Numerous Hydrogen Clouds discovered lurking among our galactic neighbours

This combined graphic shows new, high-resolution GBT imaging (in box) of recently discovered hydrogen clouds between M31 (upper right) and M33 (bottom left). 

Credit: Bill Saxton, NRAO/AUI/NSF

In a dark, starless patch of intergalactic space, astronomers have discovered a never-before-seen cluster of hydrogen clouds strewn between two nearby galaxies, Andromeda (M31) and Triangulum (M33).

The researchers speculate that these rarefied blobs of gas—each about as massive as a dwarf galaxy—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 Observatory (NRAO) in Green Bank, W.Va. The results were published in the journal Nature.

Spencer Wolfe

"We have known for some time that many seemingly empty stretches of the Universe contain vast but diffuse 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 definitive structure or represented a new type of cosmic feature."

"Now, with high-resolution images from the GBT, we were able to detect discrete 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 wavelengths, which can be detected by radio telescopes on Earth.

Though this material is abundant throughout the cosmos, in the space between galaxies it can be very tenuous and the faint signal it emits can be extremely difficult to detect.



The animation demonstrates the difference in resolution from the original Westerbork Radio Telescope data (Braun & Thilker, 2004) and the finer resolution imaging of GBT, which revealed the hydrogen clouds between M31 and M33. 

Credit: Bill Saxton, NRAO/AUI/NSF.

A little more than a decade ago, astronomers had the first speculative hints that a previously 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, smeared 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 pulled off some wispy puffs of gas, leaving a tenuous bridge between the two.