A rare glimpse back in time into the universe's early
evolution has revealed something startling: mature, fully
formed galaxies where scientists expected to discover
little more than infants.
"Up until now, we assumed that galaxies were just
beginning to form between 8 [billion] and 11 billion years
ago, but what we found suggests that that is not the case,"
said
Karl Glazebrook, associate professor of
physics and
astronomy in the Krieger School of Arts and Sciences
and co-principal author of a paper in the July 8 issue of
Nature. "It seems that an unexpectedly large
fraction of stars in big galaxies were already in place
early in the universe's formation, and that challenges what
we've believed. We thought massive galaxies came much
later."
Using the Frederick C. Gillett Gemini North Telescope
in Mauna Kea, Hawaii, Glazebrook and a multinational team
of researchers called the Gemini Deep Deep Survey employed
a special technique called the Nod and Shuffle to peer into
what had traditionally been a cosmological blind spot.
Called the Redshift Desert, this era — 8 billion to
11 billion years ago, when the universe was only 3 billion
to 6 billion years old — has remained relatively
unexplored until now, mainly because of the challenges
inherent in collecting data from the faintest galactic
light ever to be dissected into the rainbow of colors
called a spectrum. In all, the team collected and analyzed
spectra from 300 galaxies, making it the most complete
sample ever taken from the Redshift Desert.
"This was the most comprehensive survey ever done
covering the bulk of the galaxies that represent conditions
in the early universe," Glazebrook said. "We expected to
find basically zero massive galaxies beyond about 9 billion
years ago because theoretical models predict that massive
galaxies form last. Instead, we found highly developed
galaxies that just shouldn't have been there but are."
These findings challenge the dominant theory of
galactic evolution, which posits that at this early stage,
galaxies should have formed from the bottom up, with small
pieces crashing together to build small and then ever
larger galaxies. Called the hierarchical model, this
scenario predicts that normal-to-large galaxies such as
those studied by the Gemini Deep Deep Survey would not yet
exist.
"There are obviously some aspects of the early lives
of galaxies that we don't yet completely understand,"
Glazebrook said. "We do find fewer massive galaxies in the
past, but there are still more than we expected. This
result is giving us a big clue as to how stars form from
invisible gas in the hierarchical model, which is something
not well understood under current theories. Some new
ingredient is required to make more stars form earlier in
the big galaxies. But what that ingredient is, we don't yet
know."
The GDDS team, which included Nature paper
co-authors Roberto Abraham from the University of Toronto,
Patrick McCarthy from the Observatories of the Carnegie
Institution of Washington and David Crampton of the
National Research Council of Canada's Herzberg Institute of
Astrophysics, was supported by a grant from the Packard
Foundation and by institutional support from the National
Science Foundation, Canada's National Research Council, the
Natural Sciences and Engineering Research Council of Canada
and the United Kingdom's Particle Physics and Research
Council, among others.
GO TO