When Galaxies Collide: Hubble Showcases 6 Magnificent Galaxy Mergers

To celebrate a new year, the NASA/ESA Hubble Space Telescope has published a montage of six beautiful galaxy mergers. Each of these merging systems was studied as part of the recent HiPEEC survey to investigate the rate of new star formation within such systems. These interactions are a key aspect of galaxy evolution and are among the most spectacular events in the lifetime of a galaxy.

It is during rare merging events that galaxies undergo dramatic changes in their appearance and in their stellar content. These systems are excellent laboratories to trace the formation of star clusters under extreme physical conditions.

The Milky Way typically forms star clusters with masses that are 10 thousand times the mass of our Sun. This doesn’t compare to the masses of the star clusters forming in colliding galaxies, which can reach millions of times the mass of our Sun.

These dense stellar systems are also very luminous. Even after the collision, when the resulting galactic system begins to fade into a more quiescent phase, these very massive star clusters will shine throughout their host galaxy, as long-lasting witnesses of past merging events.

By studying the six galaxy mergers shown here, the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey has investigated how star clusters are affected during collisions by the rapid changes that drastically increase the rate at which new stars are formed in these galaxies.

Hubble’s capabilities have made it possible to resolve large star-forming “knots” into numerous compact young star clusters. Hubble’s ultraviolet and near-infrared observations of these systems have been used to derive star cluster ages, masses, and extinctions and to analyze the star formation rate within these six merging galaxies.

The HiPEEC study reveals that the star cluster populations undergo large and rapid variations in their properties, with the most massive clusters formed towards the end of the merger phase.

Each of the merging systems shown here has been previously published by Hubble, as early as 2008 and as recently as October 2020. To celebrate its 18th anniversary in 2008, the Hubble Space Telescope released a collection of 59 images of merging galaxies, which can be explored here.

More information

Reference: “Star cluster formation in the most extreme environments: insights from the HiPEEC survey” by A Adamo, K Hollyhead, M Messa, J E Ryon, V Bajaj, A Runnholm, S Aalto, D Calzetti, J S Gallagher, M J Hayes, J M D Kruijssen, S König, S S Larsen, J Melinder, E Sabbi, L J Smith and G Östlin, 3 September 2020, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/staa2380

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The HiPEEC survey was completed as part of the Hubble Space Telescope program GO 14066 (PI: A. Adamo). A repository with the study’s final data and catalogs is available here in the MAST Archive.

The international team of astronomers in this study consists of A. Adamo, K. Hollyhead, M. Messa, J. E. Ryon, V. Bajaj, A. Runholm, A. Aalto, D. Calzeti, J. S. Gallagher, M. J. Hayes, J. M. D. Kruijssen, S. König, S. S. Larsen, J. Melinder, E. Sabbi, L. J. Smith, and G. Östlin.

AstronomyEuropean Space AgencyHubble Space TelescopeNASAPopular
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  • George Robinson

    If galaxies all originated at the big bang singularity, and are all accelerating away from it, how can they possibly collide?

    • Torbjörn Larsson

      I’ll cut the response in as many parts that I have links in it (1 link per comment).

      First I think I need to put “the big bang singularity” in the context of modern cosmology theory which is has subsumed the old “big bang” cosmology into an inflationary hot big bang theory. It makes for an easier description.

      So what is the almost 40 year old inflationary hot big bang cosmology?

      “Among most people who study the early Universe, inflation is accepted as the new consensus theory. We might not know everything there is to know about inflation, but either it — or something so similar to it that we don’t have an observation to tell them apart — must have happened.”

      “But if at some early time, the Universe isn’t dominated by matter or radiation, but by a form of energy inherent to space itself, you get the yellow curve. Note how this yellow curve, since it’s an exponential curve, never reaches zero in size, but only approaches it, even if you go infinitely far back in time. An inflating Universe doesn’t begin in a singularity like a matter-dominated or radiation-dominated Universe does. All we can state with certainty is that the state we call the hot Big Bang only came about after the end of inflation. It says nothing about inflation’s origins.”

      “There are a lot of people who mean “the initial singularity” when they say “the Big Bang,” and to those people, I say it’s long past due for you to get with the times. The hot Big Bang cannot be extrapolated back to a singularity, but only to the end of an inflationary state that preceded it. We cannot state with any confidence, because there are no signatures of it even in principle, what preceded the very end-stages of inflation. Was there a singularity? Maybe, but even if so, it doesn’t have anything to do with the Big Bang.”

