No man is an Island, intire of itselfe; every man is a peece of the Continent, a part of the maine …

—John Donne (English clergyman and poet, 1572 – 1631),  Devotions XVII

Once upon a time — back in the early 20th century — astronomers thought of galaxies as ‘island universes’: self-contained entities swimming around in space.

As time went on they realised that galaxies are living active social lives — colliding, merging, rubbing shoulders. All this fraternising can trigger stars into bloom. It can let a galaxy grab more stars and grow, or strip it of its own precious hoard of star-making gas.

By the ‘90s astronomers were talking about the ‘cosmic web’. In this picture, galaxies, and clusters of galaxies, are the bright nodes in a network of largely invisible stuff.

Most of the invisible stuff is ‘dark matter’. But not all.

We’re just starting to glimpse the normal (non-‘dark’) matter in the cosmic web.

Recently, astronomer Spencer Wolfe and his colleagues have found clouds of very, very thin hydrogen gas lurking between a couple of nearby galaxies, Andromeda (M31) and Triangulum (M33). These clouds appear to be part of an intergalactic ‘filament’ — part of the cosmic web.

While Wolfe and colleagues have made it clearer, this structure was first detected in 2004 by Robert Braun, Chief Scientist with CSIRO’s Astronomy and Space division.

‘Secret rivers’

‘Filaments’ like this — ‘secret rivers’ of gas — are what keeps galaxies alive.

Consider this: stars form from clouds of neutral hydrogen gas — hydrogen gas in the form of single atoms.

But the total mass of stars today is five times more than the mass of neutral hydrogen gas that there was 12 billion years ago. (Astronomers are pretty sure of this number.)

So where has all the neutral hydrogen gas been coming from?

It’s actually from these ‘secret rivers’, which flow from huge reservoirs of extremely thin hydrogen gas between the galaxies.

Ninety-nine per cent of that gas isn’t neutral, it’s ionised. That is, the hydrogen atoms have had their electrons knocked off. And this ionised gas is essentially invisible to astronomers when it’s both distant and diffuse.

But Wolfe and his colleagues were able to detect the filament from the 1% of gas that isn’t ionised. This was challenging, as the radio emission from neutral hydrogen is intrinsically very faint.

As this intergalactic gas falls into galaxies, it condenses and gets its electrons back, becoming neutral. In this form, it can become the raw material for stars.

So, we are all part of an immense cosmic web … and no galaxy is an island after all.

A recent simulation of galaxy formation (by Mark Vogelsberger and colleagues), highlighting the role of the gas ‘filaments’ in intergalactic space.

Spencer A. Wolfe, D.J. Pisano, Felix J. Lockman, Stacy S. McGaugh and Edward J. Shaya. Discrete clouds of neutral gas between the galaxies M31 and M33. Nature, 497, 224-226 (9 May 2013). doi:10.1038/nature12082


  1. The small random density fluctuations occurring in the universe after the big bang act like gravitational “seeds” by pulling in more material from their surroundings to become even denser. The densest fluctuations form the galaxies, while less dense ones tend to line up as filaments between them. The “secret rivers” are produced by such moderate density fluctuations. The gas they pull in is part of the overall gas content of the universe. About two thirds of all gas in the universe is still out there between the galaxies.

  2. But from where do the secret rivers of gas come from?

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