"The chemistry we see in the stars in these dwarf galaxies is just not consistent with current cosmological models," said Amina Helmi of the Kapteyn Astronomical Institute in Groningen, The Netherlands, and lead author of the paper presenting the results. "It shows that there is plenty of astronomy to learn in our backyard."

Our Milky Way Galaxy is surrounded by a number of dwarf satellite galaxies, which because of their loosely rounded shape are referred to as 'dwarf spheroidal' galaxies. Faint and diffuse, these dwarf galaxies are a thousand times fainter than the Milky Way itself, making them the least luminous galaxies known.

Modern cosmological models predict that small galaxies form first, and later assemble into larger systems like our Galaxy. Since the Universe initially only contained hydrogen and helium (most of all other chemical elements being synthesized inside stars), dwarf galaxies should have the lowest heavy element [1] content. Not so, say the astronomers.

As part of a large observational programme, the Dwarf galaxies Abundances and Radial-velocities Team (DART), Helmi and her colleagues from institutes in 9 different countries used the FLAMES [2] instrument on ESO's Very Large Telescope to measure the amount of iron in over 2000 individual giant stars in the Fornax, Sculptor, Sextans and Carina dwarf spheroidals [3].

Their data unearthed fundamental differences in the dwarf galaxy stars' chemical composition compared with those in our galactic halo, calling into question the merger theory as the origin of large galaxies' haloes. Whilst the average abundances of elements in the dwarf spheroidals is comparable with that seen in the Galactic halo, the former are lacking the very metal-poor stars that are seen in the Milky Way - the two types of systems, contrary to theoretical predictions, are essentially of different descent.

"Our results rule out any merging of the nearby dwarf galaxies as a mechanism for building up the Galactic halo, even in the early history of the Universe," said Helmi. "More detailed chemical abundance studies of these systems are needed, as this will tell us more about what happened at those early epochs in our local Universe".

Details of this study are published in a Letter in the Astrophysical Journal of 10 November 2006 by Amina Helmi et al, entitled "A new view of the dwarf spheroidal satellites of the Milky Way from VLT/FLAMES: Where are the metal-poor stars?" (ApJ Lett, vol. 651, L121-L124). The DART team consists of Eline Tolstoy, Amina Helmi, Giuseppina Battaglia, and Bruno Letarte (University of Groningen, The Netherlands), Mike Irwin (University of Cambridge, UK), Vanessa Hill (Observatory of Paris-Meudon, France), Patrick François (Observatory of Paris-Meudon, France and ESO, Chile), Pascale Jablonka (Ecole Polytechnique Federale de Lausanne, Switzerland), Kim Venn (University of Victoria, Canada), Matthew Shetrone (University of Texas, USA), Nobuo Arimoto (National Astronomical Observatory of Japan), Tom Abel (Kavli Institute for Particle Astrophysics and Cosmology, USA), Andreas Kaufer and Thomas Szeifert (ESO, Chile), Francesca Primas (ESO, Germany), and K. Sadakane (Osaka Kyoiku University, Japan).

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Stellar Chemistry, The Universe And All Within It