The baby galaxy, I Zwicky 18, might represent the only opportunity for
astronomers to study the building blocks from which galaxies are formed
Using the Hubble Space Telescope, a University of Virginia
scientist has identified what may be the youngest galaxy ever seen in
the universe. By cosmological standards it is a mere toddler.
Called I Zwicky 18, it may be as young as 500 million years old.
Comparatively, our Milky Way galaxy is more than 20 times older – or
about 12 billion years old, the typical age of galaxies across the
universe.
The finding, reported in the Dec. 1 issue of the Astrophysical
Journal, provides new insight into how galaxies first formed. The
galaxy
I Zwicky 18 offers a glimpse of how the early Milky Way may have looked.
The baby galaxy managed to remain in an embryonic state as a
cold gas cloud of primeval hydrogen and helium for most of the
universe’s evolution. As innumerable galaxies blossomed all over
space, this late-bloomer did not begin active star formation until
some 13 billion years after the Big Bang, and went through a sudden
first starburst only about 500 million years ago.
Because it is located 45 million light-years away – much
closer than other young galaxies in the nearly 14 billion light-year
span of the universe – I Zwicky 18 might represent the only
opportunity for astronomers to study in detail the building blocks
from which galaxies are formed. It remains a puzzle why the gas in
the dwarf galaxy, in contrast to that in other galaxies, took so long
– nearly the age of the universe – to collapse under the influence of
gravity to form its first stars.
“I Zwicky 18 is a bona fide young galaxy,” said Trinh Thuan,
professor of astronomy at the University of Virginia, who co-authored
the study with Yuri Izotov from the Kiev Observatory in Ukraine.
“This is extraordinary because one would expect young galaxies to be
forming only around the first billion years or so after the Big Bang,
not some 13 billion years later. And young galaxies were expected to
be very distant, at the edge of the observable universe, but not in
the local universe.”
To prove that I Zwicky 18 is a new galaxy, Thuan and Izotov
needed to show that it was devoid of stars from the first several
billion years after the Big Bang, the period when a large fraction of
stars in the universe were formed. Though astronomers previously had
suspected that the galaxy was exceptionally young, Thuan and Izotov
had to wait for Hubble to provide the sensitivity necessary to detect
whether older stars existed within the dwarf galaxy. Hubble’s
Advanced Camera for Surveys (ACS) needs a very long exposure,
requiring 25 Hubble orbits to look for the faintest stars in the
galaxy. The presence of old stars in the galaxy would have indicated
that the galaxy itself was old, similar to all other known galaxies
in the universe.
Large galaxies such as the Milky Way are thought to grow
hierarchically, with smaller galaxies merging into bigger galaxies,
similar to tributaries merging into large rivers. I Zwicky 18 is
prototypical of this early population of small dwarf galaxies. “These
building block dwarf galaxies are too faint and too small to be
studied without the most sensitive instruments, even in the local
universe, let alone in the far reaches of the cosmos,” Thuan said.
Further evidence for the youth of I Zwicky 18 is the fact
that its interstellar gas is “nearly pristine,” Thuan said, and
composed mostly of hydrogen and helium – the primary two light
elements created in the Big Bang – during the first three minutes of
the universe’s existence. The dwarf galaxy includes only a sprinkling
of the other heavier elements such as carbon, nitrogen or oxygen that
are created later as stars develop. The near absence of such heavy
elements suggests that much of the primordial gas in the dwarf galaxy
has not managed to form stars that subsequently manufacture heavy elements.
