Wednesday, August 16, 2006

M. Dopita on "Quiescent Star Formation throughout Cosmic Time"

Starbursts have been mentioned in previous postings here. But is there also a significant amount of quiet, non-bursting SF going on? If not today, has it been existing at earlier times?

The first stars that formed in the early universe have probably been very massive and quite different from todays stars, mainly because heavy elemets are needed to cool more efficiently and this is not in place before the first stars. The IR-background and also the neutrino flux are possible means of getting a hold on this remote epoch.

Different tracers of SF measure different things, e.g. UV-continuum (GALEX) picks up older SF-regions in galaxies than the emission lines (H-alpha, [OIII]), that trace the young HII-regions (2-5Myr). Again, the attenuation from dust cannot be stressed enough, especially in the densest regions.

At low redshift, the different SF-tracers seem to agree quite well. But it is known that SF has declined by an order of magnitude since z=1 and conditions have been quite different and local calibrations are not good any more. The specific SFR peaks between z=6 and z=1 and that is the time when galaxies are mainly assembled. High-z sub-mm galaxies are real global starbursts with extreme SF-rates of several thousand solar masses per year. These objects are on average 100 times more luminous than nowadays galaxies.

The question raised in the beginning was not overly much addressed, but one of the main conclusions is that SF was all but quiescent during cosmic history.

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