      “Inflation came first, and its end heralded the arrival of the Big Bang. There are still those who disagree, but they’re now nearly a full 40 years out of date.”

      [ https://www.forbes.com/sites/startswithabang/2019/10/22/what-came-first-inflation-or-the-big-bang/?sh=147a17b54153 ; Ethan Siegel is an astrophysicist.]

    • Torbjörn Larsson

      [2/3] Second, the reason that matters is that quantum fluctuations during inflation gave us the energy density fluctuations during the hot Big Bang which lead to structure formation – the cosmic web of gas filaments and galaxy clusters. The size of those fluctuations were initially on the order of 10^-5 parts [“O(10^-5)] and not the order of 1 [“O(1)”] that you would get from an expanding singularity. And you will also get such a large universe that the idea of an expansion center makes no longer sense. Space is expanding everywhere between gravitationally unbound, free, galaxies – imagine a raisin’ raisin bread where the latter are the galaxies and Milky Way is deep into the dough.
      [ [https://www.forbes.com/sites/startswithabang/2020/11/06/ask-ethan-if-the-universe-is-expanding-are-we-expanding-too/?sh=674443563eb2 ]

      But now we have two consequences in this context. The expansion which spontaneously happened under inflation and which the new hot big bang universe inherits, that expansion rate depends on the inner state of the universe (if inflation, radiation, matter or as now dark energy dominates, see “Scale factor (cosmology)” @ Wikipedia) but it is often independent of the matter content. And the matter content is concentrated to the filaments, where the gas flows towards the galaxy cluster nodes of the web. As the galaxies streams with, they merge.

      [In fact, it is now believed that the gas initially rotates randomly with an axis parallel to the filaments for reasons of how the web comes together by gas crashing into gas so the galaxies tend to do the same. But as galaxies tend to meet and merge during the flow towards nodes the collisions will cause their spin axis to eventually tend to tip over and align perpendicular to the stream direction.]

    • Torbjörn Larsson

      [3/3] An excellent moving illustration and description can be found at https://www.forbes.com/sites/startswithabang/2021/01/06/is-the-universe-actually-a-fractal/?sh=327c64ea5415 . The context is a bit off, it is a historical question, but it tells you a lot about the balance between universe expansion and cosmic web contraction you are curious about.

      “One of the more interesting things that we started to do was to run simulations of dark matter in the Universe on a variety of scales. Perhaps surprisingly, they all yielded practically identical results.”

      ” As soon as your system isn’t perfectly uniform any longer — and perfect uniformity is unstable under the laws of gravity — the overdense regions are going to preferentially attract more matter than the surrounding regions, while the underdense regions are preferentially going to give their matter up to the surrounding regions.”

      “In this scenario, you get mini-halos inside regular halos inside giant halos, all connected by filaments that themselves, given enough time and the right properties, will produce their own halos as well, while an even grander web forms on larger scales.

      At least, that’s how it would work if we inhabited what’s known as an “-de Sitter Universe: where the only thing making up the Universe is matter, and we have enough matter to reach the critical density, where the amount of “stuff” exactly balances the initial expansion rate.”

      “But our Universe fundamentally differs from this scenario in three important ways.

      1.) We don’t just have one type of matter, but two: normal and dark matter. While dark matter behaves in this self-similar fashion, normal matter is limited.”

      “2.) Matter is joined by radiation, an incredibly important component of the Universe. Radiation, because it has an energy that depends on its wavelength, was actually more important in the early Universe.”

      “This means, for the first few hundred thousand years of the Universe (and especially in the first ~10,000 or so), the matter overdensities struggle to grow, as the radiation works to effectively wash them out. There’s a lower limit to the scales at which the Universe is self-similar even at early times: your smallest scale structures are going to have at least ~100,000 solar masses in them, which is approximately the masses of globular clusters and the smallest known dwarf galaxies. ”

      “3.) Our Universe is also made extensively of dark energy, which dominates the energy content of the Universe today. If the Universe kept expanding while gravitating, and if the expansion itself wasn’t accelerating, there would be no upper limit to how large these cosmically self-similar structures could be. But because dark energy exists, it basically sets an upper limit to the size of these structures in the Universe: roughly a few billion light-years across.”

      “When you take all of this together, it helps us realize a true but perhaps counterintuitive fact about the Universe: on both the smallest and largest cosmic scales, the Universe is not fractal-like at all, and that only the intermediate scales have any chance at exhibiting fractal-like behavior.”

      TL;DR: What Mac said.

  • Mac

    My assumption has always been that the clusters of embryonic intensely hot gases that gathered and cooled to form galaxies has created gravitational pull that while traveling away at Hubbles constant they are also being attracked to each other.

