Determining the chemical composition of 2000 stars in four of our neighboring dwarf galaxies, is a task even the biggest parallel analytical operation would probably baulk at taking on, although the fees would be enormous.

Amina Helmi

Fortunately, a chemical survey of such inter-galactic systems was possible, using the European Southern Observatory's Very Large Telescope (ESO's VLT). The results are shedding star light on our Galaxy's ancestry and revealing it to be very different from that of our neighbors. The findings cast doubt on the theory that these diminutive galaxies were the building blocks for our own Milky Way Galaxy.

The Milky Way is surrounded by spheroidal dwarf satellite galaxies. These objects are faint and diffuse. "The chemistry we see in the stars in these dwarf galaxies is just not consistent with current cosmological models," says team leader Amina Helmi of the Kapteyn Astronomical Institute in Groningen, The Netherlands, "It shows that there is plenty of astronomy to learn in our backyard."

Cosmologists have generally agreed that small galaxies form first and subsequently assemble into larger systems such as the Milky Way. If this were correct, then dwarf galaxies would have the lowest heavy element content as the Universe initially contained only hydrogen and helium with the other elements being synthesized by nuclear reactions inside stars. However, Helmi and her colleagues have shown this not to be the case.

As part of a large observational program, the Dwarf galaxies Abundances and Radial-velocities Team (DART), researchers from institutes in nine different countries, used the Fibre Large Array Multi-Element Spectrograph on ESO's VLT to measure the amount of iron in over 2000 individual giant stars in the Fornax, Sculptor, Sextans, and Carina dwarf spheroidals.

Galactic chemistry revealed

The team found that there is a fundamental difference between the chemical composition of the stars in these dwarf galaxies compared with the outer reaches, the galactic halo of our own Milky Way. The data are in direct conflict with the merger theory of galactic formation. "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," explains 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".

Astrophys J Lett, 2006, 651, L121-L124; http://dx.doi.org/10.1086/509784

http://www.astro.rug.nl/~ahelmi/

http://www.eso.org/projects/vlt/

http://www.eso.org/instruments/flames/