Thursday, August 17, 2006

T. Wilkind: Massive and old galaxies at z>5

If galaxies form hierarchically, i.e. big ones form by the merging of small ones, shouldn't big galaxies then appear rather late in the history of the universe? One cound think so, but would be mistaken. Tommy repots on their finding from last year of a massive galaxy at z=6.5 that is red in color and show no ongoing star formation at all.

Is the presence of such an object that has finished forming all its stars at so early times a threat to the lambda-CDM cosmological model? This first of all depends on how many of these really exist. They look in the K-selected GOODS-south sample and find 18 candidates out of which 5 had to be discarded as being something else.

So they have 13 galaxies at z>5 with over 10^11 solar masses in stars and no ongoing star formation (although 50% are detected in 24micron). Correcting this for completeness gives a rather high number density which indeed opposes the lamba-CDM paradigm (too many as compared to existing DM-halos at that time), unless these estimates are either flawed in redshift or stellar-mass-estimates.

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