the classical dSph Milky Way (MW) satellites (e.g., Helmi et al. 2006) and two dIrrs: NGC 6822 (Tolstoy et al. 2001) and WLM at a distance of 930 kpc (Leaman et al. 2009).”
Probing the formation of the first low‐mass stars with stellar archaeology (2007, background): “This finding has recently been investigated by Helmi et al. (2006) who consistently find a significant lack of metal-poor stars with [Fe/H] < −3.0 in several dSph galaxies.”
THE LEAST-LUMINOUS GALAXY: SPECTROSCOPY OF THE MILKY WAY SATELLITE SEGUE 1 (2008, unknown): “At red wavelengths, metallicity is often estimated via the Ca II triplet absorption lines (e.g. Helmi et al. 2006, SG07).”
THE CHEMICAL EVOLUTION OF THE DRACO DWARF SPHEROIDAL GALAXY (2009, background): “Helmi et al (2006) claim, however, that early accretion is still ruled out because of the metallicity distribution function (MDF) they deduce for four dSph galaxies.”