Using
the amplifying power of a cosmic gravitational lens, astronomers have
discovered a distant galaxy whose stars were born unexpectedly early in
cosmic history. This result sheds new light on the formation of the
first galaxies, as well as on the early evolution of the Universe.
“We
have discovered a distant galaxy that began forming stars just 200
million years after the Big Bang. This challenges theories of how soon
galaxies formed and evolved in the first years of the Universe. It could
even help solve the mystery of how the hydrogen fog that filled the
early Universe was cleared,” says Johan Richard, the lead author of a
new study.
Richard’s
team spotted the galaxy in recent observations from the NASA/ESA Hubble
Space Telescope, verified it with observations from the NASA Spitzer
Space Telescope and measured its distance using W. M. Keck Observatory
in Hawaii.
The
distant galaxy is visible through a cluster of galaxies called Abell
383, whose powerful gravity bends the rays of light almost like a
magnifying glass. The chance alignment of the galaxy, the cluster and
the Earth amplifies the light reaching us from this distant galaxy,
allowing the astronomers to make detailed observations. Without this
gravitational lens, the galaxy would have been too faint to be observed
even with today’s largest telescopes.
After
spotting the galaxy in Hubble and Spitzer images, the team carried out
spectroscopic observations with the Keck-II telescope in Hawaii.
Spectroscopy is the technique of breaking up light into its component
colours. By analysing these spectra, the team was able to make detailed
measurements of its redshift and infer information about the properties
of its component stars.
The
galaxy’s redshift is 6.027, which means we see it as it was when the
Universe was around 950 million years old. This does not make it the
most remote galaxy ever detected — several have been confirmed at
redshifts of more than 8, and one has an estimated redshift of around 10
(heic1103), placing it 400 million years earlier. However the newly
discovered galaxy has dramatically different features from other distant
galaxies that have been observed, which generally shine brightly with
only young stars.
“When
we looked at the spectra, two things were clear,” explains co-author
Eiichi Egami. “The redshift placed it very early in cosmic history, as
we expected. But the Spitzer infrared detection also indicated that the
galaxy was made up of surprisingly old and relatively faint stars. This
told us that the galaxy was made up of stars already nearly 750 million
years old — pushing back the epoch of its formation to about 200 million
years after the Big Bang, much further than we had expected.”
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Co-author
Dan Stark continues: “Thanks to the amplification of the galaxy’s light
by the gravitational lens, we have some excellent quality data. Our
work confirms some earlier observations that had hinted at the presence
of old stars in early galaxies. This suggests that the first galaxies
have been around for a lot longer than previously thought.”
The
discovery has implications beyond the question of when galaxies first
formed, and may help explain how the Universe became transparent to
ultraviolet light in the first billion years after the Big Bang. In the
early years of the cosmos, a diffuse fog of neutral hydrogen gas blocked
ultraviolet light in the Universe. Some source of radiation must
therefore have progressively ionised the diffuse gas, clearing the fog
to make it transparent to ultraviolet rays as it is today — a process
known as reionisation.
Astronomers
believe that the radiation that powered this reionisation must have
come from galaxies. But so far, nowhere near enough of them have been
found to provide the necessary radiation. This discovery may help solve
this enigma.
“It
seems probable that there are in fact far more galaxies out there in
the early Universe than we previously estimated — it’s just that many
galaxies are older and fainter, like the one we have just discovered,”
says co-author Jean-Paul Kneib. “If this unseen army of faint, elderly
galaxies is indeed out there, they could provide the missing radiation
that made the Universe transparent to ultraviolet light.”
As
of today, we can only discover these galaxies by observing through
massive clusters that act as cosmic telescopes. In coming years, the
NASA/ESA/CSA James Webb Space Telescope, scheduled for launch later this
decade, will specialise in high resolution observations of distant,
highly redshifted objects. It will therefore be in an ideal position to
solve this mystery once and for all.
The
research will appear in a paper entitled “Discovery of a possibly old
galaxy at z=6.027, multiply imaged by the massive cluster Abell 383”, to
be published in the Monthly Notices of the Royal Astronomical Society.