Ref: http://physicsweb.org/article/news/8/4/3

The rate of star formation in the universe may have peaked
about five billion years ago - which is more recent than
previous estimates - according to astronomers in the US and
UK. The team also found that stars formed earlier in more
massive galaxies, which means that that high- and low-mass
galaxies have very different histories (A Heavens et al.
2004 Nature 428 625). The results could improve our
understanding of how galactic structures form and evolve.

Alan Heavens, Benjamin Panter and James Dunlop of Edinburgh
University and Raul Jimenez of the University of
Pennsylvania analysed spectra from the Sloan Digital Sky
Survey for the stellar populations of almost 100 000 stars
to obtain a complete history of star formation over time.
The astronomers built theoretical models of how the galaxy
spectra should look and compared them to observations. "It
is a bit like fingerprint identification, one looks for the
best match," Jimenez told PhysicsWeb.

Using a computer program called Multiple Optimised Parameter
Estimation and Data Compression (MOPED), Jimenez and
colleagues were able to compare the observed and predicted
spectra of 96 545 galaxies in just four weeks. Without the
program such a feat would have taken eight years.

The astronomers calculated that star formation in the
universe reached its peak about five billion years ago. It
has since declined by around a factor of 10 to its
present-day value. Moreover, they found that galaxies with a
high mass, which include our Milky Way, formed most of their
stars much earlier than galaxies with a lower mass.

"The mass dependence of star-formation history explains why
previous surveys showed a much earlier date for star
formation, since those studies were only able to examine
more massive galaxies," said Jimenez.

The Pennsylvania-Edinburgh team now plans to look at more
spectra and develop better theoretical models. "We need to
understand how galaxies got the star formation history we
find, we really do not know yet," he said.

See: http://physicsweb.org/article/news/8/4/3