Compact Galaxies In
Early Universe Pack A Big Punch
(29 April 2008) Imagine receiving an
announcement touting the birth of a baby 20 inches long and weighing 180
pounds.
After reading this puzzling message, you would
immediately think the baby's weight was a misprint.
Astronomers looking
at galaxies in the universe's distant past received a similar perplexing
announcement when they found nine young, compact galaxies, each weighing in at
200 billion times the mass of the Sun. 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 galaxy could fit inside
the central hub of our Milky Way Galaxy.
These images taken by NASA's Hubble Space Telescope show nine compact, ultradense galaxies as they appeared 11 billion years ago. The galaxies are only 5,000 light-years across and yet are 200 billion times more massive than the Sun. They are a fraction of the size of today's grownup galaxies but contain the same number of stars. Each galaxy could fit inside the central hub of our Milky Way Galaxy. Hubble's Near Infrared Camera and Multi-Object Spectrometer snapped these images between June 2006 and June 2007. (courtesy: NASA, ESA, P. van Dokkum (Yale University), M. Franx (Leiden University, The Netherlands), and G. Illingworth (University of California, Santa Cruz, and Lick Observatory))
Astronomers used NASA's Hubble Space
Telescope and the W.M. Keck Observatory on Mauna Kea, Hawaii, to study the
galaxies as they existed 11 billion years ago, when the universe was less than
3 billion years old.
"Seeing the compact sizes of these galaxies is a
puzzle," said Pieter G. van Dokkum of Yale University in New Haven, Conn., who
led the study. "No massive galaxy at this distance has ever been observed to be
so compact. It is not yet clear how they would build themselves up to become
the large galaxies we see today. They would have to change a lot over 11
billion years, growing five times bigger. They could get larger by colliding
with other galaxies, but such collisions may not be the complete
answer."
To determine the sizes of the galaxies, the team used the Near
Infrared Camera and Multi-Object Spectrometer on Hubble. The Keck observations
were carried out with assistance of a powerful laser to correct for image
blurring caused by the Earth's atmosphere. "Only Hubble and Keck can see the
sizes of these galaxies because they are very small and far away," van Dokkum
explained.
Van Dokkum and his colleagues studied the galaxies in 2006
with the Gemini South Telescope Near-Infrared Spectrograph, on Cerro Pachon in
the Chilean Andes. Those observations provided the galaxies' distances and
showed that the stars are a half a billion to a billion years old. The most
massive stars had already exploded as supernovae.
"In the Hubble Deep
Field, astronomers found that star-forming galaxies are small," said 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."
The ultradense galaxies might comprise half
of all galaxies of that mass 11 billion years ago, van Dokkum said, forming the
building blocks of today's largest galaxies.
How did these small,
crowded galaxies form? One way, suggested van Dokkum, involves the interaction
of dark matter and hydrogen gas in the nascent universe. Dark matter is 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.
Hydrogen gas became trapped in puddles of the invisible material and began
spinning rapidly in dark matter's gravitational whirlpool, forming stars at a
furious rate.
Based on the galaxies' masses, which are derived from
their colour, the astronomers estimated that the stars are spinning around
their galactic disks at roughly 890,000 to 1 million miles an hour (400 to 500
kilometres a second). Stars in today's galaxies, by contrast, are travelling at
about half that speed because they are larger and rotate more slowly than the
compact galaxies.
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. "We hope to use the Wide Field Camera 3 to find
thousands of these galaxies. The Hubble images, together with the laser
adaptive optics at Keck and similar large telescopes, should lead to a better
understanding of the evolution of galaxies early in the life of the universe,"
said Garth Illingworth of the University of California, Santa Cruz, and Lick
Observatory.
The findings appeared in the April 10 issue of The
Astrophysical Journal Letters.
The authors of the science paper are
Pieter van Dokkum (Yale University), Marijn Franx (Leiden University, The
Netherlands), 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).
(source: Space Telescope Science
Institute)
