32 AUSTRALIAN SKY & TELESCOPE May | June 2018
NOAO / AURA / NSF / N. SMITH (UNIVERSITY OF ARIZONA)because dust grains consist mostly of
carbon, oxygen, magnesium, silicon and iron, all of which are
products of stars and are therefore scarce in these galaxies. In
the Milky Way’s disk, gas makes up 99% of interstellar matter
and dust 1%. In I Zwicky 18, however, dust accounts for a
mere 0.001% of interstellar matter.
When it exists, dust affects star formation. Dust grains
darken star-forming clouds, shielding them from harsh
radiation, and also promote star formation by emitting far-
infrared radiation, which carries heat away from gas clouds.
So do carbon and oxygen atoms. Cooler clouds have less
thermal pressure, a force that counteracts gravity and can
prevent a cloud from collapsing and becoming a star. Thus,with a shortage of dust and carbon and oxygen atoms to cool
them, gas clouds in oxygen-poor galaxies may have to be more
massive in order for gravity to take over and force collapse,
giving rise to a greater proportion of massive stars. The same
thing presumably happened in the universe’s first galaxies.
Dust grains also serve as platforms on which atoms can
meet one another and make molecules, such as molecular
hydrogen, the most abundant molecule in the Milky Way’s
gas. Big clouds of molecular hydrogen fuel many of our
galaxy’s star-forming regions. No one has ever detected any
molecular gas in a galaxy as oxygen-poor as I Zwicky 18.A new record-breaker
In 2017, Izotov and Thuan’s team discovered the current
champion: a starburst galaxy in Lynx that’s 620 million light-
years distant — 10 times farther than I Zwicky 18 — bearing
the prosaic name J0811+4730. The blue galaxy spawned 80%
of its stars during the past few million years, and its oxygen
abundance is a mere 6.98, the lowest ever seen. That’s just
1.7% the level of oxygen in the Sun.
Still, the galaxy emerged only after the astronomers
searched a million spectra from the Sloan Digital Sky
Survey. Thuan once hoped to find truly primordial galaxies
containing no oxygen at all, but he now wonders whether
he’ll ever succeed. “It’s very, very difficult,” he says. “I’ve
spent nearly 40 years of my professional life trying to find
these things, but so far to no avail.”
Perhaps, he adds, an ancient generation of massive stars
showered the whole universe with metals, setting a minimum
oxygen level in any galaxy that arose. As a result, galaxies
with much less oxygen than the current record-breaker
simply might not exist.Little galaxies, little particles
With all the new discoveries, the field is blossoming. That
may be good news not just for astronomers but also for
particle physicists, because these galaxies could reveal how
many types, or ‘flavours,’ of neutrinos there are.
Neutrinos are tiny neutral particles that whiz through
space — and our bodies — at nearly the speed of light. Hence
their name: Neutrino means ‘little neutral one’ in Italian.
Physicists recognise three flavours: electron, muon and tau.
If a fourth flavour exists, however, it would have affected the
nuclear reactions during the universe’s first three minutes
and raised the amount of helium the Big Bang produced.
But how much helium did the Big Bang actually make? We
can’t look to Earth for the answer, because the scant helium
we have here has nothing to do with the Big Bang. Instead,
the lighter-than-air gas that lifts blimps and balloons stems
from the radioactive decay of heavy elements such as thorium
and uranium.
In contrast, most of the helium on the Sun’s surface did
come from the Big Bang. Unfortunately, countless stars
that lived and died before the Sun’s birth also contributed.What Is Dust?
Cosmic dust grains are small particles that
range in size from about 10 to 100 nm. They’re
mostly silicate- or carbon-based and form
in the atmospheres of aging red giant stars
and in supernovae. Dust absorbs light with
wavelengths similar to or smaller than its grain
size, then re-emits it at infrared wavelengths.
Its presence shields interstellar molecules from
high-energy radiation and enables protostars to
radiate away excess energy.PRIMORDIAL PROXIES(continued from page 29)