The Mysterious Seven

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.

Shown on top of an illustration of M31 (upper right) and M33 (bottom left), new, high-resolution radio images (in box) from the Green Bank Telescope reveal seven hydrogen clouds, each the size of a dwarf galaxy. Click to zoom.
Bill Saxton (NRAO / AUI / NSF)
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.

Galaxies, News

About Camille M. Carlisle

Assistant Editor Camille M. Carlisle covers science news for Sky & Telescope. She specializes in black holes, galactic cosmology, and whatever she happens to be writing about at the time.

13 thoughts on “The Mysterious Seven

  1. Bruce

    It’s nice to hear the suggestion that the universe may have been equipped with an extra 200% of the gas that’s already been incorporated into galaxies. It can be a real bummer when you run out of gas. (See Peter’s “Strategic reserves?” comment about future star formation from the second link.) But these seven clumps of gas are somewhat aligned with the center of M31, and they have been detected via X-rays. How can cold intergalactic gas clumps glow in X-rays, unless they are being illuminated by something very powerful? I would therefore assume that these clumps of gas are ether being lit up by a beam from Andromeda’s SMBH, or perhaps they aren’t from extra-galactic space but were blown out from M31 via galactic jet activity. Has the latter possibility been eliminated?

  2. Mike W. Herberich

    If "… two-thirds of the universe’s regular matter …" should be (hard to track down) in between galaxies, is that (partly) a candidate for (dark/ missing) matter? Or is it to be thought of as 2/3 of the 4% matter "known" to us? That would leave 1/3 of 4% (= 1.33%) of all matter to galaxies.I’m confused.

  3. starfart

    While it is possible that vestiges of the original primordial filaments along which galaxies may form (according to simulations of early galaxy dynamics) there is no need to invoke them as an explanation for the observed existence of cloud pockets such as these – in this case – stringing two galaxies together which have already been suspected of having interacted. In this present era within the Local Group of galaxies, the members must already have undergone a fair degree of stirring about one another and it its alternatively possible that such clouds may be attributable to a previous relatively close encounter between M31 and M33. Besides, many of the same and best simulations suggest that small compact irregular systems are apt to shunt between larger systems (such as m33 and M31 now are) and that initial intergalactic filamentary configurations can be drastically altered by evolutionary dynamics between growing galaxies, not to mention the classic tidal effects between systems as a dynamical matter of course. This particular observation is by no means trivial, to be sure, but it would be far more surprising if nothing was found between M33 and M31…but it should not necessarily suggest a surviving structure from a period when all the members of the Local Group as they are now were still in the construction phase. On the scale of our Local Group the primordial filaments that may initially have fed the growth of its current member galaxies would have long since been stirred beyond recognition. By the way: it hasn’t yet been determined if M33 will fall (back?) toward or into M31 before the latter is so popularly scheduled to hit the Milky Way head on. The whole scenario seems to ignore what simulations actually indicate.

  4. Bruce

    Interesting comments, which prompted me to reread the article, whereupon I discover that these gas clouds are observed in radio, not X-Rays. Doh, again. The question remains though, why are these clouds observable at all? Are they glowing due to internal heating or are they being lit up from an outside power source, such as M31’s SMBH?
    If they where being lit up externally then clouds like these could be very common, but only a tiny few would be discernable at all. Mike, good question about what the theoretical existence of all this extra-galactic gas would mean for the universal energy percentages. Is our place in the universal scheme of things about to shrink once again? Or, did this extra baryonic mass inferred by simulations take a bite out of dark matter’s share? If so the baryonic share would move up to 12%. Starfart, you make a good case for the tidal interaction side of the argument. I wonder though, since this material looks to be aligned somewhat along a line perpendicular to M31’s axis of rotation. Wouldn’t tidally formed clouds be strung out along an arc between the two galaxies?

  5. Lou

    Bruce, the gas clouds are detectable because of the hydrogen ‘superfine transition’ emission line at 21-cm (1420 MHz) – this is something that neutral hydrogen does by itself with no outside help (funny thing is that this was one of the very first things I learned about astronomy, because I got interested through SETI@home!). Regarding missing baryons, the CMB tells us that they would be part of the 4%.

