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# Interacting galaxies and mergers

E. Athanassoula, Albert Bosma

► To cite this version:

E. Athanassoula, Albert Bosma. Interacting galaxies and mergers. Nature Astronomy, 2019, 3 (7), pp.588-589. 10.1038/s41550-019-0822-z. hal-03577963

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# Interacting galaxies and mergers

Two landmark papers in the 1970s contributed strongly to establishing the importance of galaxy interactions and mergers in the formation and evolution of galaxies, using only gravity, and the ensuing dynamical friction.

E. Athanassoula and Albert Bosma

Some nearby galaxies look peculiar and also spectacular, and in the mid-1950s and 1960s several compilations of such objects were made, the best known one being Arp's Atlas of Peculiar Galaxies¹. Nowadays, it is well established that a galaxy can be strongly perturbed by a close passage of another galaxy, particularly if the latter approaches on an appropriate orbit. This can lead not only to the formation of structures such as bridges (that is, straight narrow strips of gas and/or stars joining the two galaxies) or tails (similar strips but of spiral shape in a direction roughly opposite to that of the other galaxy), but also to a merging of the two systems whose remnant would then be considered as another galaxy, perhaps of a different type. Hence, this process can bring about a galaxy transformation.

In the 1960s, bridges and tails were debated vigorously at conferences, and, to explain them, forces other than plain gravity, such as magnetic forces, were invoked though hardly substantiated (see references in the introduction of ref. ²). Two papers²,³ (hereafter TT72 and T77, respectively) went a long way to getting the present picture established. In the first paper, written with his brother Juri, Alar Toomre's main purpose was to prove that bridges and tails are due to gravity alone. TT72 used a restricted three-body code for this, that is, two heavy point masses representing the galaxies, and massless particles representing material in their disks. A survey of test cases was made, and the case studies of well-known individual systems, such as the 'Antennae' (NGC 4038/39), or the 'Mice' (NGC 4676), produced computed images so convincingly close to the observations that the figures became nearly self-explanatory. In addition, the authors explored a set of wider implications, "on the pretext that here, as in medicine, pathology seems instructive" (TT72). Topics addressed ranged from 'eccentric bound orbits', 'orbit decay', 'stoking the furnace' to 'certain exceptional spirals'.

The work on orbital decay led to Alar's contribution to a famous conference on galaxy evolution in Yale (T77), organized by Beatrice Tinsley and Richard Larson in

1977, and whose proceedings, including discussions, were published and widely available already three months later. The core of Alar's argument was the construction of a galaxy sequence, showing objects all having spectacular tails, but forming a sequence starting with cases where the two galaxies were clearly distinct, and ending with cases where there was only a single main object (see Fig. 1a). This 'Toomre sequence' has been well studied⁴,⁵, and single-object studies of merger products, such as NGC 7252,⁶ show that the end result of the interaction and merging is indeed a galaxy with a photometric profile similar to that of an elliptical galaxy. A summary figure for this galaxy is given in Fig. 1b and c.

What is important to understand here is the notion of dynamical friction. In an ideal theoretical case, if a massive compact object, such as a $10^6 M_\odot$ black hole, travels through a homogeneous sea of lighter particles such as stars of $1 M_\odot$ each, it will deflect the orbits of these stars, and create a wake behind it. This wake will exert a drag force on the massive object and slow it down. Likewise, when a compact companion penetrates into the sea of stars of its main galaxy, it will slow down, and eventually migrate to the centre of the latter, if it is dense enough to retain its own stars. When two spirals collide, such as envisaged for the formation of NGC 7252, their central parts will undergo dynamical friction and thus slow down, and merge in a time that can be quite short in case of sufficient interpenetration, comparable to or shorter than the lifetime of the tail structures. The latter was estimated in T77 to be "hardly much greater than half a billion years".

