GOODS South
The universe suddenly looks a lot more crowded, thanks to a deep-sky
census assembled from surveys taken by NASA's Hubble Space Telescope and
other observatories.
Astronomers came to the surprising conclusion that there are at least
10 times more galaxies in the observable universe than previously
thought.
The results have clear implications for galaxy formation, and also
helps shed light on an ancient astronomical paradox — why is the sky
dark at night?
In analyzing the data, a team led by Christopher Conselice of the
University of Nottingham, U.K., found that 10 times as many galaxies
were packed into a given volume of space in the early universe than
found today. Most of these galaxies were relatively small and faint,
with masses similar to those of the satellite galaxies surrounding the
Milky Way. As they merged to form larger galaxies the population
density of galaxies in space dwindled. This means that galaxies are not
evenly distributed throughout the universe's history, the research team
reports in a paper to be published in The Astrophysical Journal.
"These results are powerful evidence that a significant galaxy
evolution has taken place throughout the universe's history, which
dramatically reduced the number of galaxies through mergers between
them — thus reducing their total number. This gives us a verification
of the so-called top-down formation of structure in the universe,"
explained Conselice.
One of the most fundamental questions in astronomy is that of just
how many galaxies the universe contains. The landmark Hubble Deep
Field, taken in the mid-1990s, gave the first real insight into the
universe's galaxy population. Subsequent sensitive observations such as
Hubble's Ultra Deep Field revealed a myriad of faint galaxies. This
led to an estimate that the observable universe contained about 100
billion galaxies. The new research shows that this estimate is at least
10 times too low.
Conselice and his team reached this conclusion using deep-space
images from Hubble and the already published data from other teams.
They painstakingly converted the images into 3-D, in order to make
accurate measurements of the number of galaxies at different epochs in
the universe's history. In addition, they used new mathematical models,
which allowed them to infer the existence of galaxies that the current
generation of telescopes cannot observe. This led to the surprising
conclusion that in order for the numbers of galaxies we now see and
their masses to add up, there must be a further 90 percent of galaxies
in the observable universe that are too faint and too far away to be
seen with present-day telescopes. These myriad small faint galaxies
from the early universe merged over time into the larger galaxies we can
now observe.
"It boggles the mind that over 90 percent of the galaxies in the
universe have yet to be studied. Who knows what interesting properties
we will find when we discover these galaxies with future generations of
telescopes? In the near future, the James Webb Space Telescope will be
able to study these ultra-faint galaxies," said Conselice.
The decreasing number of galaxies as time progresses also contributes
to the solution for Olbers' paradox (first formulated in the early
1800s by German astronomer Heinrich Wilhelm Olbers): Why is the sky
dark at night if the universe contains an infinity of stars? The team
came to the conclusion that indeed there actually is such an abundance
of galaxies that, in principle, every patch in the sky contains part of a
galaxy. However, starlight from the galaxies is invisible to the human
eye and most modern telescopes due to the other known factors that
reduce visible and ultraviolet light in the universe. Those factors are
the reddening of light due to the expansion of space, the universe's
dynamic nature, and the absorption of light by intergalactic dust and
gas. All combined, this keeps the night sky dark to our vision.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight
Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the
Association of Universities for Research in Astronomy in Washington,
D.C.
Contact
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Mathias Jäger
ESA/Hubble, Garching, Germany
011-49-176-6239-7500
mjaeger@partner.eso.org
Christopher Conselice
University of Nottingham, Nottingham, United Kingdom
011-44-115-951-5137
conselice@nottingham.ac.uk
Source: HubbleSite
