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.

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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

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Distant Galaxies Reveal The Clearing of the Cosmic Fog

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Artist’s impression of galaxies at the end of the era of reionisation

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

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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

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