http://www.skyandtelescope.com.au 27ASTRONOMERS OFTEN PURSUE EXTREMES: the
biggest, the brightest, the farthest, the oldest.
Recently observers have discovered the most
extreme members of an already extreme
celestial class that promises to teach us
much about the cosmos: small blue
galaxies that spawn new stars yet possess
scarcely any oxygen.
A galaxy without oxygen is like a
forest without fallen leaves. Massive
stars create lots of oxygen during
their bright but brief lives, then hurl
the element into space when they
explode. So it’s no surprise that
within a billion light-years of Earth,
astronomers have spotted fewer than
ten extremely oxygen-poor star-
forming galaxies.
These galaxies have somehow
survived for eons without acquiring
much oxygen. Such oddballs are telling
us some fundamental things about the
early universe, because these galaxies
resemble the first ones that ever arose.
Primordial galaxies were also small, and
because they formed soon after the Big
Bang, they should have consisted of the
three elements it created: hydrogen, helium
and lithium — with little if any oxygen.
Furthermore, the first galaxies changed
the universe. Radiation from their hottest stars
reionised space, transforming the neutral gas that
once existed between the galaxies into the ionised
form that pervades space today. Alas, no one can see
these primordial galaxies in detail yet, because they’re
billions upon billions of light-years distant.
In 1971, however, two British-born astronomers in
California, Leonard Searle and Wallace Sargent, found an
easier way. They discovered that a much closer galaxy in Ursa
Major named I Zwicky 18 has almost no oxygen. A mere 60
million light-years away, the galaxy is a lot easier to study
than its distant cousins. Indeed, we now have beautiful
Hubble images that show it to be a splotchy blue dwarf galaxy
brimming with gas and rambunctious young stars. Yet its
abundance of ‘metals’ — elements heavier than hydrogen and
helium — is just a few percent of the Sun’s.
“We realised that these galaxies are very, very, very metal-
deficient and hence probably the closest proxies to primordial
galaxies,” says Trinh Thuan (University of Virginia), whoSPRIMORDIAL PRETENDER Despite hosting a collection of newly
formed stars, the dwarf galaxy I Zwicky 18 (larger blue object) has an
anaemic level of star-produced oxygen. It appears here with a companion
I ZWICKY 18: NASA / ESA / A. ALOISI (ESA / STSCI) called Component C (upper left), which might be a separate galaxy.