First stars formed even later than previously thought

ESA's Planck satellite has revealed that the first stars in the Universe started forming later than previous observations of the Cosmic Microwave Background indicated. This new analysis also shows that these stars were the only sources needed to account for reionising atoms in the cosmos, having completed half of this process when the Universe had reached an age of 700 million years. 

Cosmic reionization

Credit: ESA – C. Carreau

With the multitude of stars and galaxies that populate the present Universe, it's hard to imagine how different our 13.8 billion year cosmos was when it was only a few seconds old. At that early phase, it was a hot, dense primordial soup of particles, mostly electrons, protons, neutrinos, and photons – the particles of light.

 In such a dense environment the Universe appeared like an 'opaque' fog, as light particles could not travel any significant distance before colliding with electrons.

As the cosmos expanded, the Universe grew cooler and more rarefied and, after about 380 000 years, finally became 'transparent'. By then, particle collisions were extremely sporadic and photons could travel freely across the cosmos.

History of the Universe

Credit: ESA

Today, telescopes like Planck can observe this fossil light across the entire sky as the Cosmic Microwave Background, or CMB. Its distribution on the sky reveals tiny fluctuations that contain a wealth of information about the history, composition and geometry of the Universe.

The release of the CMB happened at the time when electrons and protons joined to form hydrogen atoms. This is the first moment in the history of the cosmos when matter was in an electrically neutral state.

After that, a few hundred million years passed before these atoms could assemble and eventually give rise to the Universe's first generation of stars.

As these first stars came to life, they filled their surroundings with light, which subsequently split neutral atoms apart, turning them back into their constituent particles: electrons and protons.

Scientists refer to this as the 'epoch of reionisation'. It did not take long for most material in the Universe to become completely ionised, and – except in a very few, isolated places – it has been like that ever since.

Observations of very distant galaxies hosting supermassive black holes indicate that the Universe had been completely reionised by the time it was about 900 million years old. The starting point of this process, however, is much harder to determine and has been a hotly debated topic in recent years.

"The CMB can tell us when the epoch of reionisation started and, in turn, when the first stars formed in the Universe," explains Jan Tauber, Planck project scientist at ESA.

To make this measurement, scientists exploit the fact that a fraction of the CMB is polarised: part of the light vibrates in a preferred direction. This results from CMB photons bouncing off electrons – something that happened very frequently in the primordial soup, before the CMB was released, and then again later, after reionisation, when light from the first stars brought free electrons back onto the cosmic stage.

"It is in the tiny fluctuations of the CMB polarisation that we can see the influence of the reionisation process and deduce when it began," adds Tauber.

Polarisation of the Cosmic Microwave Background

Credit: ESA and the Planck Collaboration

A first estimate of the epoch of reionisation came in 2003 from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), suggesting that this process might have started early in cosmic history, when the Universe was only a couple of hundred million years old. This result was problematic, because there is no evidence that any stars had formed by then, which would mean postulating the existence of other, exotic sources that could have caused the reionisation at that time.

This first estimate was soon to be corrected, as subsequent data from WMAP pushed the starting time to later epochs, indicating that the Universe had not been significantly reionised until at least some 450 million years into its history.

This eased, but did not completely solve the puzzle: although the earliest of the first stars have been observed to be present already when the Universe was 300 to 400 million years old, it remained unclear whether these stars were the main culprits for reionising fully the cosmos or whether additional, more exotic sources must have played a role too.

In 2015, the Planck Collaboration provided new data to tackle the problem, moving the reionisation epoch even later in cosmic history and revealing that this process was about half-way through when the Universe was around 550 million years old. The result was based on Planck's first all-sky maps of the CMB polarisation, obtained with its Low-Frequency Instrument (LFI).

Now, a new analysis of data from Planck's other detector, the High-Frequency Instrument (HFI), which is more sensitive to this phenomenon than any other so far, shows that reionisation started even later – much later than any previous data have suggested.

