Galaxy Growth Not Always Result of Violent Collisions


Until recently, it was thought the galactic equivalent of a motorway pile-up was the only way galaxies got bigger. But startling new evidence from a European team of astronomers suggests that violent galactic collisions are not the only way that galaxies evolve and grow, and instead there seems to be something else happening that has affected the majority of galaxies — a kinder, gentler action which is not quite so disruptive.

For some years, astronomers have struggled to understand why the mass of galaxies seems to have increased dramatically just a few billion years after the Big Bang. We know from observation that galaxies collide but this is an incredibly violent activity and one that is not particularly common.

A new study using the Very Large Telescope (VLT) at the European Southern Observatory (ESO), by a team led by Giovanni Cresci, looked for evidence that galaxies might be accreting material from the hydrogen and helium gas that filled the early Universe and permeates the space between the galaxies. We know that they are surrounded by halos of unseen material but Cresci’s team wanted to see if there was any evidence of material being sucked into the galaxy from the surrounding environment.

Their study focused on a group of distant galaxies which would represent those in the early Universe, about 2 billion years after the big bang, to see if they could detect any evidence of this gas accretion.

Using the SINFONI (Spectrograph for Integral Field Observation in the Near Infrared) attached to the VLT, Cresci and his team mapped the distribution of elements within the target galaxies. Their findings showed that instead of heavier elements being concentrated around the core as we find in today’s galaxies, the core was surprisingly abundant of the lighter elements hydrogen and helium. This can only be as a result of accretion of lighter elements from the surrounding area boosting the rate of star formation in the core. The accretion process itself relies on cool gas being transferred directly into the core of the galaxy.

“The primordial gas in the halo of galaxies, especially at great distances, is mostly shock heated and therefore very hot,” Cresci told Universe Today. “To be accreted it has to be cooled and this is not an efficient process. Recent theoretical models have shown that narrow streams of cold gas can form, and that they are able to penetrate the hot gas and to provide fresh gas to the centre of the galaxy. Unlike more destructive and violent mergers between galaxies, the streams are likely to keep the rotating disk configuration intact, although turbulent.”

This new discovery means astronomers have perhaps found an answer to a long standing question but with the major consequence of needing to rewrite our current theories of the evolution of the Universe.

Source: ESO, email exchange with Cresci

Mark Thompson is a writer and the astronomy presenter on the BBC One Show. See his website, The People’s Astronomer, and you can follow him on Twitter, @PeoplesAstro

10 Replies to “Galaxy Growth Not Always Result of Violent Collisions”

  1. I never believed collisions alone could explain the growth of large galaxies. Too many collisions needed and calming down after collision taking too long.

  2. Correct me if I am wrong, but do not most galaxy collisions result in elliptical galaxies? Can collisions between galaxies result in a spiral galaxy at some later stage in the post collision evolution?


  3. This is not a very surprising finding, at least to someone who has read anything about how galaxies evolve. There has been solid evidence for quite some time now that galaxy clusters form along filaments of gas. Active, still intact, galaxies can be seen “falling” into clusters along these filaments before getting stripped of their gas by superheated “winds” within the cluster. It stands to reason then that these same filaments are feeding nascent galaxies in the same way that a gas line feeds an engine. Fresh gas from the intergalactic medium should then feed into the center of galaxies, provinging fresh “fuel” to form new stars and also “power up” the central black hole which then pushes out material with its periodic outbursts. Such observation when extrapolated make alot of sense when considering how spiral galaxies might form.

  4. Exciting!

    If galaxies are feeding on an interstellar medium, what is the byproduct in the wake of the galaxy? (Like an organism might turn co2 to oxygen.) Is there one. ?

    How do galaxies connect in their local clusters?
    Is there a variance in the density of interstellar space. If so do galaxies travel randomly or follow higher densities.

    To me this paints a far more pleasing hypothesis of the universe, less random chance, more ecosystem on a grand scale. With lots of new questions to consider.


  5. Damian, I guess it depends whether the gas is collapsing to form stars, or whether it is just being accreted onto a supemassive black hole . The former will leave the usual stardust, some of which will escape the gravity of te galaxy from SuperNova explosions. What’s in between galaxies is hot ionised gas at 10^7-10^8K

  6. Seems to me the by-product of a galaxy is light, heat and heavy nucleai.

    Metaphorically, the galaxy is sucking up the energy (mass) of the interstellar medium and storing it by conversion to heavier elements. ?

    The singularities remain the mystery here, the Milky way has a Circumnuclear Disk of molecular gas that orbits the Galactic center at two parsecs that is considered favorable for star formation. The hypothesis is that Star bursts may occur every 500 million years in this region.

    Perhaps thats how long it takes for our galaxy to accrue enough ionized gas from the interstellar medium for this to occur.?

    However I’m dubious about the idea that the ISM is being eaten by the black hole.
    Perhaps collected would be a better analogy.?


  7. Damian does indicate something which might be relevant to galaxy formation. These filamentary structures connect clusters of galaxies and “walls,” where it might not be unreasonable to suspect that material in them is gravitationally accreted onto galaxies. A mental picture of these structures might be spider webs in an attic. The sunlight which glints off the points on these webs are galaxy clusters. Further, the walls of the attic are moving outwards, to simulate the expansion of the universe. So the filaments are being drawn and thinned with time, while material also gravitationally accretes on the galaxy clusters. The dark matter will not so much accumulate there, but luminous matter which strongly interacts will lose energy so as to accumulate.

    The SDSS and related research has found large scale filaments and structures. It is worth mentioning that John Hurcha, who lead initial the research on the map demonstrating filaments and domain walls, died recently. These structures have a fractal structure to them and the question has occurred to me whether it has the same Hausdorff dimension as the structure of CMB anisotropy. I have been thinking about how to structure algorithms which would use current data (SDSS and WMAP) to illustrate this.

    The wastes produced by this process, if we call it that, are heavy elements and collapsed stellar cores. From a thermodynamic perspective the generation of heavy elements is a sort of cosmic chemical potential term in the thermodynamic first law (conservation) equation. The integration of this conservation law with the loss of free energy generates entropy, which frankly includes the formation of heavier element, or the accumulation of “metals.” Of course a byproduct of this process is where lots of energy gets cycled through complex systems, which includes our planet, life and our selves.


  8. LC

    I recall a story stating that collisions may indeed create spiral galaxies and not just ellipticals. I think it had something to do with barred spirals. The story was probably here in UT, so a search may answer your question. I don’t recall if the study was confirmed, or it was just a computer model created by someone.

    Our Milky way is a spiral, and several galactic collisions have taken place in the past, and IIRC two or three are happening now. This would support the idea that ellipticals are not the only outcome of a galactic collision.

    Sorry I could not be more specific, or cite specific papers. 🙂

  9. Very nice comments here. I will add a few comments addressing some issues above. It is thought that collisions of dwarf galaxies generate spirals and that the collision of spirals generates elipticals. Infalling gas from the interstellar medium would have been a phenomenon much more commonly seen in an younger universe, but still may go on today to some extent. Such infalling gas would concentrate around the core of the galaxy and could provide material that generates star formation there. It could also provide an alternative explanation for the frequency of quasar outbursts in the early universe as it accretes upon the supermassive black hole, especially when there is no evidence of a recent galaxy merger. Such streams would be very difficult, if not impossible, to spot in the early universe given the current resolving power of our biggest telescopes. Naturally it would occur on a larger scale in galaxy clusters which is why this process has been seen on that scale.

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