Artist’s impression of CR7: the brightest galaxy in the early Universe
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VLT discovers CR7, the brightest distant galaxy, and signs of Population III stars
Astronomers using ESO’s Very Large
Telescope have discovered by far the brightest galaxy yet found in the
early Universe and found strong evidence that examples of the first
generation of stars lurk within it. These massive, brilliant, and
previously purely theoretical objects were the creators of the first
heavy elements in history — the elements necessary to forge the stars
around us today, the planets that orbit them, and life as we know it.
The newly found galaxy, labelled CR7, is three times brighter than the
brightest distant galaxy known up to now.
Astronomers have long theorised the existence of a first generation of stars — known as Population III stars — that were born out of the primordial material from the Big Bang [1].
All the heavier chemical elements — such as oxygen, nitrogen, carbon
and iron, which are essential to life — were forged in the bellies of
stars. This means that the first stars must have formed out of the only
elements to exist prior to stars: hydrogen, helium and trace amounts of
lithium.
These Population III stars would have been enormous — several hundred
or even a thousand times more massive than the Sun — blazing hot, and
transient — exploding as supernovae after only about two million years.
But until now the search for physical proof of their existence had been
inconclusive [2].
A team led by David Sobral, from the Institute of Astrophysics and
Space Sciences, the Faculty of Sciences of the University of Lisbon in
Portugal, and Leiden Observatory in the Netherlands, has now used ESO’s Very Large Telescope (VLT) to peer back into the ancient Universe, to a period known as reionisation,
approximately 800 million years after the Big Bang. Instead of
conducting a narrow and deep study of a small area of the sky, they
broadened their scope to produce the widest survey of very distant
galaxies ever attempted.
Their expansive study was made using the VLT with help from the W. M. Keck Observatory and the Subaru Telescope as well as the NASA/ESA Hubble Space Telescope. The team discovered — and confirmed — a number of surprisingly bright very young galaxies. One of these, labelled CR7 [3], was an exceptionally rare object, by far the brightest galaxy ever observed at this stage in the Universe [4].
With the discovery of CR7 and other bright galaxies, the study was
already a success, but further inspection provided additional exciting
news.
The X-shooter and SINFONI
instruments on the VLT found strong ionised helium emission in CR7 but —
crucially and surprisingly — no sign of any heavier elements in a
bright pocket in the galaxy. This meant the team had discovered the
first good evidence for clusters of Population III stars that had
ionised gas within a galaxy in the early Universe [5].
“The discovery challenged our expectations from the start,” said David Sobral, “as
we didn’t expect to find such a bright galaxy. Then, by unveiling the
nature of CR7 piece by piece, we understood that not only had we found
by far the most luminous distant galaxy, but also started to realise
that it had every single characteristic expected of Population III
stars. Those stars were the ones that formed the first heavy atoms that
ultimately allowed us to be here. It doesn’t really get any more
exciting than this.”
Within CR7, bluer and somewhat redder clusters of stars were found,
indicating that the formation of Population III stars had occurred in
waves — as had been predicted. What the team directly observed was the
last wave of Population III stars, suggesting that such stars should be
easier to find than previously thought: they reside amongst regular
stars, in brighter galaxies, not just in the earliest, smallest, and
dimmest galaxies, which are so faint as to be extremely difficult to
study.
Jorryt Matthee, second author of the paper, concluded: “I have
always wondered where we come from. Even as a child I wanted to know
where the elements come from: the calcium in my bones, the carbon in my
muscles, the iron in my blood. I found out that these were first formed
at the very beginning of the Universe, by the first generation of stars.
With this discovery, remarkably, we are starting to actually see such
objects for the first time.”
Further observations with the VLT, ALMA,
and the NASA/ESA Hubble Space Telescope are planned to confirm beyond
doubt that what has been observed are Population III stars, and to
search for and identify further examples.
Notes
[1] The name Population III arose because astronomers had already
classed the stars of the Milky Way as Population I (stars like the Sun,
rich in heavier elements and forming the disc) and Population II (older
stars, with a low heavy-element content, and found in the Milky Way
bulge and halo, and globular star clusters).
[2] Finding these stars is very
difficult: they would have been extremely short-lived, and would have
shone at a time when the Universe was largely opaque to their light.