"The highly sensitive measurements from HFI have clearly demonstrated that reionisation was a very quick process, starting fairly late in cosmic history and having half-reionised the Universe by the time it was about 700 million years old," says Jean-Loup Puget from Institut d'Astrophysique Spatiale in Orsay, France, principal investigator of Planck's HFI.

"These results are now helping us to model the beginning of the reionisation phase."

"We have also confirmed that no other agents are needed, besides the first stars, to reionise the Universe," adds Matthieu Tristram, a Planck Collaboration scientist at Laboratoire de l'Accélérateur Linéaire in Orsay, France.

The new study locates the formation of the first stars much later than previously thought on the cosmic timeline, suggesting that the first generation of galaxies are well within the observational reach of future astronomical facilities, and possibly even some current ones.

In fact, it is likely that some of the very first galaxies have already been detected with long exposures, such as the Hubble Ultra Deep Field observed with the NASA/ESA Hubble Space Telescope, and it will be easier than expected to catch many more with future observatories such as the NASA/ESA/CSA James Webb Space Telescope.

Notes for Editors

'Planck intermediate results. XLVII. Planck constraints on reionization history' and 'Planck intermediate results. XLVI. Reduction of large-scale systematic effects in HFI polarization maps and estimation of the reionization optical depth' by the Planck Collaboration are published in Astronomy and Astrophysics. 

Launched in 2009, Planck was designed to map the sky in nine frequencies using two state-of-the-art instruments: the Low Frequency Instrument (LFI), which includes three frequency bands in the range 30-70 GHz, and the High Frequency Instrument (HFI), which includes six frequency bands in the range 100-857 GHz.

HFI completed its survey in January 2012, while LFI continued to make science observations until 3 October 2013, before being switched off on 19 October 2013. Seven of Planck's nine frequency channels were equipped with polarisation-sensitive detectors.

The Planck Scientific Collaboration consists of all the scientists who have contributed to the development of the mission, and who participate in the scientific exploitation of the data during the proprietary period.

These scientists are members of one or more of four consortia: the LFI Consortium, the HFI Consortium, the DK-Planck Consortium, and ESA's Planck Science Office. The two European-led Planck Data Processing Centres are located in Paris, France and Trieste, Italy.

The LFI consortium is led by N. Mandolesi, Università degli Studi di Ferrara, Italy (deputy PI: M. Bersanelli, Università degli Studi di Milano, Italy), and was responsible for the development and operation of LFI. The HFI consortium is led by J.L. Puget, Institut d'Astrophysique Spatiale in Orsay (CNRS/Université Paris-Sud), France (deputy PI: F. Bouchet, Institut d'Astrophysique de Paris (CNRS/UPMC), France), and was responsible for the development and operation of HFI. 

 
For further informatin, please contact:

Jan Tauber
ESA Planck Project Scientist
Scientific Support Office
Directorate of Science
European Space Agency
Email: jan.tauber@esa.int
Phone: +31-71-565-5342

Jean-Loup Puget
Principal Investigator, High Frequency Instrument
Institut d'Astrophysique Spatiale
Orsay, France
Email: jean-loup.puget@ias.u-psud.fr
Phone: +33-169858665

Matthieu Tristram
CNRS - IN2P3
Laboratoire de l'Accélérateur Linéaire
Université Paris-Sud 11
Orsay, France
Email: tristram@lal.in2p3.fr
Phone: +33-164468388 

Source: ESA/PLANCK

Milky way's first stars killed the satellite galaxies

Two researchers from Observatoire Astronomique de Strasbourg have revealed for the first time the existence of a new signature of the birth of our galaxy's first stars. More than 12 billion years ago, their intense light dispersed the gas of the Milky Way's satellite galaxies. By computing the observable consequences of this process, Pierre Ocvirk and Dominique Aubert demonstrated their prevailing role. This result confirms that reionisation is indeed an essential process in the standard model of galaxy formation. The study took place within the LIDAU collaboration (Light In the Dark Ages of the Universe). It is published in the october issue of the letters of the Monthly Notices of the Royal Astronomical Society.