  • Prakash

    Please post worthy comments, like the one’s with astronomical background.pl do not post silly,irrational,unreasonable comments which really cause.irritation to read.Thanks

    • matthew mcgehee

      Who judges silly because personally, I find every person who believes in God to be silly. That would be silly right to infer that either one or with the greeks multiple gods exist. that means only 1 group of beliefs and people are right and that’s silly. But if I want to ask with this inflation it seems the explanation given is what came first the chicken or the egg. When is it possible the bang or bangs are really simpler than it is being made. inflation expansion then it reaches its limit it contracts and compresses until was bam it did it again that singular bang that is at bang number 2.

      With that in mind a loop theory would be easily deducted and explained would it not. on top of the NASA guy who says in time travel things move backwards …..Put your self on a race track oval. on the front stretch if you look left. you “DOPPELGANGER” to you would be moving as he said backwards. uh oh, that’s 2 blues clues. 1 the reoccurring yet singular bang, time travel is backwards and the 3rd clue is really the reason the physics equations aren’t exactly right I mean the answers they don’t get are called paradoxes, quit using time. We made time up if we were smarter we wouldn’t need a leap year we would have gotten it right not just real close. 6 hours a year off 30 seconds a month a second a day almost exact. use something we don’t makeup. einstien meant distance when he space and time its distance. see cause speed doesn’t matter if our bodies can’t take the pressure of the speed if you don’t shorten the distance-time doesn’t matter anyway.

      plus can we stop trying to travel in time, we made it up there is no going backwards, it’s one plane it has its “plan” and how can you travel back in something that we made up not that long ago.. I assure you there were no 60 x60x24 = 86400 sec in day clocks in that there cave was there??? So stop it. And if you could travel forward guess what again simple silly theory, to travel forward you would have to get up to faster than light and you whats there. DArkness. light has not arrived you will be waiting for light and time to catch up.

      Is that silly if so mmcgehee2010 (at) that google mail site dot coms

      tell me me why its 1. silly
      2 wrong and prove it
      3 who made you fricken god people post and ask that’s how they learn. we make mistakes and we are corrected we learn the worst comment is the one not shared because it may bring upon change , challenge, and celebration. But people who don’t like those comments would rather not read them. so don’t read them. and now send me my email tell me why I’m wrong.

    • slim shady

      im sorry 1 more silly is this you

      Prakash Astrologer
      Harborne, Birmingham, West Midlands
      UKTel: 0044 121 427 4653

      silly would be the con man snake oil salesman hocking horoscopes

      point and laugh poiunt and laugh

  • Fred Smith

    Prackdash, isn’t it nice to be intolerant towards others. Did you take lessons on that?

  • Vickie

    Do you think there are “earth like” planets within these galaxies? And Those people are probably all dying during these mergers….🤔☹


    … the one should be able to figure out the workings of the dark matter from this…

  • Alexander

    Andromeda galaxy will have head collision with milky way galaxy in 3.6 billion years!! True factual information! The sun will stay in its own orbit and earth and other planets will be sling shoted into deep space

  • Binyamin

    Actually. The multiple awesome sync, order, beauty of the universe obviously needed the beyond the universe wisdom of G-d in its creation. G-d of course does not need billions of years for its creation. He performed this all 5781 years ago. Time same as space being part of Creation, was created all together. Prior to this date, time (the presumption of) billions of years – did not exist.

  • Benjamin

    “In the beginning G-d created he heavens and the earth”.(The Bible 1.1)
    This was 5781 years ago.
    G-d is the source of all wisdom we encounter in every detail of the natural order.
    The parameters of the material world are space and time.
    Time being part of the world, was and is part of the world.
    Time goes back 5781 years ago.
    There was no time prior to creation.
    The theme of “billions of years” is a figment of the imagination.
    Finally, G-d the infinite source of wisdom we see on earth and in the sky did not need ‘billions of years” for Him to develop what we see.
    He created everything in an instant.

  • Benjamin

    Time is a parameter of the physical world, created at the time of earth’s creation 5781 years ago by G-d Almighty. The concept of ‘billions of years’ is a figment of the imagination. Did not exist.

  • Lee berry

    I was just wondering about the voids in outer space could they actually be the place blacksphere’s go to die. Or could they be full of blacksphere’s from ancient times in our verse just a thought but I would if I could have a look at that. Just saying have a look and don’t forget the name.

  • Paco taco

    Space and dinosaurs are Jew hoaxes