IT’S a bridge like no other. The intergalactic void between the Andromeda and Triangulum galaxies is spanned by a 782,000-light-year-long hydrogen link. The vast “bridge” looks like the strongest evidence yet of the elusive final third of the universe’s normal matter, which has so far escaped direct detection.
Computer simulations of how the universe formed suggest such enormous structures should exist. Astronomers did not come close to seeing one until 2004, though, when and reported something strange during a survey of a cluster of galaxies, including our own Milky Way, called the Local Group (illustrated on opposite page).
The duo saw clumps of neutral hydrogen atoms stretching out between Andromeda (galaxy M31, pictured) and Triangulum (M33). Hydrogen exists in haloes around galaxies, but these clumps were different: they seemed to extend like a bridge between the two galaxies.
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The detection was so faint that many doubted the entity even existed. Braun, now at CSIRO, Australia’s national science agency, in Epping, New South Wales, and Thilker, at Johns Hopkins University in Baltimore, Maryland, had made the find using the in the Netherlands. The telescope was configured to sacrifice resolution for sensitivity. In other words, the researchers saw the faint clumps of hydrogen – but with eyes too blurry to discern the fine detail.
“There were people that doubted it was real, and it was difficult to confirm,” says at the US National Radio Astronomy Observatory in Green Bank, West Virginia.
Difficult, but not impossible. All that was needed was a closer look. Lockman and his colleagues have now carried out a more detailed examination using the world’s largest fully steerable single-dish radio telescope – the 100-metre-wide .
Lockman’s survey was as sensitive as Braun and Thilker’s, but with five times the spatial resolution. The picture it paints vindicates Braun and Thilker: Lockman’s team also found the blobs of neutral hydrogen between M31 and M33, suggesting the two are indeed connected by a gas bridge ().
Braun is delighted that the bridge was not a figment of his astronomical imagination. “While we had great confidence in its reality, it was a surprising discovery,” he says. “So it’s vital that an independent confirmation has now been made.”
The bridge probably spans the full distance between M31 and M33, about 782,000 light years, or 7.4 × 1018 kilometres. Long enough, then, to suggest it might contain lots of hydrogen. To find out, Lockman’s team focused in on two sections of the bridge, each less than 6500 light years long. They estimate that each section contains neutral hydrogen with the mass of 100,000 suns.
This neutral hydrogen is in intergalactic space, though, which is so suffused with radiation that most of the hydrogen atoms there should have lost their only electron. The bridge will not have escaped this ionisation process. In fact, it probably consists mainly of ionised hydrogen, which does not emit the radio waves that neutral hydrogen does, and so cannot be detected by this type of astronomical survey.
Braun estimates that there must be so much more ionised hydrogen in the bridge than neutral hydrogen that the whole structure weighs in at about 500 million solar masses. If so, there are implications for our understanding of how normal, or baryonic, matter is distributed in the local universe.
“There is so much hydrogen in the bridge that it weighs in at about 500 million times the mass of the sun”
A third of all baryonic matter is tied up in stars and galaxies. Another third is very diffuse and thought to be distributed in the form of filamentary networks spread through space. The rest is something of a mystery. It should exist around galactic neighbourhoods, less dense than the galaxies themselves but more dense than the filaments, but it has never been seen.
“The M31-M33 bridge seems to be an example of this ‘final third’,” says Braun, “and what’s special is that this is arguably the first direct [evidence] of this elusive medium.”
Astronomers have a name for this missing matter: warm-hot intergalactic medium (WHIM). Physicist of Princeton University, who studies the distribution of matter in the universe, agrees that the M31-M33 bridge fits the bill. “This is confirmatory evidence for warm-hot intergalactic medium,” he says.
Computer simulations of how our universe evolved suggest that WHIM bridges should exist between many or all individual galaxies and even clusters of galaxies, says Braun. There are hints that they might: streams of hydrogen have also been seen between the M81, M82 and NGC 3077 galaxies, which are about 12 million light years from Earth. The wispy hydrogen streams connecting them are likely remnants of ancient collisions between these galaxies.
The M31-M33 bridge may have formed in a similar way. Measurements of the velocities of M31 and M33, combined with computer simulations, suggest that the two came close to colliding a few billion years ago.
“The gas that was pulled out of the two galaxies by [gravitational interaction] during that near miss is now falling back into the two galaxies along the bridge that connects them,” says Braun.
If bridges of hydrogen stretch between galaxies, there are tantalising possibilities. Could bridges eventually be harnessed for intergalactic travel by providing a harvestable source of hydrogen fuel in the otherwise near-empty vacuum of space?
“Could these bridges eventually be harnessed as hydrogen fuel for intergalactic travel?”
Although this is unlikely, given that the gas in the bridge is probably sparsely distributed, Lockman says: “I suppose that if you wanted to collect gas between galaxies those would be the places to search.”
Dark matter crisis averted?
As the last third of normal matter is spotted (main story), physicists can also rest easy that the search for its invisible counterpart is worthwhile.
Dark matter is thought to make up about 83 per cent of the matter in the universe. It has aided our understanding of how structures in the universe formed and move, yet has so far evaded detection.
A study published last month suggested an explanation: dark matter doesn’t exist – at least, not in our cosmic neighbourhood. Christian Moni-Bidin of the University of Concepción in Chile and colleagues tracked the motions of stars near Earth to estimate the total mass in the sun’s neighbourhood. All the motions could be accounted for by the gravitational effects of the visible matter alone, the team concluded (91av, 28 April, p 6).
But they made a subtle error, claim Jo Bovy and Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey. Moni-Bidin’s team considered stars whose orbits take them far above or below the Milky Way’s main bright disc, and used the speed at which they orbit the centre of the galaxy to figure out how much of a pull they feel from the nearby mass of stars and dark matter.
They assumed that the stars’ speeds would be constant, say Bovy and Tremaine, but stars that orbit high above or far below the disc have highly elliptical orbits, and their speeds are not the same at all distances from the galactic centre. On average, their orbiting speeds should be slower than assumed by Moni-Bidin and his colleagues.
Bovy and Tremaine reanalysed the data and found that the amount of dark matter in our neighbourhood agrees with previous predictions – if anything, there might be a little more nearby dark matter than we thought ().
That means “the prospects of detecting dark matter on Earth are good”, Bovy says.