 

The first stars of the Universe appeared about 150 million years after the Big Bang. Back then, the hydrogen and helium gas filling the universe was cold enough to have its atoms be electrically neutral. As the intense light of the first stars propagated through this gas, it broke the hydrogen atoms, returning them to the plasma state they experienced in the first moments of the Universe. This process, known as reionisation, also results in significant heating, which can have dramatic consequences: the gas becomes so hot that it escapes the weak gravity of the lowest mass galaxies, thereby depriving them of the material needed to form stars. It is now widely admitted that this photo-evaporation process explains the small number and large ages of the stars seen in the dwarf galaxies satellites of the Milky Way. It also offers a credible solution to the missing satellites problem. On the other hand, their sensitivity to UV radiation means satellite galaxies are good probes of the reionisation epoch. Moreover, they are relatively nearby, from 30000 to 900000 light-years, which allows us to study them in great details, especially with the forthcoming generation of telescopes. In particular, the study of their stellar content with respect to their position could give us precious insight into the structure of the local UV radiation field during the reionisation epoch. 

 

Tiff Image

 

Until now, satellite galaxies models assumed that the radiation leading to the photo-evaporation of their gas was produced collectively by the large galaxies nearby, resulting in a quasi-uniform background at the scale of the Milky Way. The new model built by the two french researchers proves this assumption wrong. It is based on a high resolution numerical simulation (the Via Lactea II) describing the dynamics of the dark matter haloes that populated our galaxy and its neigbourhood from the Big Bang to present times. This dataset is completed by a description of the formation of stars from the gas trapped in these haloes, and in paricular a detailed model of the reaction of this gas to UV radiation. 

 

It is the first time that a model accounts for the effect of the radiation emitted by the first stars formed at the center of the Milky way, on its satellite galaxies. Indeed, contrary to previous models, the radiation field produced in this configuration is not uniform, but decreases in intensity as one moves away from the source. On one hand, the satellite galaxies close to the galactic center see their gas evaporate very quickly. They form so few stars that they can be undetectable with current telescopes. On the other hand, the more remote satellite galaxies experience on average a weaker irradiation. Therefore they manage to keep their gas longer, and form more stars. As a consequence they are easier to detect and appear more numerous. 

 

 

Previous models assumed a uniform UV background during reionisation. In contrast, the influence of the first stars of the Milky Way results in fewer satellite galaxies in the inner parts of our galaxy, and an excess in the outer parts. Comparing the observed spatial distribution of the satellite galaxies with the predictions of the new model, it appears that the latter matches the observations much better than older models. This suggests that the first stars of our galaxy played a major role in the photo-evaporation of the satellite galaxies' gas. It is not the large nearby galaxies, but our own, who caused the demise of her tiny sisters, asphyxiating them through her intense radiation. 

 

This new scenario has deep consequences on the formation of galaxies and the interpretation of the large astronomicals surveys to come. Indeed, satellite galaxies are affected by our galaxy's tidal field, and can be slowly digested into our galaxy's stellar halo. They can also be stretched into filaments and form stellar streams. These will be the main science goals of the Gaia space mission, scheduled for launch in 2013. Therefore we need to understand as soon as possible how they are affected by radiative processes during reionisation.  

 

Notes The LIDAU project is funded by the french Agence Nationale pour la Recherche (ANR). The collaboration comprises the 2 researchers from Observatoire Astronomique de Strasbourg, as well as Benoit Semelin, Patrick Vonlanthen et Kenji Hasegawa, who belong to LERMA (Observatoire de Paris).

