In a dark, starless patch of intergalactic（星系间的） space, astronomers² have discovered a never-before-seen cluster of hydrogen clouds strewn between two nearby galaxies³, Andromeda (M31) and Triangulum (M33). The researchers speculate that these rarefied blobs of gas -- each about as massive as a dwarf⁴ galaxy⁵ -- condensed out of a vast and as-yet undetected reservoir of hot, ionized gas, which could have accompanied an otherwise invisible band of dark matter. The astronomers detected these objects using the National Science Foundation's Green Bank Telescope (GBT) at the National Radio Astronomy Observatory⁶ (NRAO) in Green Bank, W.Va. The results were published in the journal Nature.

 

"We have known for some time that many seemingly empty stretches of the Universe contain vast but diffuse⁷ patches of hot, ionized hydrogen," said Spencer Wolfe of West Virginia University in Morgantown. "Earlier observations of the area between M31 and M33 suggested the presence of colder, neutral hydrogen, but we couldn't see any details to determine if it had a definitive⁸ structure or represented a new type of cosmic feature. Now, with high-resolution images from the GBT, we were able to detect discrete⁹（不连续的） concentrations of neutral hydrogen emerging out of what was thought to be a mainly featureless field of gas."

 

Astronomers are able to observe neutral atomic hydrogen, which is referred to as HI (H and the Roman numeral one), because of the characteristic signal it emits at radio wavelengths¹⁰, which can be detected by radio telescopes on Earth. Though this material is abundant throughout the cosmos¹¹, in the space between galaxies it can be very tenuous¹²（稀薄的） and the faint signal it emits can be extremely difficult to detect.

 

A little more than a decade ago, astronomers had the first speculative¹³ hints that a previously¹⁴ unrecognized reservoir of hydrogen lay between M31 and M33. The signal from this gas, however, was too faint to draw any firm conclusions about its nature, origin, or even certain existence. Last year, preliminary data taken with the GBT confirmed that there was indeed hydrogen gas, and a lot of it, smeared¹⁵ out between the galaxies. These preliminary observations, however, lacked the necessary sensitivity to see any fine-grain structure in the gas or deduce whence it came and what it signified. The most likely explanation at the time was that a few billion years earlier, these two galaxies had a close encounter and the resulting gravitational perturbations（扰动，不安） pulled off some wispy¹⁶ puffs¹⁷ of gas, leaving a tenuous bridge between the two.

 

New and more thorough studies of this region with the GBT, however, revealed that the hydrogen gas was not simply in the form of wispy streamers, as would be expected by the interactions of two galaxies in a gravitational ballet. Instead, a full 50 percent of the gas was conspicuously¹⁸ clumped¹⁹ together into very discrete and apparently²⁰ self-gravitating blobs that -- apart for their lack of stars -- would be dead ringers for dwarf galaxies. Dwarf galaxies, as their name implies, are relatively²¹ small collections of stars bound together by gravity. They can contain anywhere from a few thousand to a few million stars.

 

The GBT was also able to track the motion of these newly discovered clouds, showing that they were traveling through space at velocities²² similar to M31 and M33. "These observations suggest that they are independent entities²³ and not the far-flung suburbs of either galaxy," said Felix J. Lockman, an astronomer¹ at the NRAO in Green Bank. "Their clustered orientation²⁴ is equally compelling and may be the result of a filament²⁵ of dark matter. The speculation²⁶ is that a dark-matter filament, if it exists, could provide the gravitational scaffolding upon which clouds could condense from a surrounding field of hot gas."

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