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Post Info TOPIC: Glow From The First Stars In The Universe


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Helium Reionisation
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Title: The History and Morphology of Helium Reionisation
Authors: Steven Furlanetto (Yale, UCLA), S. Peng Oh (UCSB)

A variety of observations now indicate that intergalactic helium was fully ionised by z~3. The most recent measurements of the high-redshift quasar luminosity function imply that these sources had produced ~2.5 ionising photons per helium atom by that time, consistent with a picture in which the known quasar population drives HeII reionisation. Here we describe the distribution of ionised and neutral helium gas during this era. Because the sources were rare and bright (with the photon budget dominated by quasars with luminosities L>L_\star), random fluctuations in the quasar population determined the morphology of ionised gas when the global ionised fraction x_i was small, with the typical radius R_c of a HeIII bubble ~15-20 comoving Mpc. Only when x_i>0.5 did the large-scale clustering of the quasars drive the characteristic size of ionised regions above this value. Still later, when x_i>0.75, most ionising photons were consumed by dense, recombining systems before they reached the edge of their source's ionised surroundings, halting the bubble growth when R_c~35-40 Mpc. These phases are qualitatively similar to those in hydrogen reionisation, but the rarity of the sources makes the early stochastic phase much more important. Moreover, the well-known characteristics of the z=3 intergalactic medium allow a much more robust description of the late phase in which recombinations dominate.

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RE: Glow From The First Stars In The Universe
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Recent claims that the Spitzer Space Telescope has detected light from the universe's first stars or black holes have been met with scepticism by many astronomers. Instead, they say the light simply comes from faint, relatively nearby galaxies.
Since the earlier Spitzer studies imply a higher number of stars than predicted in some cosmological models, the more prosaic explanation suggests existing theories about the early universe do not need to be revised

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New observations from NASA's Spitzer Space Telescope strongly suggest that infrared light detected in a prior study originated from clumps of the very first objects of the Universe. The recent data indicate this patchy light is splattered across the entire sky and comes from clusters of bright, monstrous objects more than 13 billion light-years away.

"We are pushing our telescopes to the limit and are tantalisingly close to getting a clear picture of the very first collections of objects. Whatever these objects are, they are intrinsically incredibly bright and very different from anything in existence today" - Dr. Alexander Kashlinsky of NASA's Goddard Space Flight Centre, Greenbelt, Md., lead author on two reports to appear in the Astrophysical Journal Letters.

Astronomers believe the objects are either the first stars -- humongous stars more than 1,000 times the mass of our sun -- or voracious black holes that are consuming gas and spilling out tons of energy. If the objects are stars, then the observed clusters might be the first mini-galaxies containing a mass of less than about one million suns. The Milky Way galaxy holds the equivalent of approximately 100 billion suns and was probably created when mini-galaxies like these merged.
This study is a thorough follow-up to an initial observation presented in Nature in November 2005 by Kashlinksy and his team. The new analysis covered five sky regions and involved hundreds of hours of observation time.
Scientists say that space, time and matter originated 13.7 billion years ago in a tremendous explosion called the Big Bang. Observations of the cosmic microwave background by a co-author of the recent Spitzer studies, Dr. John Mather of Goddard, and his science team strongly support this theory. Mather is a co-winner of the 2006 Nobel Prize for Physics for this work. Another few hundred million years or so would pass before the first stars would form, ending the so-called dark age of the universe.
With Spitzer, Kashlinsky's group studied the cosmic infrared background, a diffuse light from this early epoch when structure first emerged. Some of the light comes from stars or black hole activity so distant that, although it originated as ultraviolet and optical light, its wavelengths have been stretched to infrared wavelengths by the growing space-time that causes the universe's expansion. Other parts of the cosmic infrared background are from distant starlight absorbed by dust and re-emitted as infrared light.

"There's ongoing debate about what the first objects were and how galaxies formed. We are on the right track to figuring this out. We've now reached the hilltop and are looking down on the village below, trying to make sense of what's going on" - Dr. Harvey Moseley of Goddard, a co-author on the papers.

The analysis first involved carefully removing the light from all foreground stars and galaxies in the five regions of the sky, leaving only the most ancient light. The scientists then studied fluctuations in the intensity of infrared brightness, in the relatively diffuse light. The fluctuations revealed a clustering of objects that produced the observed light pattern.

"Imagine trying to see fireworks at night from across a crowded city. If you could turn off the city lights, you might get a glimpse at the fireworks. We have shut down the lights of the universe to see the outlines of its first fireworks" - Dr. Alexander Kashlinsky.