 

The missing satellite problem:

 

The missing satellties problem was formulated about 10 years ago, as a disagreement between the expected and observed numbers of satellite galaxies of the Milky Way. While standard numerical simulations predicted as many as 500, only 10 of them were known, and still only about 20 at present. This means that, either thes galaxies do not exist, thereby ruling out the standard cosmological model, or they do exist, but are rendered undetectable for some unknow reason. This problem finds a credible solution in the processes resulting from the UV background pervading the Universe during reionization. Its intensity may be sufficient to photo-evaporate the gas of low mass satellite galaxies and stop their star formation very early on. The paucity of their stars eventually makes them difficult to detect today, and explains why we see so few of them.

 

Reference

A signature of the internal reionisation of the Milky Way?, Pierre Ocvirk, Dominique Aubert, in press in Monthly Notices of the Royal Astronomical Society - Letters : http://arxiv.org/abs/1108.1193

Distant Galaxies Reveal The Clearing of the Cosmic Fog

PR Image eso1138a
Artist’s impression of galaxies at the end of the era of reionisation

PR Image eso1138b
A galaxy seen when the Universe was only 820 million years old

PR Image eso1138c
A galaxy seen when the Universe was only 840 million years old

PR Video eso1138a
Animation of artist’s impression of galaxies at the end of the era of reionisation

Scientists have used ESO’s Very Large Telescope to probe the early Universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history — known as reionisation — occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionisation for the first time. They have also demonstrated that this phase must have happened quicker than astronomers previously thought.

An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang [1].
The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation [2] for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.

The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.

“Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil. Astronomers can go one better: we can look directly into the remote past and observe the faint light from different galaxies at different stages in cosmic evolution,” explains Adriano Fontana, of INAF Rome Astronomical Observatory who led this project. “The differences between the galaxies tell us about the changing conditions in the Universe over this important period, and how quickly these changes were occurring.”

Different chemical elements glow brightly at characteristic colours. These spikes in brightness are known as emission lines. One of the strongest ultraviolet emission lines is the Lyman-alpha line, which comes from hydrogen gas [3]. It is bright and recognisable enough to be seen even in observations of very faint and faraway galaxies.

Spotting the Lyman-alpha line for five very distant galaxies [4] allowed the team to do two key things: first, by observing how far the line had been shifted toward the red end of the spectrum, they were able to determine the galaxies’ distances, and hence how soon after the Big Bang they could see them [5]. This let them place them in order, creating a timeline which shows how the galaxies’ light evolved over time. Secondly, they were able to see the extent to which the Lyman-alpha emission — which comes from glowing hydrogen within the galaxies — was reabsorbed by the neutral hydrogen fog in intergalactic space at different points in time.

“We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample,” says lead author Laura Pentericci of INAF Rome Astronomical Observatory. “When the Universe was only 780 million years old this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe’ volume. But only 200 million years later the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionisation must have happened quicker than astronomers previously thought.”

As well as probing the rate at which the primordial fog cleared, the team’s observations also hint at the likely source of the ultraviolet light which provided the energy necessary for reionisation to occur. There are several competing theories for where this light came from — two leading candidates are the Universe’s first generation of stars [6], and the intense radiation emitted by matter as it falls towards black holes.

"The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed," says Eros Vanzella of the INAF Trieste Observatory, a member of the research team. "These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent."

The highly accurate measurements required to confirm or disprove this hypothesis, and show that the stars can produce the required energy, require observations from space, or from ESO’s planned European Extremely Large Telescope, which will be the world’s largest eye on the sky once completed early next decade.

Studying this early period in cosmic history is technically challenging because accurate observations of extremely distant and faint galaxies are needed, a task which can only be attempted with the most powerful telescopes. For this study, the team used the great light-gathering power of the 8.2-metre VLT to carry out spectroscopic observations, targetting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.