Previous findings include: Nagao, et al., 2008, where no ionised helium was detected; De Breuck et al., 2000, where ionised helium was detected, but alongside carbon and oxygen, as well as clear signatures of an active galactic nucleus; and Cassata et al., 2013, where ionised helium was detected, but of a very low equivalent width, or weak intensity, and alongside carbon and oxygen.
[3] CR7’s nickname is an abbreviation of COSMOS Redshift
7, a measure of its place in terms of cosmic time. The higher the
redshift, the more distant the galaxy and the further back in the
history of the Universe it is seen. A1689-zD1, one of the oldest galaxies ever observed, for example, has a redshift of 7.5.
CR7 is located in the COSMOS field, an intensely studied patch of sky in the constellation of Sextans (The Sextant).
The nickname was inspired by the great Portuguese footballer, Cristiano Ronaldo, who is known as CR7.
[4] CR7 is three times brighter in terms of ultraviolet light emission than the previous titleholder, Himiko, which was thought to be one of a kind at this very early time. Dusty galaxies, at
later stages in the history of the Universe, may radiate far more total
energy than CR7 in the form of infrared radiation from warm dust. The
energy coming from CR7 is mostly ultraviolet/visible light.
[5] The team considered two alternate theories: that the source of the light was either from an AGN or Wolf–Rayet
stars. The lack of heavy elements, and other evidence strongly refutes
both these theories. The team also considered that the source may be a direct-collapse black hole,
which are themselves exceptional exotic and purely theoretical objects.
The lack of a broad emission line and the fact that the hydrogen and
helium luminosities were much greater than what has been predicted for
such a black hole indicate that this, too, is unlikely. A lack of X-ray
emissions would further refute this possibility, but additional
observations are needed.
More Information
This research was presented in a paper entitled “Evidence for PopIII-like stellar populations in the most luminous Lyman-α emitters at the epoch of re-ionisation: spectroscopic confirmation”, by D. Sobral, et al., is accepted for publication in The Astrophysical Journal.
The team is composed of David Sobral (Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Lisbon, Portugal; Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal; Leiden Observatory, Leiden University, Leiden, The Netherlands), Jorryt Matthee (Leiden Observatory), Behnam Darvish (Department of Physics and Astronomy, University of California, Riverside, California, USA), Daniel Schaerer (Observatoire de Genève, Département d’Astronomie, Université de Genève, Versoix, Switzerland; Centre National de la Recherche Scientifique, IRAP, Toulouse, France), Bahram Mobasher (Department of Physics and Astronomy, University of California, Riverside, California, USA), Huub J. A. Röttgering (Leiden Observatory), Sérgio Santos (Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa; Departamento de Física, Universidade de Lisboa, Portugal) and Shoubaneh Hemmati (Department of Physics and Astronomy, University of California, Riverside, California, USA).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
This research was presented in a paper entitled “Evidence for PopIII-like stellar populations in the most luminous Lyman-α emitters at the epoch of re-ionisation: spectroscopic confirmation”, by D. Sobral, et al., is accepted for publication in The Astrophysical Journal.
The team is composed of David Sobral (Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, Lisbon, Portugal; Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal; Leiden Observatory, Leiden University, Leiden, The Netherlands), Jorryt Matthee (Leiden Observatory), Behnam Darvish (Department of Physics and Astronomy, University of California, Riverside, California, USA), Daniel Schaerer (Observatoire de Genève, Département d’Astronomie, Université de Genève, Versoix, Switzerland; Centre National de la Recherche Scientifique, IRAP, Toulouse, France), Bahram Mobasher (Department of Physics and Astronomy, University of California, Riverside, California, USA), Huub J. A. Röttgering (Leiden Observatory), Sérgio Santos (Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa; Departamento de Física, Universidade de Lisboa, Portugal) and Shoubaneh Hemmati (Department of Physics and Astronomy, University of California, Riverside, California, USA).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
Links
Contacts
David Sobral
Universidade de Lisboa and Leiden University
Lisbon / Leiden, Portugal / The Netherlands
Tel: +351 916 700 769
Email: sobral@iastro.pt
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
João Retrê
Coordinator, Science Communication and Outreach Office, Instituto de Astrofísica e Ciências do Espaço
Lisbon, Portugal
Tel: +351 21 361 67 49
Email: jretre@iastro.pt