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The First Galaxies
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The first galaxies were small - about 10,000 times less massive than the Milky Way. Billions of years ago, those mini-furnaces forged a multitude of hot, massive stars. In the process, they sowed the seeds for their own destruction by bathing the universe in ultraviolet radiation. According to theory, that radiation shut off further dwarf galaxy formation by both ionising and heating surrounding hydrogen gas. Now, astronomers Stuart Wyithe (University of Melbourne) and Avi Loeb (Harvard-Smithsonian Centre for Astrophysics) are presenting direct evidence in support of this theory.

Wyithe and Loeb showed that fewer, larger galaxies, rather than more numerous, smaller galaxies, dominated the billion-year-old universe. Dwarf galaxy formation essentially shut off only a few hundred million years after the Big Bang.

"The first dwarf galaxies sabotaged their own growth and that of their siblings. This was theoretically expected, but we identified the first observational evidence for the self-destructive behaviour of early galaxies" - Avi Loeb.

Their research is being reported in the May 18, 2006 issue of Nature.
Nearly 14 billion years ago, the Big Bang filled the universe with hot matter in the form of electrons and hydrogen and helium ions. As space expanded and cooled, electrons and ions combined to form neutral atoms. Those atoms efficiently absorbed light, yielding a pervasive dark fog throughout space. Astronomers have dubbed this era the "Dark Ages."

The first generation of stars began clearing that fog by bathing the universe in ultraviolet radiation. UV radiation splits atoms into negatively charged electrons and positively charged ions in a process called ionisation. Since the Big Bang created an ionised universe that later became neutral, this second phase of ionisation by stars is known as the "epoch of reionisation." It took place in the first few hundred million years of existence.

"We want to study this time period because that's when the primordial soup evolved into the rich zoo of objects we now see" - Avi Loeb.

During this key epoch in the history of the universe, gas was not only ionised, but also heated. While cool gas easily clumps together to form stars and galaxies, hot gas refuses to be constrained. The hotter the gas, the more massive a galactic "seed" must be to attract enough matter to become a galaxy.
Before the epoch of reionisation, galaxies containing only 100 million solar masses of material could form easily. After the epoch of reionisation, galaxies required more than 10 billion solar masses of material to be assembled.
To determine typical galaxy masses, Wyithe and Loeb looked at light from quasars - powerful light sources visible across vast distances. The light from the farthest known quasars left them nearly 13 billion years ago, when the universe was a fraction of its present age. Quasar light is absorbed by intervening clouds of hydrogen associated with early galaxies, leaving telltale bumps and wiggles in the quasar's spectrum.
By comparing the spectra of different quasars along different lines of sight, Wyithe and Loeb determined typical galaxy sizes in the infant universe. The presence of fewer, larger galaxies leads to more variation in the absorption seen along various lines of sight. Statistically, large variation is exactly what Wyithe and Loeb found.

"As an analogy, suppose you are in a room where everybody is talking. If this room is sparsely populated, then the background noise is louder in some parts of the room than others. However if the room is crowded, then the background noise is the same everywhere. The fact that we see fluctuations in the light from quasars implies that the early universe was more like the sparse room than the crowded room" - Stuart Wyithe.

Astronomers hope to confirm the suppression of dwarf galaxy formation using the next generation of telescopes - both radio telescopes that can detect distant hydrogen and infrared telescopes that can directly image young galaxies. Within the next decade, researchers using these new instruments will illuminate the "Dark Ages" of the universe.

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Posts: 131433
Date:
Dark Ages
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A team of astronomers has uncovered new evidence about the stars whose formation ended the cosmic "Dark Ages" a few hundred million years after the Big Bang.

In a presentation at the 2006 annual winter meeting of the American Astronomical Society (AAS), California Institute of Technology graduate student George Becker discussed his team's investigation of several faraway quasars and the gas between the quasars and Earth. The paper on which his lecture is based will be published in the Astrophysical Journal in March.

One quasar in the study seems to reveal numerous patches of "neutral" gas, made up of atoms where the nucleus and electrons cling together, floating in space when the universe was only about 10 percent of its present age. This gas is thought to have existed in significant quantities only within a certain time-frame in the early universe. Prior to the Dark Ages, all material would have been too hot for atomic nuclei to combine with their electrons; after, the light from newly-formed stars would have reached the atoms and stripped off the electrons.

