Monday, August 14, 2006

D. Kunth: "Are the most metal-poor galaxies young?"

This is interesting for myself since it relates to the galaxies I work with, Blue Compact Galaxies (BCGs). "Young" means that it forms its first generation of stars. Only two nearby galaxies with metallicity of around a 50th of the solar value are known: IZw18 and SBS0335-052, two BCGs.

But even if we trust the metallicity determinations, it remains the question if metallicity acts as an "arrow of time". Especially, a young galaxy does not need to have low metallicity since the gas may be preenriched during the denser early phases of the universe, where also SF was stronger.

In the luminosity-metallicity relation (more luminous galaxies are usually more metal-rich) the objets mentioned above fall out of the line, since the strong short burst of SF only boosts the luminosity, while the metallicity stays the same for a longer time, because intermediate-mass stars have not yet had time to release their metals.

The claim of these galaxies being young galaxies has been challenged many times and indeed, a weak old stellar population can be found in most of the galaxies, presumed to be "young".


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