How Can Growing Galaxies Stay Silent?


Beginning around 2005, astronomers began discovering the presence of very large galaxies at a distance of around 10 billion lightyears. But while these galaxies were large, they didn’t appear to have a similarly large number of formed stars. Given that astronomers expect galaxies to grow through mergers and mergers tend to trigger star formation, the presence of such large, undeveloped galaxies seemed odd. How could galaxies grow so much, yet have so few stars?

One of the leading propositions is that the galaxies have undergone frequent mergers, but each one was very small and didn’t encourage large scale star formation. In other words, instead of mergers between galaxies of similar size, large galaxies developed quickly and early in the universe, and then tended to accumulate through the integration of minor, dwarf galaxies. While this solution is straightforward, testing it is difficult since the galaxies in question are at vast distances and detecting the minor galaxies as they are devoured would require exceptional observations.

Seeking to test this hypothesis, a team of astronomers led by Andrew Newman from the California Institute of Technology combined observations from Hubble and the United Kingdom Infra-Red Telescope (UKIRT), to search for these diminutive companions. The team examined over 400 galaxies that didn’t display signs of active star formation (called “quiet” galaxies) in search of possible companion galaxies from distances of 10 billion light years to a relatively close 2 billion lightyears in order to determine how this minor merger rate has evolved over time.

From their study, they determined that around 15% of quiet galaxies had a nearby counterpart that had at least 10% the mass of the larger galaxy. This took into account the possibility that some galaxies may have been more distant but along the line of sight by ensuring that both galaxies had similar redshifts. Over time, the partner galaxies became rarer suggesting that they were becoming rarer as more were consumed by the larger brethren. Using this as a rate at which mergers must occur, the team was able to answer the question of whether or not these minor mergers could account for the galaxy growth discovered six years earlier.

For galaxies closer than a distance of roughly 8 billion light years, the rate of minor mergers was able to completely explain the overall growth of galaxies. However, for the growth rate of galaxies at times earlier than this, such minor mergers could only account for around half of the apparent growth.

The team proposes several reasons this may be the case. Firstly, many of the basic assumptions could be flawed. Teams may have overestimated the sizes of the massive galaxies, or underestimated the rate of star formation. These key properties were often derived from photometric surveys which are not as reliable as spectroscopic observations. In the future, if better observations can be made, these values may be revised and the problem may resolve itself. The other option is that there are simply additional processes at work that astronomers have yet to understand. Either way, the question of how growing galaxies avoid advertising their growth is unanswered.

9 Replies to “How Can Growing Galaxies Stay Silent?”

  1. Somehow I seem to have a problem with the idea that there was this one “big bang” at a single location, yet galactic collisions are common place . . . . it does not seem to add up. Explosions radiate outward linearly.

    1. Any physicist would have problem with that, not only because it didn’t happen that way but because it couldn’t happen that way.

      – Inflationary cosmologies such as the standard cosmology, which replaced classical “big bang” cosmologies, have no “explosion” or single location which they originate in.

      – It is impossible to have an “explosion” into a preexisting spacetime, because that spacetime can’t exist without a preceding cosmological explanation.

      What happens instead is that spacetime expands everywhere, solving the cosmological conundrum.

      First spacetime expands under the inflation era of inflationary cosmologies, which is nothing but nonlinear (exponential).

      Then it continues to expand at a much slower rate under the earlier “big bang” expansion, which has a complicated behavior depending on which of several mechanisms stands for the main expansion at the time. You can recognize 3 more periods here, one of which is indeed linear.

      The last picture in this starting article depicts the resulting expansion from all of that.

      It is, I think, quite easy to imagine such an expansion if not the mechanisms. Imagine a raising loafer of raisin bread. The dough would stand for the expanding spacetime. Each raisin would stand for a galaxy group, which are separated during the raising.

      Bound systems like galaxies and local groups does not expand just because unbound spacetime does. Hence they stay close, and gravitational interaction would make them aggregate.

      1. what is not easy to imagine is the preexisting nullspace that everything expanded into.
        was it solid or was it nothing?
        was it time that was disconnected with space?
        (or vice versa? ha !)

      2. The critical part of “nullspace” is the null. No space. None. Niet. There was no universe so nothing solid, gaseous, foamy, aqueous, nor even darkness. Our universe did not push another out of the way. As far as we know, ours is the only one and uses up all the space there is. Remember, it’s not the universe that’s expanding, but only space.

      3. What about the “other” universes? How do they fit into the Big Bang Zero Space theory??? it is nothing but a theory, right(!?)

      4. Other universes are posited by mathematics, not what we can see or determine through optics or sensors. As stated, this, dearest Professor, is IT!. I don’t know anything about a BBZS theory but as we can only conjecture facts post BB, anything is possible before, including other universes. All evidence, including red shifts, age appropriate sightings of quasars etc, points to an origin time. The current expansion of the visible universe naturally would run in reverse as we dial back time: ei a massive contraction. It’s a pretty solid theory.

    2. The occurrence of these galaxies probably tells us there is some power law to how matter clumped. Such a law would be that N(m) = kL^a, N = number (or number per cubic billion lys), m the mass, k a constant and L the length scale. The term a is some power law term which tells us how matter clumping scales. This may be connected to the fractal structure of the CMB anisotropy, or with the scaling of filaments and domain wall as seen by the SDSS.


  2. Don’t forget: what we see is 10 Billion light years away, hence, a lot has happened since.

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