Astronomers have discovered an unexpected cache of spiral
galaxies that appear to have formed recently, long after the period early in
the history of the universe that most galaxies were thought to have been
created.
These younger galaxies are big and bright, like our
own Milky Way. The reigning hypothesis
of galaxy formation holds that such well-established spirals would have
formed about 13 billion years ago, shortly after the Big Bang.
But the new discovery of a group of 15 spirals that look to
be much
younger may upset that thinking.
Though they are just as luminous and large as normal
spirals, these galaxies appear to have all the hallmarks of youth. They don't
have nearly as many heavy elements, called metals, as would be expected for
older galaxies. All elements heavier than helium are created through nuclear
reactions inside stars, and elements heavier than iron are made when stars die
in supernovae. Thus, the longer a galaxy has been alive and forming stars, the
longer its stars have been churning out heavy metals, so the greater abundance
you would expect.
But the chemical abundances of the newly discovered galaxies
would suggest they are only about 3 billion or 4 billion years old.
"We're not saying there's a complete breakdown in the
theory of galaxy evolution, but that these objects do run counter to the
standard model," said Indiana University astronomer John Salzer, the lead
author of a paper detailing the study in the April 10 issue of the Astrophysical
Journal Letters. "These potentially could have formed much more
recently. The significance is that they give us the opportunity to study galaxy
formation and evolutionary processes which would otherwise be veiled at the
vast distances involved in looking at things at these early stages of the
universe."
If this age estimate proves to be correct, these galaxies
could present an unprecedented opportunity to study massive spirals at a
younger point in their evolution, and at
closer range, than is generally possible.
"These objects may represent a unique window on the
process of galaxy formation, allowing us to study relatively nearby systems
that are undergoing a phase in their evolution that is analogous to the types
of events that, for most galaxies, typically occurred much earlier in the
history of the universe," Salzer said. Another possible explanation for
the unusual galaxies is that they are the product of collisions between two
smaller galaxies. This could explain why they have low levels of metals, since dwarf
galaxies tend to have few heavy elements to begin with. Also, the chaos of a
collision can stimulate a burst of star formation, which might explain why the
galaxies appear so bright. However, this scenario would require the galaxies'
luminosity to multiply by about 30 times after merging — a boost much greater
than is usually seen.
"It's just hard for me to fathom, and hard for models
to account for, an increase by a factor of 30," Salzer told SPACE.com.
Salzer discovered the galaxies through the Kitt Peak
National Observatory International Spectroscopic Survey (KISS), a multi-year
project to observe more than 2,400 star-forming galaxies. He and his team have
requested time on the Hubble Space Telescope to try to distinguish between the
two possible explanations by hunting for evidence that the galaxies have
undergone collisions.
The KISS project was funded by the National Science
Foundation.
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