Notes

[1] The most distant galaxy that has been reported with a distance measured by spectroscopy is at a redshift of 8.6, placing it 600 million years after the Big Bang (eso1041). There is a candidate galaxy thought to be at a redshift of about 10 (480 million years after the Big Bang) identified by the Hubble Space Telescope, but this is awaiting confirmation. The most distant galaxy in this study is at a redshift of 7.1, placing it 780 million years after the Big Bang. The Universe today is 13.7 billion years old. The new sample of five confirmed galaxies with Lyman-alpha detections (out of 20 candidates) includes half of all galaxies known at z>7.

[2] At the time the first stars and galaxies formed, the Universe was filled with electrically neutral hydrogen gas, which absorbs ultraviolet light. As the ultraviolet radiation from these early galaxies excited the gas, making it electrically charged (ionised), it gradually became transparent to ultraviolet light. This process is technically known as reionisation, as there is thought to have been a brief period within the first 100 000 years after the Big Bang in which the hydrogen was also ionised.

[3] The team measured the effects of the hydrogen fog using spectroscopy, a technique which involves splitting and spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow.

[4] The team used the VLT to study the spectra of 20 candidate galaxies at redshifts close to 7. These come from deep imaging studies of three separate fields. Of these 20 targets five were found to have clearly detected Lyman-alpha emission. This is currently the only set of spectroscopically confirmed galaxies around z=7.

[5] Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, the characteristic red colour this gives to extremely distant objects has become known as ‘redshift’. Although it is technically a measure of how the colour of an object’s light has been affected, it is also by extension a measure both of the object’s distance, and of how long after the Big Bang we see it.

[6] Astronomers classify stars into three categories, known as Population I, Population II and Population III. Population I stars, like our Sun, are rich in heavier elements synthesised in the hearts of older stars and in supernova explosions: as they are made up from the wreckage of previous generations of stars, they only came into existence later in the Universe. Population II stars have fewer heavy elements in them and are predominantly made up of the hydrogen, helium and lithium created during the Big Bang. These are older stars, though there are still many of them in existence in the Universe today. Population III stars have never been directly observed, though they are thought to have existed in the early years of the Universe. As these contained only the material created during the Big Bang, they contained no heavier elements at all. Because of the role of heavier elements in the formation of stars, only very large stars with very short lifespans were able to form at this stage, and so all the Population III stars quickly ended their lives in supernovae in the early years of the Universe. Up to now, no solid evidence of Population III stars has been confirmed even in observations of very distant galaxies.
More information

This research was presented in a paper “Spectroscopic Confirmation of z∼7 LBGs: Probing the Earliest Galaxies and the Epoch of Reionization”, to appear in the Astrophysical Journal.

The team is composed of L.Pentericci (INAF Osservatorio Astronomico di Roma, Rome, Italy [INAF-OAR]), A. Fontana (INAF-OAR), E. Vanzella (INAF Osservatorio Astronomico di Trieste, Trieste, Italy [INAF-OAT]), M. Castellano (INAF-OAR), A. Grazian (INAF-OAR), M. Dijkstra (Max-Planck-Institut für Astrophysik, Garching, Germany), K. Boutsia (INAF-OAR), S. Cristiani (INAF-OAT), M. Dickinson (National Optical Astronomy Observatory, Tucson, USA), E. Giallongo (INAF-OAR), M. Giavalisco (University of Massachusetts, Amherst, USA), R. Maiolino (INAF-OAR), A. Moorwood (ESO, Garching), P. Santini (INAF-OAR).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. 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 the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links
Research paper
Photos of the VLT

Contacts

Dr. Laura Pentericci
INAF Rome Astronomical Observatory
Rome, Italy
Tel: +39 06 94 286 450
Email: laura.pentericci@oa-roma.inaf.it

Dr. Adriano Fontana
INAF Rome Astronomical Observatory
Rome, Italy
Tel: +39 06 94 286 456
Email: adriano.fontana@oa-roma.inaf.it

Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org

Clearing the Cosmic Fog

PR Image eso1041a

Galaxies during the era of reionisation in the early Universe (simulation)