"There should have been a period when most of the atoms in the universe were neutral. This would have continued until stars and galaxies began forming" - George Becker.

In other words, the universe went from a very hot, very dense state following the Big Bang where all atomic nuclei and electrons were too energetic to combine, to a less dense and cooler phase-albeit a dark one-where the nuclei and the electrons were cool enough to hold onto each other and form neutral atoms, to today's universe where the great majority of atoms are ionised by energetic particles of light.
Wallace Sargent, who coined the term Dark Ages in 1985 and who is Becker's supervising professor, adds that analysing the quasars to learn about the early universe is akin to looking at a lighthouse in order to study the air between you and it. During the Dark Ages, neutral atoms filling the universe would have acted like a fog, blocking out the light from distant objects. To end the Dark Ages, enough stars and galaxies needed to form to burn this "cosmic fog" away.

"We may have detected the last wisps of the fog" - Wallace Sargent, Bowen Professor of Astronomy at Caltech.

The uniqueness of the new study is the finding that the chemical elements of the cool, un-ionised gas seem to have come from relatively ordinary stars. The researchers think this is so because the elements they detect in the gas- oxygen, carbon, and silicon-are in proportions that suggest the materials came from Type II supernovae.
These particular explosions are caused when massive stars collapse and then rebound to form a gigantic explosion. The stars needed to create these explosions can be more than ten times the mass of the sun, yet they are common over almost the entire history of the universe.
However, astronomers believe that the very first stars in the universe would have been much more massive, up to hundreds of times the mass of the sun, and would have left behind a very different chemical signature.

"If the first stars in the universe were indeed very massive stars, then their chemical signature was overwhelmed by smaller, more typical stars very soon after"- George Becker.

Becker and his colleagues believe they are seeing material from stars that was blown into space by the supernovae explosions and mixed with the pristine gas produced by the Big Bang. Specifically, they are looking at the spectra of the light from quasars as it is absorbed during its journey through the mixed-up gas.
The quasars in this particular study are from the Sloan Digital Sky Survey, an ongoing mapping project that seeks, in part, to determine the distances of 100,000 quasars. The researchers focused on nine of the most distant quasars known, with redshifts greater than 5, meaning that the light we see from these objects would have been emitted when the universe was at most 1.2 billion years old.
Of the nine, three are far enough away that they may have been at the edge of the dark period. Those three have redshifts greater than 6, meaning that the universe was less than 1 billion years old when they emitted the light we observe. By comparison, the present age of the universe is believed to be about 13.7 billion years.

The study in part promises a new tool to investigate the nature of stars in the early universe.

"Now that we've seen these systems, it's reasonable to ask if their composition reflects the output of those first very massive stars, or whether the mix of chemicals is what you would expect from more ordinary stars that ended in Type II supernovae. It turns out that the latter is the case. The chemical composition appears to be very ordinary"- George Becker.

Thus, the study provides a new window into possible transitions in the early universe.

"The relative abundance of these elements gives us in principle a way of finding out what the first stars were. This gives us insight into what kind of stars ended the Dark Ages" - Wallace Sargent.

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L

Posts: 131433
Date:
RE: Glow From The First Stars In The Universe
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The top panel is an image from NASA's Spitzer Space Telescope of stars and galaxies in the constellation Draco, covering about 50 by 100 million light-years (6 to 12 arcminutes).


Expand (1.6Mb, 1200x1200)

This is an infrared image showing wavelengths of 3.6 microns, below what the human eye can detect. The bottom panel is the resulting image after all the stars, galaxies and artefacts were masked out.
The remaining background has been enhanced to reveal a glow that is not attributed to galaxies or stars.
This might be the glow of the first stars in the universe.

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Anonymous

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From BBC:

Astronomers have detected a faint glow from the first stars to form in the Universe, Nature journal reports.
This earliest group of stars, called Population III, probably formed from primordial gas less than 200 million years after the Big Bang.
These objects cannot be seen by any present or planned telescopes.
Nasa scientists detected the stars from the imprint they have left on the general glow of infrared radiation dispersed throughout the cosmos.
These first stars were huge thermonuclear furnaces; few and far between, but they burned ferociously
The observations used in the latest study were made by the Infrared Array Camera (Irac) on the US space agency's Spitzer Space Telescope.
The results present the first evidence for cessation of the so-called cosmic Dark Ages.
The term, coined by the English Astronomer Royal, Sir Martin Rees, refers to the period in cosmic history when hydrogen and helium atoms had formed but had not yet had the opportunity to condense and ignite as stars.

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