A History of Mergers

Recent observations of the Hubble Deep Field [1] have shown that the fraction of interacting and merging objects seen is significantly higher than it is among nearby galaxies. For some time, the hosts of quasars have been thought to be galaxies in the process of merging [2]. On a more fundamental level, mergers are thought to play an important role in structure formation in the early matter dominated epoch of the Universe.

However, mergers are not confined to exotic circumstances in history; if we look at the local Universe, there are numerous examples of disturbed galaxies that quite obviously point to galaxy interactions. Such a view was not always held; for many years, right up until the early 1970's, many held the opinion that simple gravity acting on stars could not produce the observed features - bridges and tails being the prime examples - of the peculiars.

The Hubble Sequence

In 1920, a Great Debate roared between Harlow Shapley and Heber Curtis; what was the scale of the Universe?. Considering the "spiral nebulae", what we now know to be independent galaxies in their own right, Shapley argued that these were merely nearby gas clouds in a Universe that consisted solely of our Galaxy. Curtis disagreed; he expounded the view that the Sun was but a member of a small Galaxy of which there were many.

The true nature of our Galaxy and the Universe was resolved by Edwin Hubble. Using the 100 m Hooker telescope he was able to gauge the distance to Cepheids in M31, the Andromeda Galaxy; this distance was greater than that proposed by Shapley for the dimensions of our Galaxy and thus M31 must lie beyond its boundaries, a galaxy in its own right.

Hubble is perhaps better known for his observations which demonstrated that the Universe is expanding (borne out in the by now infamous Hubble Law), but he was also responsible for another important development of extragalactic astronomy - the Hubble Sequence. It should not be surprising that Hubble spent many years surveying thousands of galaxies, and during the course of his work, he noticed a trend - galaxies could be ordered into a sequence based on their morphology, the so called Hubble Sequence, an illustration of which is shown below.

On the left hand side of the diagram, which is commonly called a Tuning Fork diagram, we have the ellipticals which range from E0 up to E7 based on their ellipticity. E0s are spherical and ball like while an E7 is shaped like a rugby ball. At the vertex we have the S0s, lenticulars - galaxies which have disks like spirals but unlike spirals are gas poor. Then we branch out into the spirals, Sa, Sb and Sc and the barred spirals, SBa, SBb and SBc.

The obvious question to ask is why the dichotomy of shapes among galaxies? An obvious answer would lie in the formation of galaxies, and indeed, the ELS (Eggen, Lynden-Bell, Sandage) hypothesis [3] seemed to offer a solution. Stars in ellipticals formed far earlier and more quickly during the initial collapse of a primoridal gas cloud that would form the galaxy than their counterparts in spirals. However, this now introduced the question, why should this be so? It is at this point that the importance of galaxy mergers becomes clear.

Early References to Mergers : Lundmark (1926) and Lindblad (1926)

The first studies of pairs of interacting galaxies was undertaken in the early 1920's. Knut Lundmark, then at Upsaala Observatory in Sweden, carried out investigations of "double nebulae". (1920, 1926, 1927)[4,5,6] Assuming that both of the galaxies are at the same distance from the observer, Lundmark deduced the relative properties such as size and luminosity of the different morphological types of nebulae. Based on this research, he was able to conclude that given the large sizes of the "nebulae" relative to their separations in space, "collisions or encounters must be rather common among these objects.

Lindblad (1926) [7] further theorized that "sharp encounters between nebulae...must be considered as highly inelastic and must tend to convert translational (i.e. orbtial) into rotational (i.e. internal) kinetic energy. An encounter of this kind may even lead to a fusion of the respective bodies."

Holmberg (1941)

Erik Holmberg was interested in the clustering tendencies of galaxies; galaxies arise in a variety of locations. A few percent can be found in clusters of hundreds; a still greater fraction can be found in groups of tens. Our own Galaxy and M31 are both part of a group of galaxies called the Local Group. In particular, Holmberg wanted to explain the origins of these groups and clusters, and the mechanism he had in mind was the mutual tidal capture of galaxies during initial hyperbolic passages. Initially the galaxies approach each other with sufficient energy to escape to infinity but lose energy during the encounter and thus become bound. By assuming a uniform distribution of galaxies, each with a peculiar velocity of a few hundred kilometers per second, Holmberg envisaged the gradual accumulation of galaxies into groups and clusters.

Unfortunately for Holmberg, there is a fundamental problem with this hypothesis; when the probability for an encounter is estimated, we find that only one galaxy in ten thousand will have had sufficient time to be tidally captured during the lifetime of the Universe, quite at odds with observations.

