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A leaky galaxy might be offering up clues about a vast
cosmic makeover foisted on the universe during its youth.

Within about a billion years after the Big Bang, something
stripped nearly all of the hydrogen atoms in the universe of their electrons.
This “reionization” puzzles astronomers, who can’t account for all
of the energy needed to make such a sweeping change
(SN: 2/6/17).

A galaxy dubbed the Sunburst Arc might help. It appears to
be blasting ionizing
ultraviolet radiation through a small hole (or holes) carved out of the gas

that permeates the galaxy, researchers report in the Nov. 8 Science. Similar channels in the
earliest generation of galaxies could have provided an escape hatch for harsh
light to zap intergalactic hydrogen.

While massive youthful stars can produce ionizing radiation,
the light has trouble navigating the thickets of gas and dust within the host
galaxy. “Not all of it can get outside the galaxy, let alone reionize the
intergalactic medium,” says Brant Robertson, an astronomer at the University of
California, Santa Cruz, who was not involved with the research. And yet, get
out it must, given what happened to the cosmic inventory of hydrogen.

Directly looking for ionizing radiation from the first
galaxies is out of the question. Intervening gas clouds absorb that faint light
long before it reaches Earth. “The way to go about it is to look for [closer]
analogs,” says Joanna Bridge, an astronomer at the University of Louisville in
Kentucky also not a part of this study. “We look for galaxies that are similar …
and gain an understanding of the physical processes that might have occurred.”

Suburst Arc galaxy
Ionizing radiation blasts out of a single spot in the Sunburst Arc galaxy, made visible here using a special filter on the Hubble Space Telescope. The six spots of light are all from the same source, and are distorted and replicated thanks to the gravity of an intervening galaxy cluster (not shown).T.E. Rivera-Thorsen, Hubble Space Telescope

Meet the Sunburst Arc, a galaxy in the small southern
constellation Apus whose light takes nearly 11 billion years to reach Earth — far,
but not quite as far as the galaxies responsible for reionization. Part of what
makes the Sunburst Arc special is that it hides behind a much closer cluster of
galaxies. The gravity from that cluster amplifies and smears the Sunburst Arc’s
light into an arc — hence its nickname — creating 12 distorted images of the
galaxy smeared across the sky.

Without this gravitational assist, “we probably would not
have noticed it,” says Thøger Emil Rivera-Thorsen, an astronomer at Stockholm
University. “It would have been just one more speck out of millions.”


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Two years ago, Rivera-Thorsen and colleagues noticed that a
particular wavelength of ultraviolet light from this galaxy appeared to
sneak out through small gaps in its hydrogen gas
, like water through a
sieve. This light is not energetic enough to ionize hydrogen. But through those
gaps, the team hypothesized, more energetic ionizing light might slip out as
well.

To test their idea, the team pointed the Hubble Space
Telescope at the Sunburst Arc. In all 12 of the gravitationally distorted
images, the researchers saw ultraviolet light capable of ionizing hydrogen
blasting out of a small region within the galaxy. The source of the ultraviolet
light coincides with a splotch of bright light seen in previous Hubble images,
light that the team suspects radiates from a pocket of intense star formation
no more than about 520 light-years across. The scientists think that the ionizing
radiation from these young stars is using one or a few holes in the surrounding
gas to escape into intergalactic space.

“This object is a unique lab for understanding the detailed
way in which ionizing photons get out of a galaxy,” Robertson says. The find is
also reminiscent of another much closer galaxy, where a few years ago astronomers
reported a similar leak of ionizing light
(SN: 10/10/14).

Whether this actually is a missing piece in the reionization
puzzle remains to be seen. “In the part of the universe we studied, this is an
atypical galaxy,” Rivera-Thorsen says. Out of hundreds of thousands the team
has looked at, no other galaxy appears to behave this way. Whether such open
pathways in the gas were more common in earlier galaxies is unknown. “That’s
still an open question,” he says.

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