Spirals, Tides, and M51

Article Updated: 24 Dec , 2015

Spiral galaxies are undoubtedly one of the most beautiful structures in the universe. Yet, their simple elegance belies a complex nature. How do such structures not “wind up” and what causes them in the first place? The answers to these questions is a long standing challenge. Under one model, spiral structure is created by spiral density waves. In another, they are induced by tidal interactions. It is this approach that is explored in a new paper by Dobbs et al., accepted for publication in the Monthly Notices of the Royal Astronomical Society. Specifically, the authors attempted to use modeling of tidal forces to recreate the structure of the spiral arms on the grand design spiral, M51.

M51sim1To model the interaction, they began with a model of a simple galaxy with a mass distribution (broken into a disc, bulge, and halo) similar to that for M51. Their initial galaxy was initially free of spiral structure, but “gravitational instabilities in the stars [Note: as opposed to the galactic gas. Not in individual stars.] produce a multi-armed” and patchy spiral structure (known as a flocculent spiral). This flocculent nature was first predicted in a 1964 paper by Toomre and has been simulated numerous times since then. Dobbs’ team then introduced a point source to represent the smaller galaxy (NGC 5195) along the orbital parameters derived by previous simulations of Theis and Spinneker in 2003.

For the first 60 million years, significant new structure was not evidence. The disc showed some perturbation due to the approaching companion, but no new spiral structure arose. However, by 120 million years from the start of the simulation, hints of a spiral arm on the side of the galaxy closest to the companion begin forming and by 180 million years, two pronounced “grand design” spiral arms dominate the face of the galaxy, spanning over 15,000 light years.

But the arms were too good to last. By 240 million years, the arms only stretch to a mere 6,500 light years as the gravitational forces from the companion seem to shepherd the galaxy’s gas as it is pulled around in its orbit. By 300 million years, the spiral arms have grown again and the pair looks remarkably similar to the present state of the M51/NGC 5195 system.

Comparison of simulation at 300 million years to HST image.

Comparison of simulation at 300 million years to HST image.

The authors note several features their simulation has in common with the observed galaxy. On the side where the companion first approached the galaxy, they note a “kink” in one arm (labeled as A in image to left). Another similarity is a splitting of one of the spiral arms although, again, the exact positioning is different (labeled B).

Another comparison the authors made was to the strengths (or amplitude) of various arm patterns (1 arm, 2 arm, 3 arm, etc…) over time. They found that the two armed pattern was the most predominant, but from the mechanics, they determined there were underlying higher armed structures that never fully took hold. However, these higher armed patterns did come close to the strength of the 2 arm spiral. The authors note that this is consistent with the observational findings of another group studying M51 in a work that has yet to be prepared for publication.

However, there are also some differences. A plume of gas extended from the simulated M51 which has no counterpart in actual observations (labeled C). Actual observations show large amounts of gas in front of the companion galaxy which are not present to the same degree in the simulation (labeled D). Lastly, real observations show a noticeable flattening of M51’s arms closest to the companion. Again, these do not appear in the simulation. The authors suggest discrepancies may be due to the over simplistic modeling of NGC 5195 as a point source instead of an extended body, or slight differences in initial parameters when compared to the actual system.

Even with these differences, the authors suggest that their modeling of the interaction shows that spiral structure, at least in this case, is most likely the result of the tidal interaction on M51 by NGC 5195. They also note that spiral density waves are likely not the culprit since other studies have not been able to determine a consistent “pattern speed” for the galaxy (the pattern speed is the angular speed at which the arms would rotate if viewed as a coherent structure). Instead, observations showed that the arms should have different pattern speeds at different radii.

Although their work does not suggest that all spiral structure is formed by tidal interactions with companions, this work makes a strong case for the possibility in many galaxies which would have such companions and M51 in specific. Furthermore, the simulations also reveal that these tidally induced arms are a temporary phenomenon. Since they do not have a fixed speed, they will slowly wind up and as the interaction progresses, the galaxies will be further distorted and eventually merge.

(Thanks to Claire Dobbs for permission to reproduce images from the paper as well as clarification on a few points.)


5 Responses

  1. I would like to see a three dimensional map of the mass of the galaxy. Including the distribution of dark matter. This might make understanding the spiral structure easier.

  2. Torbjorn Larsson OM says:

    I should really read the paper before I comment, but assuming that the post somewhat maps it, I must say that I’m neither terribly impressed with identifying qualitative characteristics (such as “kinks”) nor bothered by failure to do so, as they may be both spurious and unquantifiable.

    I _would_ be impressed if simulations succeeds in reproducing quantifiable statistics of distributions within galaxies. Especially if they can do so with parameter change, so that one goes from testing correlation to testing causation.

    So again, as I haven’t read the paper, maybe they did. In any case this is interesting. AFAIU it seems that bulges and ellipticals get their explanation from galaxies gobbling up satellites, or at least that is the impression that later press releases gives me. I assume that means that one should see more flat spirals as one goes further back. At the same these simulations, while starting with bulges, seems to indicate that (likely many) spirals are temporary signs of nearby satellites.

    To paraphrase:

    “Big galaxies have little galaxies

    That feed them their massivity,

    And little galaxies have lesser galaxies

    And so on to passivity.”

  3. Torbjorn Larsson OM says:

    D’oh! “maps it” – maps to it. A horse of a different color, indeed.

  4. RUF says:

    I think this clearly illustrates the problem with using computer modelling. Unless one accounts for ALL variables (including those that may not even be known) reliance upon computer modelling only exposes the bias of he who enters the data.

  5. Lawrence B. Crowell says:

    It is my understanding that the density wave model is sort of the standard model for galaxies. The mutual gravitational interaction between galaxies would then produce tidal distortions of the density wave distributions.


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