Apr 19, 2002

Great Intergalactic Cobwebs

Astronomers using NASA's FUSE spacecraft have spotted vast clouds between galaxies that might permeate the early Universe like a tangled spider's web.


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April 19, 2002: Much has been written about the first three minutes after the birth of the Universe in the Big Bang. But what do astronomers know about the next few billion years of our Universe's childhood?

Astronomers agree that matter somehow gathered together during that epoch to form stars and galaxies -- but how? Did stars appear first, or galaxies? And what was left behind in the void after they formed: empty space or vast drifting clouds of now-invisible gas?

Above: Almost everything in this Hubble "deep field" image is a young distant galaxy. [more]

How our Universe started -- and how it will end -- depends on the answers.


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To explore such questions, astronomers turned an orbiting NASA telescope named "FUSE" (short for Far-Ultraviolet Spectroscopic Explorer) toward a distant quasar -- a brilliant, active nucleus of a galaxy some 10 billion light-years away, near the edge of the known Universe. For 20 days in August and October, 2000, FUSE collected the quasar's light in the far ultraviolet region of the electromagnetic spectrum. (Far ultraviolet radiation is about 5 times more energetic than the sunburn-causing ultraviolet rays familiar to beach-goers.) Other researchers using the Hubble Space Telescope and the gigantic ground-based Keck telescope in Hawaii monitored the quasar at ordinary ultraviolet and visual wavelengths.

Then the analyses -- and discoveries -- began.

Actually, the astronomers were not interested in the quasar itself (known only by its star-catalogue designation HE2347-4342). They were trying to see whatever might lie between Earth and the quasar -- in this case, rarefied clouds of helium gas in intergalactic space.

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Helium is not the most abundant element in the Universe -- hydrogen is. But during those first few billion years after the Big Bang, much of the hydrogen between galaxies was in pieces. In those days, quasars radiating as much energy per second as thousands of normal galaxies were abundant. Quasar radiation split intergalactic hydrogen atoms into protons and electrons. Such "free-ranging" sub-atomic particles are essentially transparent; they are hard to detect.

Above: Quasars like the one pictured here bathed the young Universe with ionizing radiation. [more]

Helium, however, is not so stealthy. It has two electrons that are difficult to pry away. Even as radiation from quasars split hydrogen atoms in two, many helium atoms would have remained more or less intact. (A helium atom might lose one electron, but rarely two.) According to Philippe Crane, Origins Theme Scientist at NASA headquarters, this is why helium fascinates cosmologists: it is a useful tool for probing the early Universe.

Helium gas between us and quasar HE2347-4342 revealed its presence to FUSE by absorbing specific "colors" or wavelengths of the quasar's light in the far-ultraviolet region of the spectrum. Meanwhile, Hubble and Keck observers working at longer wavelengths found hydrogen in many of the same clouds. (The combination of hydrogen and helium data can tell researchers much more about the state of a cloud than either one alone.)

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Cosmologists have long thought that gas in the early Universe condensed into cobweb-like structures pervading intergalactic space. Tiny ripples in that distribution of matter became the stars and galaxies we see today. The clouds detected by FUSE, Keck and Hubble are samples of those tenuous intergalactic structures, says Crane.

Right: This false-color computer model of the early Universe is 30 million light years on each side. The green-shaded cobwebs are tenuous clouds of intergalactic gas. [more]

Looking far out in space is equivalent to looking far back in time. So, when astronomers saw clouds near the quasar HE2347-4342, they were seeing them as they appeared some 10 billion years ago -- impressively antique, as the Universe is thought to be 14 or 15 billion years old. At least, that's what astronomers calculated from how much spectral lines (specific wavelengths or colors) were red-shifted (moved toward longer wavelengths) from their normal positions by the expansion of the Universe.

Thus, by studying different red-shifted wavelengths of the quasar's spectrum, investigators were able to trace through time how the transparency (ionization) of the intergalactic "cobwebs" waxed and waned as the Universe expanded and evolved.

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Helium atoms, which trace the cobwebs for FUSE, were forged during the first few minutes of the Big Bang itself. At first the helium nuclei were bare. But then, as the Universe expanded, the nuclei captured electrons and became neutral helium atoms. These gaseous atoms created a nearly-opaque fog that dimmed the light from distant quasars.

Left: An artist's concept of FUSE peering into a distant and foggy past.

Some time later torrents of radiation poured into space, presumably from quasars that reheated and re-ionized some of the helium gas. Because helium ions absorb fewer photons of light, the gas became transparent again, once more allowing light from distant quasars to shine through -- rather like energy from the rising sun burns off morning fog to reveal distant hills.

By comparing FUSE's observations of helium with Keck and Hubble observations of hydrogen, the astronomers realized that the transparent periods -- that is, times when helium was substantially ionized -- were caused by radiation both from brilliant quasars and from a firestorm of star birth in more ordinary galaxies. Before, the leading candidate was quasars alone.

These new results tell scientists a great deal about the radiation environment of that early era. Indeed, says Crane, FUSE data hint that "radiation pressure was as important as gravity in causing matter to condense into stars and galaxies."

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Above: (top) the helium absorption spectrum of quasar HE2347-4342 as observed by FUSE. (bottom) the hydrogen absorption spectrum of the same quasar captured by Keck. Green-shaded areas denote helium absorption lines with no corresponding hydrogen. [more information]

Perhaps, though, the biggest find is what Hubble and Keck didn't see. Some clouds that revealed themselves to FUSE by means of their internal helium were invisible to Hubble and Keck -- telescopes that could only see hydrogen. The hydrogen is surely there, notes Crane, but it is hiding: the atoms are transparent because they have been split apart by energetic radiation.

Drifting between the galaxies, then, could be vast clouds of unseen hydrogen -- an important repository of mass in our Universe. One line of sight and a single quasar isn't enough to reveal how much matter is hiding in this fashion, but now that FUSE has found some, the hunt for more can begin.

Web Links and more...

FUSE - a NASA-supported astronomy mission that was launched on June 24, 1999, to explore the Universe using the technique of high-resolution spectroscopy in the far-ultraviolet spectral region

The First Starlight -- (Science@NASA) By peering through a giant cosmic lens, scientists have found some of the first-born stars in our Universe

The intergalactic medium -- (FUSE) a science summary by Gerard Kriss.

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Ultraviolet radiation -- We can't see it, but it's there: UV radiation that permeates the cosmos. Click on the image for more information.

New View of Primordial Helium Traces the Structure of Early Universe -- (Space Telescope Science Institute) press release

Resolving the Structure of Ionized Helium in the Intergalactic Medium using the Far Ultraviolet Spectroscopic Explorer -- a pre-print of the original research paper later published by Science magazine.

Quasars and Active Galaxies -- learn more about quasars from the Chandra X-ray Field Guide

Timeline of the Universe -- a tutorial from NASA's Origins Program. See also "How did the first galaxies form?" -- a pdf document from the Origins Roadmap.

FUSE exhibit at Maryland Science Center

Cosmology Solved. Not! -- (APOD); see also "Frequently Asked Questions in Cosmology" from UCLA.

Stellar 'Fireworks Finale' Came First in the Young Universe -- The deepest views of the cosmos from NASA's Hubble Space Telescope suggest that the very first stars may have burst into the universe as brilliantly and spectacularly as a fireworks finale

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