YSM Issue 90.4

FEATURE
astronomy
S T A R R Y
FUEL TANKS
What fuels the
starburst phases
of galaxies?
A long time ago, in galaxies far, far away, stars were
churned out at unprecedented rates; over 100 solar masses
were produced annually. These luminous, dusty starburst
galaxies were 1000 times more common in the very early
universe than they are today. But the light from these stars
is only now reaching the earth, ending its multi-billion year
journey through the cosmos as it reaches our telescopes.
By looking out into the universe and back into the past, we
gain a better understanding of how these starburst galaxies
formed and how they sustained such rapid rates of star
production.
In the early universe, galaxies differed in their abilities to
produce stars. Many of the galaxies at this time were unable
to support such rapid star formation, as the process blasted
away the hydrogen gas, preventing the creation of future
stars. Yet others continued to produce hundreds of stars per
year for periods up to 100 million years, long after other
galaxies had subsided. Why did some galaxies remain fertile
while others died down? Recent research suggests that we
can find the answer with the help of CH+, a rare but useful
cation (a positively-charged, electron-deficient molecule).
Using the Atacama Large Millimeter Array (ALMA)
radio telescopes in Northern Chile, astronomers studied six
early starburst galaxies. They found that CH+ is abundantly
present in all of them, providing insight into what set these
galaxies apart from the others. CH+ is very rare and was
only discovered in 1941, as it can only form in extremely cold
temperatures of about 20 Kelvin and with extremely high
energy inputs equivalent to 4000 Kelvin. With such a high
by ELIZABETH RUDDY || art by SONIA RUIZ
energy requirement, CH+ must be formed in the presence
of strong ultraviolet radiation or mechanical energy. It also
has an extremely short lifespan, so it cannot be transported
far. Therefore, its presence in these galaxies suggests that they
must have recently undergone violent energy shocks.
“CH+ traces how energy flows in a galaxy. Think of plankton
fluorescence which is excited by little shocks that generate
turbulence in the water. When you throw a stone in, you
light up trails of fluorescent plankton,” said Edith Falgarone
of the École Normale Supérieure and Observatoire, where
the research was conducted.
Falgarone and her team of researchers studied the emission
and absorption spectral lines of this CH+ cation in samples
from the six starburst galaxies. Each spectral line emitted
by the interstellar gas corresponds to a different compound
present in the galaxy, so the CH+ spectral lines can tell us a
lot about how the CH+ was formed. The lines revealed the
presence of extremely turbulent hydrogen gas surrounding
the galaxies, extending far outwards from the cores where
stars form.
The discovery of this turbulent gas elucidates how galaxies
grow and how these star-forming engines are fueled.
The width of the CH+ lines are broader than 1000 kilometers
per second, suggesting that it was born in enormous
shock waves. The researchers suggest that these motions
are powered by energetic outflows originating in the core
of the galaxy. These outflows exit the galaxy in such a way
that leaves matter trapped within the galaxy’s gravitational
pull. This culminates in vast, turbulent reservoirs of cool,
28 Yale Scientific Magazine October 2017 www.yalescientific.org