Despite this, it is interesting to follow Holmberg's experiments into tidal capture of galaxies during close encounters [8]. These were numerical experiments in which each galaxy was modeled as 37 mass points arranged in concentric circles traveling around a common centre. What perhaps was most ingenious about this arrangement was that the mass points were represented by light bulbs! By using the inverse square behaviour of light, Holmberg had uncovered a method to mimic gravitational forces - this was the most computationally intensive part of the problem. Photocells were used to detect the magntiude and direction of the "gravitational force", i.e. the light, and so it was possible to map the trajectories of the mass points by graphical integration.

Holmberg simulated planar encounters between disk galaxies using this technique; he was able to estimate the efficiency of tidal capture for a variety of approach velocities, rotations and minimum separations. Whatsmore he correctly noted that the maximum tidal distortion occurs after the passage of the interloper through pericenter. However, he did not report any problems with bar instabilities, something which should have plagued such a cold disk, and he also noted that tidal capture was most efficient during retrograde passages when the disks counterrotated in an opposite sense to their orbital motion. As noted in Barnes (1996) [9], this is "curious".

In addition to the hyperbolic passages, Holmberg did investigate parabolic encounters and used them to illustrate tidal deformations, but because of the number of mass points involved and the approximations made, felt that this was not sufficient for a detailed study of tidal response. Hence an opportunity was missed to uncover the tidal origins of bridges and tails.

Zwicky (1953,56)

Fritz Zwicky was the first to start systematically photographing peculiar galaxies using his 18 inch Schmidt camera at Palomar. Using these images, he quickly concluded that many of the narrow filaments that he observed must be due to tides stemming from gravitational interactions. In a 1953 issue of Physics Today[10], he described these filaments as tides and countertides and furthermore, correctly deduced that the narrowest of them must be sheets of matter seen edge on. He emphasized that much of the visible matter in the bridges and tails must consist of stars. Shown below is one of his sketches - "the possible formation of an intergalactic bridge between two galaxies passing each other"[11].

In the sketch, Zwicky considers the possible bridge formation between two interacting galaxies; the resulting configuration closely resembles Arp 96. Note that the lower galaxy in instances (a) and (b) becomes the upper galaxy in instances (c) and (d).

Vorontsov-Velyaminov (1959) and Arp (1966)

Inspired by both Zwicky's sketches and faint images of peculiar and distorted galaxies, such as those of Vorontsov Velyaminov, Halton Arp spent four years photographing many such objects with the Palomar 200 inch. He believed that "the peculiarities...represent perturbations, deformations and interactions which should enable us to analyze the nature of real galaxies which we observe and which are too remote to experiment on directly". In 1966, he published his Atlas of Peculiar Galaxies, containing high resolution images of 338 systems [12].

Toomre and Toomre (1972)

During the 1960's, Pfleiderer [13,14] and Siedentopf [15] investigated how spiral patterns in disk galaxies could be excited by gravitational interactions between disk galaxies and concluded that chance encounters between field galaxies are not sufficiently common enough to produce the observed population of spirals. However, they did produce, albeit in passing, the first plots of tail and bridge building.

This was followed in the early 1970's by a spate of papers, but one in particular stands out. Galactic Bridges and Tails by Toomre and Toomre (1972) is seen as many as the seminal paper in the field of galaxy interactions and mergers. Although it was not unusual at this time for a paper to be published that dealt with computer modeling on interacting galaxies, what separates TT from other papers is


  1. A Morphological Catalog of Galaxies in the Hubble deep Field, Van den Bergh, S., Abraham, R.G., Ellis, R.S., Tanvir, N.R., Santiago, B.X. and Glazebrook, K.G. 1996, AJ, 112, 359.
  2. Identification of the Radio Sources in Cassiopeia, Cygnus A and Puppis A. Baade, W. and Minkowski, R. 1954, ApJ 119, 206
  3. Evidence from the motions of old stars that the Galaxy collapsed. Eggen, O.J., Lynden-Bell, D. and Sandage, A. 1962, ApJ 136, 748.
  4. On the Clustering Tendencies among the Nebulae. II. A Study of Encounters between Laboratory Models of Stellar Systems by a New Integration Procedure. 1941, ApJ, vol.94, p.385.
  5. Galaxies : Interactions and Induced Star Formation, Saas-Fe Advanced Course 26, Springer. Edited by Kennicutt, R.C., Schweizer, F. and Barnes, J.E.
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  7. Zwicky, F., Ergebnisse d. exakten Naturw. 1956, 29,344.
  8. The Atlas and Catalogue of Interacting Galaxies, Vorontsov-Velyaminov, B.A. 1959
  9. Atlas Of Peculiar Galaxies Arp, H. 1966
  10. Pfleiderer, J. and Siedentopf, H. Zs. f. Ap, 51, 201. 1961
  11. Pfleiderer, J. Zs. f. Ap, 58, 12. 1963
    Chris Power, Last Updated 6th September 1999

    N-Body Simulations of the Antennae