PR Image eso1041b

Hubble image of the distance-record galaxy UDFy-38135539

PR Video eso1041a
ESOcast 22: The most distant galaxy ever measured

PR Video eso1041b

Video News Release: The most distant galaxy ever measured

PR Video eso1041c
Zooming in on the most distant galaxy ever measured

PR Video eso1041d

The era of reionisation (simulation)

PR Video eso1041e
The era of reionisation (artist’s impression)

PR Video eso1041f
The era of reionisation (artist’s impression)

The Most Distant Galaxy Ever Measured

A European team of astronomers using ESO’s Very Large Telescope (VLT) has measured the distance to the most remote galaxy so far. By carefully analysing the very faint glow of the galaxy they have found that they are seeing it when the Universe was only about 600 million years old (a redshift of 8.6). These are the first confirmed observations of a galaxy whose light is clearing the opaque hydrogen fog that filled the cosmos at this early time. The results were presented at an online press conference with the scientists on 19 October 2010, and will appear in the 21 October issue of the journal Nature.

“Using the ESO Very Large Telescope we have confirmed that a galaxy spotted earlier using Hubble is the most remote object identified so far in the Universe” [1], says Matt Lehnert (Observatoire de Paris) who is lead author of the paper reporting the results. “The power of the VLT and its SINFONI spectrograph allows us to actually measure the distance to this very faint galaxy and we find that we are seeing it when the Universe was less than 600 million years old.”

Studying these first galaxies is extremely difficult. By the time that their initially brilliant light gets to Earth they appear very faint and small. Furthermore, this dim light falls mostly in the infrared part of the spectrum because its wavelength has been stretched by the expansion of the Universe — an effect known as redshift. To make matters worse, at this early time, less than a billion years after the Big Bang, the Universe was not fully transparent and much of it was filled with a hydrogen fog that absorbed the fierce ultraviolet light from young galaxies. The period when the fog was still being cleared by this ultraviolet light is known as the era of reionisation [2]. Despite these challenges the new Wide Field Camera 3 on the NASA/ESA Hubble Space Telescope discovered several robust candidate objects in 2009 [3] that were thought to be galaxies shining in the era of reionisation. Confirming the distances to such faint and remote objects is an enormous challenge and can only reliably be done using spectroscopy from very large ground-based telescopes [4], by measuring the redshift of the galaxy’s light.

Matt Lehnert takes up the story: “After the announcement of the candidate galaxies from Hubble we did a quick calculation and were excited to find that the immense light collecting power of the VLT, when combined with the sensitivity of the infrared spectroscopic instrument, SINFONI, and a very long exposure time might just allow us to detect the extremely faint glow from one of these remote galaxies and to measure its distance.”

On special request to ESO’s Director General they obtained telescope time on the VLT and observed a candidate galaxy called UDFy-38135539 [5] for 16 hours. After two months of very careful analysis and testing of their results, the team found that they had clearly detected the very faint glow from hydrogen at a redshift of 8.6, which makes this galaxy the most distant object ever confirmed by spectroscopy. A redshift of 8.6 corresponds to a galaxy seen just 600 million years after the Big Bang.

Co-author Nicole Nesvadba (Institut d’Astrophysique Spatiale) sums up this work, “Measuring the redshift of the most distant galaxy so far is very exciting in itself, but the astrophysical implications of this detection are even more important. This is the first time we know for sure that we are looking at one of the galaxies that cleared out the fog which had filled the very early Universe.”

One of the surprising things about this discovery is that the glow from UDFy-38135539 seems not to be strong enough on its own to clear out the hydrogen fog. “There must be other galaxies, probably fainter and less massive nearby companions of UDFy-38135539, which also helped make the space around the galaxy transparent. Without this additional help the light from the galaxy, no matter how brilliant, would have been trapped in the surrounding hydrogen fog and we would not have been able to detect it”, explains co-author Mark Swinbank (Durham University).

