Published online 22 April 2009 | Nature | doi:10.1038/news.2009.382

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Galaxies grow fat on 'big blob' meals

Discovery of a glowing gas cloud casts doubt on theories of galaxy formation.

Image of a distance blobA distant blob could prompt astronomers to rethink their models of galaxy formation.Ouchi, M. et al

Using some of the biggest telescopes in the world, astronomers have discovered a huge, amorphous cloud of hot hydrogen gas that was glowing when the Universe was just 800 million years old.

The cloud — called a blob — may be collapsing, fuelling the birth of new stars that lie within but are obscured by the cloud. If so, the blob would be a rare glimpse of galaxy formation in action 12.9 billion years ago, in the Universe's infancy. But the blob — which the astronomers named Himiko after a legendary Japanese queen — is no baby. It is at least half the size of the Milky Way — big enough to fly in the face of standard theories of galaxy formation, which propose that galaxies grow slowly over time through the merger of smaller building blocks.

"To find [a blob] that's already large and forming stars is a bit of a surprise," says Patrick McCarthy, an astronomer at the Observatories of the Carnegie Institution based in Pasadena, California, and one of the authors who describe the discovery1. The findings will be published in a forthcoming issue of the Astrophysical Journal. The discovery, combined with others (see 'Early galaxies surprise with size'), is adding weight to the idea that the Universe's first galaxies found a way to grow fat more quickly than previously thought. "The evidence has been piling up for several years," says McCarthy.

Mysterious blob

In 2000, Charles Steidel of the California Institute of Technology in Pasadena was one of the first people to discover the lumpy objects — technically called Lyman-α emitters, after the part of the hydrogen spectrum in which the gas glows. "We just jokingly started calling them blobs," says Steidel, who was not involved with the latest study. "And it kinda stuck."

“We just jokingly started calling them blobs. And it kinda stuck.”

Charles Steidel
Caltech

The blobs glow by re-radiating energy from other sources. They are often driven by the activity of supermassive black holes at galactic centres, or the collective activity of stars. These sources excite the hydrogen gas to a higher energy state. When the gas relaxes to its ground energy state, it fluoresces in the ultraviolet range.

Because the Earth's atmosphere blocks most ultraviolet light, and because few space-based telescopes have worked in that part of the spectrum, blobs have not been studied extensively in the nearby, contemporary Universe. Instead, astronomers have found them in ancient corners of the cosmos, where their light has been redshifted — shifted to lower frequencies — by the expansion of the Universe.

To find such a distant blob — the oldest yet — the astronomers in the latest study had to combine a suite of observations from several of the world's top telescopes. The team initially spotted the blob with Suburu, an 8-metre-diameter Japanese telescope atop Mauna Kea in Hawaii. They then used the twin 10-metre Keck telescopes, also at Mauna Kea, and the 6.5-metre Magellan telescopes in Las Campanas, Chile, for follow-up observations.

Monolithic collapse

The researchers were not able to find an obvious black hole nearby. Finding one would have led them to consider the blob as a by-product of a mature galaxy, rather than the progenitor of a young one. If the blob does mark the birth of an early galaxy, then it will be more evidence that astronomers should adjust their models, many of which currently suggest that large spiral galaxies should require long periods of time and mergers to grow.

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It could be, says McCarthy, that "nature knows how to do things more efficiently than we've been able to figure out yet". He says the theory of the blob as a 'galactic embryo' recalls a popular theory from the 1960s and 1970s that has since fallen out of favour: that of "monolithic collapse", in which giant clouds collapse gravitationally and form galaxies in one massive burst of star formation.

But Steidel isn't convinced just yet. Previous searches to validate the monolithic collapse theory proved largely unsuccessful. And in the case of this particular blob, a black hole may have been missed: the blobs scatter light, so the things that they envelop, such as an active black hole, aren't necessarily along the line of sight. Moreover, the black hole may have become dormant recently. If monolithic collapse is the norm, Steidel says, astronomers should be finding more blobs like this. "It would surprise me that these things were so rare if they were just a normal phase in the galaxy formation process." 

  • References

    1. Ouchi, M. et al. Preprint at http://lanl.arxiv.org/abs/0807.4174 (2009).
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