Green pea galaxies could help astronomers understand early universe

A new analysis shows
they may be leaking
ionizing radiation,
thereby casting light on a
key stage in the early
evolution of the cosmos.
The rare Green Pea
galaxies that the general
public discovered in 2007
could help confirm
astronomers'
understanding of
reionization, a pivotal
stage in the evolution of
the early universe. This is
according to new findings
from the University of
Michigan.
Reionization occurred a
few hundred million
years after the Big Bang
as the first stars were
turning on and forming
the first galaxies. During
this period, the space
between the galaxies
changed from an opaque,
neutral fog to a
transparent charged
plasma, as it is today.
Plasma is gas that is
electrically charged.
As for how this
happened, the prevailing
theory holds that
massive stars in the early
galaxies produced an
abundance of high-
energy ultraviolet light
that escaped into
intergalactic space. There,
the UV light interacted
with the neutral
hydrogen gas it met,
blasting electrons off the
hydrogen atoms and
leaving behind a plasma
of negatively charged
electrons and positively
charged hydrogen ions.
"We think this is what
happened, but when we
looked at galaxies
nearby, the high-energy
radiation doesn't appear
to make it out. There's
been a push to find some
galaxies that show signs
of radiation escaping,"
said Anne Jaskot, a
doctoral student in
astronomy.
Jaskot and Sally Oey, an
associate professor of
astronomy in the College
of Literature, Science and
the Arts, have found that
the Green Peas could hold
that evidence. Their
findings are published in
the current edition of The
Astrophysical Journal.
"The Green Peas are
compact, highly star-
forming galaxies that are
very similar to the early
galaxies in the universe,"
Jaskot said. "Our analysis
shows they may be
leaking ionizing
radiation."
The researchers focused
on six of the most
intensely star-forming
Green Pea galaxies, which
are between one billion
and five billion light-
years away. They studied
their emission lines as
observed by the Sloan
Digital Sky Survey.
Emission lines show how
light interacts with
matter, and in this case,
they helped the
astronomers understand
the relationship between
the stars and gas in these
galaxies.
The emission lines told
Jaskot and Oey how
much light the galaxies
absorbed. Then, to
determine how much
light was there to start
with, they ran models to
estimate, for example,
how old the galaxies are
and how many stars they
contain. The galaxies, the
researchers determined,
produced more radiation
than the researchers
detected, so they infer
that some of it must have
escaped.
"An analogy might be if
you have a tablecloth and
you spill something on it.
If you see the cloth has
been stained all the way
to the edges, there's a
good chance it also
spilled onto the floor,"
she said. "We're looking
at the gas like the
tablecloth and seeing
how much light it has
absorbed. It has absorbed
a lot of light. We're
seeing that the galaxy is
saturated with it and
there's probably some
extra that spilled off the
edges."
Jaskot says the Green
Peas are exciting
candidates to help
astronomers understand
a major milestone in the
development of the
cosmos 13 billion years
ago.
The paper is called
"Origin and Optical Depth
of Ionizing Radiation in
the 'Green Pea' Galaxies."
The research is funded by
the National Science
Foundation.