Seven clouds of hydrogen dotting the space between two iconic galaxies might be crumbs from a past encounter or evidence for the elusive cosmic web theorized to fuel galaxy growth.
Despite appearances, galaxies don’t float in total voids. There’s a lot of diffuse gas milling around the cosmos — in fact, computer simulations suggest that two-thirds of the universe’s regular matter exists not in galaxies but in the stuff between the galaxies. This gas is hard to track down, although previous studies have revealed hints.Following up on radio observations of the Andromeda (M31) and Triangulum (M33) Galaxies that detected neutral hydrogen gas between the two spiral galaxies, astronomers working with the 100-meter Green Bank Telescope in West Virginia now think they’ve detected seven clumps in that gas. The clouds are each a few thousand light-years wide, the size of dwarf galaxies, the team reports in the May 9th Nature.
Spencer Wolfe (West Virginia University) and his colleagues are cautious in their interpretation, but they think the best explanation for these clouds might be that they’re condensations in the elusive cosmic filaments proposed to funnel gas into galaxies. While galaxies do grow by cannibalizing one another, astronomers suspect that the universe’s vast star cities also grow by feeding on gas siphoned into them by a giant cosmic web. This gas would be the fuel galaxies need to form stars and beef themselves up.
The detection of cosmic filaments is an attractive conclusion, but other astronomers hesitate to back that interpretation. The two galaxies are known to be an interacting pair — the Triangulum travels around its big brother in a wide orbit, and it’s possible that the interaction stripped out material to form a string of clouds between the two galaxies, says Gurtina Besla (Columbia University). The clouds move at similar speeds to the galaxies, which would make sense if the gas is a part of the pair’s system.
Mary Putman (also at Columbia) favors the tidal leftovers scenario, too. The denser parts of filaments would typically be closer to the galaxies than these clouds are, she says. “Still, getting a few clumps out there like this is definitely possible,” she adds. The cosmic web could indeed be the source.
While his team doesn’t rule out the interaction scenario, Wolfe thinks it’s less likely than the filament one. “If the clouds were due to an interaction between M31 and M33, then it would have occurred a few billion years ago,” he says. “It's not obvious that these clouds would persist for that long a time.” Furthermore, the clumps don’t have any stars, which would likely form in gas kneaded by the tidal stripping process.
Either way, the results might help astronomers understand gas flows in the Local Group, the bunch of galaxies of which Andromeda, Triangulum, and the Milky Way are members. “The Local Group is the only place these kinds of clouds can be detected and mapped in detail,” Putman says. “And how gas accretes onto galaxies to fuel star formation depends heavily on what happens with the gas between galaxies, as this is where the majority of the baryons lie.”
Further observations of the hydrogen distribution around other parts of the M31-M33 system will help distinguish between the scenarios. One question that needs answering is whether the pressure of the surrounding environment is high enough that interaction leftovers would last this long, Wolfe says.
Below, you'll find an animation showing where the clouds lie between the galaxies and how improved resolution revealed what previously looked like diffuse gas.
Reference: S.A. Wolfe et al. "Discrete clouds of neutral gas between the galaxies M31 and M33." Nature, 9 May 2013.