  6. Bruce

    Thanks Lou for both answers. So these gas clouds are observed to be starless, while the question is: Are they from an fully extragalactic source or have they been tidally pulled from the vicinities of M31 or M33? If they truly were extragalactic then their gas would be primordial in origin, having zero metals, and if stars were to form in them they would be members of the elusive Population III class. Since no Pop III stars (or any others) have been found this gas must be too diffuse to form stars. The absence of Pop II and I stars also leads me to doubt the tidally extracted side of this question.

  7. ctj

    these gas clouds remind me a lot of hanny’s voorwerp. it makes me wonder if voorwerps are a common feature of many galaxies, but aren’t easily seen unless they are "lit up" by an AGN. although i seem to recall that the voorwerp contained oxygen, so perhaps they are truly distinct objects.

    then again, maybe the oxygen IS present in these clouds, but we just don’t have an AGN to "illuminate" it.

  8. Bruce

    Yes Ctj, after looking up "Hanny’s Voorwerp" I can see why you made the comparison. These voorwerp objects (redundant, since voorwerp is Dutch for "object") would have to be of galactic origin since they contain metals. Also some star formation has started in Hanny’s Voorwerp, but not in the gas clouds of this news item. You questions are interesting and bring more insight into this discussion. Thanks for making your comment.

  9. Anthony Barreiro

    Aside from all the cosmological speculation engendered by this finding, I have a tiny mythological quibble. Camille refers to M31 as M33′s big brother. But M31 is commonly called the Andromeda galaxy after the princess Andromeda from Greek mythology — daughter of Cepheus and Cassiopeia, damsel in distress rescued by Perseus of the winged sandals, etc. Surely M31 is M33′s big sister. Of course the imagined gender of a galaxy has no bearing on whether those clouds of intergalactic hydrogen are primordial stuff or the detritus of more recent galactic interactions. But for those of us who came into astronomy through the study of mythology and history, seeing Andromeda referred to with a masculine epithet is passing strange.

  10. Joe S.

    For Mike, this material cannot be dark matter since it is radiating in the radio or long microwave, i.e., it’s not dark (Bruce mentioned 21-cm hydrogen hyperfine). So it is normal matter, probably almost entirely hydrogen and helium. As you phrased it, it is "2/3 of the 4% matter "known" to us". If this were evidence that the normal matter fraction of the universe differed from the accepted 4-5%, this would be front page news! That the visible matter in galaxies is only 1/3 of the total normal matter is not surprising, since not everything is glowing brightly enough for us to see. We get that 4-5% value not from observations of starlight, but from analysis of the cosmic background radiation (see COBE, WMAP, and Planck). The energy for this radio-wave radiation probably is thermal – the heat coming either from the energy of the galactic interactions that might have created these clouds or from gravitational potential energy as they collapse under their own self-attraction (this last can occur if they are cool enough to be contracting – and if they are too hot for that, then there is enough energy already there for this glow). I haven’t read the original article so take my opinions with sufficient salt to make them palatable.

  11. Bruce

    Those answers taste great as they are Joe S., except that it was Lou who first mentioned the 21cm hyperfine emission line. So my first question as to why these gas clouds glow has been well answered, as has Mike’s question about where all this extra gas fits in the universal energy percentages. As to the question Camille raised in her article’s subtitle, Starfart has made a rather convincing argument that these objects aren’t “evidence for the elusive cosmic web” since as he put it, “On the scale of our Local Group the primordial filaments that may initially have fed the growth of its current member galaxies would have long since been stirred beyond recognition.” Since galaxies inside clusters orbit around each other his point is hard to refute. Perhaps the filamentary web of dark matter mainly exists at the larger scales, stringing galactic clusters together? Since dark matter far out weighs “ordinary” baryonic matter ordinary matter won’t be pulling much on dark matter, it’s the other way around.