Color
image of the cluster taken with Hubble Space Telescope (images in three
different filters were combined to make an RGB image). In the inset we
show three spectra of the multiply imaged systems. They have peaks at
the same wavelength, hence showing that they belong to the same source. Credit: Bradac/HST/W. M. Keck Observatory
MAUNAKEA,
Hawaii – An international team of scientists has detected and
confirmed the faintest early-Universe galaxy ever using the W. M. Keck
Observatory on the summit on Maunakea, Hawaii. In addition
to using the world’s most powerful telescope, the team relied on
gravitational
lensing to see the incredibly faint object born just after the Big Bang.
The results are being published in The Astrophysical Journal Letters today.
The team detected the galaxy as it was 13 billion years ago, or when
the Universe was a toddler on a cosmic time scale.
The detection
was made using the DEIMOS
instrument fitted on the ten-meter Keck II telescope, and was made
possible
through a phenomenon predicted by Einstein in which an object is
magnified by
the gravity of another object that is between it and the viewer. In this
case,
the detected galaxy was behind the galaxy cluster MACS2129.4-0741, which
is massive enough to create three different images of the object.
"Keck
Observatory's telescopes are simply the best in the world for this
work," said Marusa Bradac, a proefssor at University of California,
Davis who led the team. "Their power, paired with the gravitational
force of a massive cluster of galaxies, allows us to truly see where no
human has seen before."
“Because you see three of them and the
characteristics are exactly the same, that means it was lensed,” said Marc
Kassis, staff astronomer at Keck Observatory who assists the discovery team at
night. “The other thing that is particularly interesting is that it is small.
The only way they would have seen it is through lensing. This allowed them to
identify it as an ordinary galaxy near the edge of the visible Universe.”
“If
the light from this galaxy was not
magnified by factors of 11, five and two, we would not have been able to
see
it,” said Kuang-Han Huang, a team member from UC Davis and the lead
author of
the paper. “It lies near the end of the reionization epoch, during which
most of the hydrogen gas between galaxies transitioned from being
mostly neutral to being mostly ionized (and lit up the stars for the
first time). That shows how gravitational lensing is important for
understanding
the faint galaxy population that dominates the reionization photon
production.”
The galaxy’s magnified images were originally
seen separately in both Keck Observatory and Hubble Space Telescope data. The
team collected and combined all the Keck Observatory/DEIMOS spectra from all
three images, confirming they were the same and that this is a triply-lensed
system.
“We now have good constraints on when the
reionization process ends – at redshift around 6 or 12.5 billion years ago –
but we don’t yet know a lot of details about how it happened,” Huang said. “The
galaxy detected in our work is likely a member of the faint galaxy population
that drives the reionization process.”
“This
galaxy is exciting because the team infers a very low stellar mass, or only one
percent of one percent of the Milky Way galaxy,” Kassis said. “It’s a very,
very small galaxy and at such a great distance, it’s a clue in answering one of
the fundamental questions astronomy is trying to understand: What is causing
the hydrogen gas at the very beginning of the Universe to go from neutral to
ionized about 13 billion years ago. That’s when stars turned on and matter
became more complex.”
The core of the team consisted of Bradac,
Huang, Brian Lemaux, and Austin Hoag of UC Davis who are most directly involved
with spectroscopic observation and data reduction of galaxies at redshift above
seven.
Keck Observatory astronomers Luca Rizzi and Carlos Alvarez were
instrumental in helping the team collect the DEIMOS data. Tommaso Treu from
University of California, Los Angeles and Kasper Schmidt of Leibniz Institute
for Astrophysics Potsdam were also part of the team. They lead the effort that
obtains and analyzes spectroscopic data from the WFC3/IR grism on Hubble.
The W. M. Keck Observatory operates the
largest, most scientifically productive telescopes on Earth. The two, 10-meter
optical/infrared telescopes near the summit of Maunakea on the Island of Hawaii
feature a suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field spectrographs and
world-leading laser guide star adaptive optics systems.
DEIMOS (the
DEep Imaging and Multi-Object Spectrograph) boasts the largest field of view
(16.7 arcmin by 5 arcmin) of any of the Keck instruments, and the largest
number of pixels (64 Mpix). It is used primarily in its multi-object mode,
obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers
study fields of distant galaxies with DEIMOS, efficiently probing the most
distant corners of the universe with high sensitivity.
Keck Observatory is a private 501(c) 3
non-profit organization and a scientific partnership of the California
Institute of Technology, the University of California and NASA.
By Steve Jefferson
Source: W.M. Keck Observatory
