Low mass stars (< 1 solar mass) live longer than the age of the Universe, and therefore reliably trace the star formation activity experienced by a galaxy over time. Indeed, the Universe initially only contained hydrogen and helium, most of all other chemical elements being synthesized inside stars and fed back into the galactic gas when stars die (supernovae, stellar winds). The chemical composition of stars of various ages therefore reflects the chemical enrichement of galactic gas by the stellar life and death cycles over time (star formation history). 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. The four dwarf spheroidal galaxies Sculptor, Fornax, Sextans and Carina are all located between 80 and 150kpc away from the Milky Way, and display various star formation histories, as reflected by the distribution of the iron content (metallicity) of their stars. For example, Sculptor only made stars for the first 2-3 billion years of its life and its most metal-rich stars reach only to 1/10th of the solar metallicity, whereas Fornax made stars for the best of 10 billion years, reaching up to 1/3 of the solar metallicity. This is illustrated in figure 1 where the iron content distribution of the 2000 stars measured in the four galaxies are displayed.
On the other hand, modern cosmological models predict that small galaxies form first, and later assemble into larger systems like our Galaxy. If so, dwarf galaxies should have the same early epoch fossil content (before the assembly epoch) as larger galaxies, despite the diversity that may arise later in these systems. So, if dwarf spheroidal galaxies are survivors from the building blocks that assembled to make up our Milky Way, we expect that at early epochs (when the metallicity was still very low), they should have the same characteristics as the Milky Way halo (where ancient low metallicity stars are found). There is however a common denominator to these 4 dwarf galaxies : contrary to what was expected, they lack stars with very low iron content, i.e. with a metallicity <-3. Whereas for each 100 stars with metallicities <-2.5, the Milky Way halo contains 25 stars with iron <-3.0 and 10 stars with iron <-3.5, the dwarf spheroidals contain no stars below -3.0 of metallicity. This is illustrated in figure 2 where the distributions of the iron content of stars in Sculptor, Fornax, Sextans et Carina are compared to the Milky Way halo in the most metal-poor regime (<-2.5).
This fundamental differences in the dwarf galaxy stars’ chemical composition compared with those in our galactic halo calls into question the merger theory as the origin of large galaxies’ haloes. The dwarf spheroidals 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.
Reference
- A New View of the Dwarf Spheroidal Satellites of the Milky Way from VLT FLAMES : Where Are the Very Metal-poor Stars ? Helmi, A. ; Irwin, M. J. ; Tolstoy, E. ; Battaglia, G. ; Hill, V. ; Jablonka, P. ; Venn, K. ; Shetrone, M. ; Letarte, B. ; Arimoto, N. ; Abel, T. ; Francois, P. ; Kaufer, A. ; Primas, F. ; Sadakane, K. ; Szeifert, T. 2006, Astrophysical Journal 651, L121
Contact
- Vanessa Hill
Observatoire de Paris, GEPI, UMR 8111 - Patrick François
Observatoire de Paris, GEPI, UMR 8111