 | This image shows how one of the newly discovered young galaxies looks relative to the "grownup" Milky Way galaxy. |
— By Janet Rettig Emanuel
T H I S
Yale astronomers discover nine
young ultra-dense galaxies
A team of astronomers looking at the
universe’s distant past found nine young, unusually compact galaxies,
each weighing in at 200 billion times the mass of the Sun.
The findings appeared in the April 10 issue of The Astrophysical Journal Letters.
These young galaxies are the equivalent of a human baby that is 20 inches long,
yet weighs 180 pounds.
“Seeing the compact sizes of these galaxies is a puzzle,” says Pieter
G. van Dokkum of Yale, who led the study. “No massive galaxy at this distance
has ever been observed to be so compact, and it is not yet clear how one of these
would build itself up to be the size of the galaxies we see today.”
The galaxies, each only 5,000 light-years across, are a fraction of the size
of today’s “grownup” galaxies but contain approximately the
same number of stars. Each could fit inside the central hub of the Milky Way. “These
ultra-dense galaxies, forming the building blocks of today’s largest
galaxies, might comprise half of all galaxies of that mass at this early time,” van
Dokkum says.
But, van Dokkum noted that they would have to change a lot over 11 billion
years — they would have to grow five times bigger, “While they
could get larger by colliding with other galaxies, such collisions may not
be the complete answer,” he says.
Astronomers used NASA’s Hubble Space Telescope and the W.M. Keck Observatory
on Mauna Kea, Hawaii, to study the galaxies whose light has been traveling
toward the Earth for 11 billion years. “What we see now is the way these
compact galaxies existed 11 billion years ago, when the universe was less than
3 billion years old,” van Dokkum explains. “Only Hubble and Keck
can see the sizes of these galaxies because they are very small and far away.”
In 2006, the research team also studied the galaxies with the Gemini South
Telescope Near-Infrared Spectrograph, on Cerro Pachon in the Chilean Andes.
Those observations provided information on the galaxies’ distances and
showed that the stars are a half a billion to a billion years old, and that
the most massive stars had already exploded as supernovae.
“In the Hubble Deep Field, astronomers found that star-forming galaxies
are small,” says Marijn Franx of Leiden University, The Netherlands. “However,
these galaxies were also very low in mass. They weigh much less than our Milky
Way. Our study, which surveyed a much larger area than in the Hubble Deep Field,
surprisingly shows that galaxies with the same weight as our Milky Way were also
very small in the past. All galaxies look really different in early times, even
massive ones that formed their stars early.”
Speculating on how these small, crowded galaxies formed, van Dokkum says one
way could have involved an interaction in the emerging universe between hydrogen
gas and dark matter — an invisible form of matter that accounts for most
of the universe’s mass. Shortly after the Big Bang, the universe contained
an uneven landscape of dark matter, he explains, noting that hydrogen gas could
have been trapped in puddles of the invisible material which began spinning
rapidly in dark matter’s gravitational whirlpool, forming stars at a
furious rate.
The astronomers estimated that the stars in the compact galaxies are spinning
around their galactic disks at roughly one million miles an hour (500 kilometers
a second). Stars in today’s galaxies, by contrast, are traveling at about
half that speed because they are larger and rotate more slowly.
These galaxies are ideal targets for the Wide Field Camera 3, which is scheduled
to be installed aboard Hubble during Servicing Mission 4 in the fall of 2008.
The team says that the new images should lead to a better understanding of
the evolution of galaxies early in the life of the universe.
Other authors of the paper are Mariska Kriek (Princeton University), Bradford
Holden, Garth Illingworth, Daniel Magee and Rychard Bouwens (University of
California, Santa Cruz and Lick Observatory), Danilo Marchesini (Yale University),
Ryan Quadri (Leiden University), Greg Rudnick (National Optical Astronomical
Observatory, Tucson), Edward Taylor (Leiden University), and Sune Toft (European
Southern Observatory, Germany).
Images and more information on the compact galaxies are available on line at
http://hubblesite.org/news/2008/15.
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency and is managed by NASA’s Goddard Space
Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute
conducts Hubble science operations. The institute is operated for NASA by the
Association of Universities for Research in Astronomy Inc., Washington, D.C.
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