Co-author Jean-Gabriel Cuby (Laboratoire d’Astrophysique de Marseille) remarks: “Studying the era of reionisation and galaxy formation is pushing the capability of current telescopes and instruments to the limit, but this is just the type of science that will be routine when ESO’s European Extremely Large Telescope — which will be the biggest optical and near infrared telescope in the world — becomes operational.”

Notes

[1] An earlier ESO result (eso0405) reported an object at a larger distance (a redshift of 10). However, further work failed to find an object of similar brightness at this position, and more recent observations with the NASA/Hubble Space Telescope have been inconclusive. The identification of this object with a galaxy at very high redshift is no longer considered to be valid by most astronomers.

[2] When the Universe cooled down after the Big Bang, about 13.7 billion years ago, electrons and protons combined to form hydrogen gas. This cool dark gas was the main constituent of the Universe during the so-called Dark Ages, when there were no luminous objects. This phase eventually ended when the first stars formed and their intense ultraviolet radiation slowly made the hydrogen fog transparent again by splitting the hydrogen atoms back into electrons and protons, a process known as reionisation. This epoch in the Universe’s early history lasted from about 150 million to 800 million years after the Big Bang. Understanding how reionisation happened and how the first galaxies formed and evolved is one of the major challenges of modern cosmology.

[3] These Hubble observations are described at:
http://www.spacetelescope.org/news/heic1001/

[4] Astronomers have two main ways of finding and measuring the distances to the earliest galaxies. They can take very deep images through differently coloured filters and measure the brightness of many objects at different wavelengths. They can then compare these with what is expected of galaxies of different types at different times in the Universe’s history. This is the only way currently available to discover these very faint galaxies and is the technique employed by the Hubble team. But this technique is not always reliable. For example, what may seem to be a faint, very distant galaxy can sometimes turn out to be a mundane, cool star in our Milky Way.

Once candidate objects are found more reliable estimates of the distance (measured as the redshift) can be obtained by splitting the light from a candidate object up into its component colours and looking for the telltale signs of emission from hydrogen or other elements in the galaxy. This spectroscopic approach is the only means by which astronomers can obtain the most reliable and accurate measurements of distance.

[5] The strange name indicates that it was found in the Ultra Deep Field search area and the number gives its precise position on the sky.

More information

An online press conference to announce the new results and offer journalists the opportunity for discussion with the scientists will be held at 16:00 CEST on Tuesday, 19 October 2010. To participate in the teleconference, bona-fide members of the media must get accredited by contacting Douglas Pierce-Price by email (dpiercep@eso.org). Reporters will need access to a computer with a recent version of Adobe Flash Player installed and a broadband internet connection.

This research was presented in a paper, Spectroscopic confirmation of a galaxy at redshift z=8.6, Lehnert et al., to appear in Nature on 21 October 2010.

The team is composed of M. D. Lehnert (Observatoire de Paris – Laboratoire GEPI / CNRS-INSU / Université Paris Diderot, France), N. P. H. Nesvadba (Institut d’Astrophysique Spatiale / CNRS-INSU / Université Paris-Sud, France), J.-G.Cuby (Laboratoire d’Astrophysique de Marseille / CNRS-INSU / Université de Provence, France), A. M. Swinbank (Durham University, UK), S. Morris (Durham University, UK), B. Clément (Laboratoire d’Astrophysique de Marseille / CNRS-INSU / Université de Provence, France), C. J. Evans (UK Astronomy Technology Centre, Edinburgh, UK), M. N. Bremer (University of Bristol, UK) and S. Basa (Laboratoire d’Astrophysique de Marseille / CNRS-INSU / Université de Provence, France).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. 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 VISTA, the world’s largest survey telescope. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

Research paper: Nature paper
More info about reionisaton
Marcelo Alvarez’ simulations of reionisation

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