The importance of such galaxy transformations was not readily accepted, as is often the case with bold views that present a novel interpretation of known facts. Gerard de Vaucouleurs⁷ remarked that "after a collision a car is a wreck, not a new type of car!". Ostriker⁸ and Sandage⁹ came up with a number of arguments, concluding that elliptical galaxies did not form that way. Rereading the exchange of opinions at that time, one can see that there were only few worthwhile arguments against the idea that

gravity alone can account for the formation and structures of bridges and tails, due to the success of TT72 in reproducing spectacular examples. Transforming disk galaxies into ellipticals, however, is a more revolutionary idea, and has many wider implications for the general picture of galaxy evolution, touching not only the dynamical and kinematical aspects, but also the colour luminosity relation¹⁰, or the dependence of the fraction of ellipticals on the environment⁹. It was thus met with considerable scepticism. With time, however, and as more observational facts and simulations came to the fore, this picture became much more widely accepted.

This area of study got a tremendous boost in the 1980s by the discovery with the Infrared Astronomical Satellite of strong infrared emission associated with peculiar galaxies, often active galactic nuclei, but also starburst galaxies, such as M82, Arp 220 and other ultra-luminous infrared galaxies. These were often attributed to 'wet' mergers, that is, mergers of gas-rich galaxies. Some of those galaxies were found to have a large amount of CO emission, and therefore molecular hydrogen, which could be the main reservoir fuelling the huge starbursts. In the 1990s, it became clear from Hubble Space Telescope data that there were a lot of perturbed galaxies at higher redshifts, presumably due to interactions. Moreover, the numerical simulations of merging galaxies became more realistic, fully self-consistent and included gas. It was then found¹¹ that dwarf galaxies, without much dark matter, could be formed in the tidal tails, while the existence of such features near the tips of the tails requires quite extended haloes¹², at least ten times more so than the disks. Currently, merging is routinely seen to occur in the large-scale ab initio cosmological simulations, which also include recipes for star formation, feedback and cooling. All the above and much more have contributed to making interactions and mergers of galaxies an integral part of our science, rather than peculiarities.

One key point in T77 merits renewed emphasis in the current debate regarding dark matter. Alar Toomre clearly understood that the dynamical friction necessary for
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**Fig. 1** | NGC 7252, a transformed galaxy as end point of the 'Toomre sequence' of galaxies in different stages of merging. **a**, Sketch of the sequence in T77. **b**, H I distribution¹, overlaid on an image by the European Southern Observatory (ESO). **c**, Hubble Space Telescope image. The main body has the photometric profile of an elliptical galaxy⁶. Panel **a** adapted from ref. ³, Yale University Observatory. Image in panel **b** courtesy of ESO; overlay adapted from ref. ⁴, AAS/IOP. Image in panel **c** courtesy of NASA and ESA, acknowledgement: Judy Schmidt (Geckzilla).

the rapid merging could hardly be achieved by the visible parts of the galaxies alone. Thus, the last paragraph of T77 contains a suggestion: dark matter provides a lot of this friction. A “strange way to make ellipticals” perhaps, but there it was: a now solid argument for the presence of these “extensive halos” starting to be discussed then. Trials with N-body codes¹³,¹⁴ based on modified gravity (MOND¹⁵), where the dynamical

friction is necessarily weaker, show that one can in this way produce an object with tails and two separate hulks (as in NGC 4038/9), but making an object with two tails and a single hulk (as in NGC 7252) is still elusive.

A last point arises from the discussion itself (T77): after being asked about a possible smooth transition from ellipticals to spiral galaxies, and where the line should be drawn between galaxies having experienced

one or more mergers, and those that have not, Alar answered that any compact parts should sink deep into the centre and contribute to what is called a central bulge. This is quite close to what is considered now, several decades later, the most promising way to make ‘classical bulges’, as has been witnessed in a number of simulations¹⁶,¹⁷.

Today, the possibility of forming galaxies via mergers and the corresponding transformation is widely accepted, but the nature of the merger remnant has come anew under discussion. It is now realized that the hot gas around large spirals, whose presence has been difficult to prove, could be so important as to form a new disk in the remnant by cooling and collapse, and thus lead to a spiral galaxy. This has been argued using numerical simulations¹⁷. Hence the merger remnant could be an elliptical, or a spiral, depending on the relative orbit of the two merging systems (and thus the angular momentum of the remnant), the environment (cluster, group, or field), the properties of the circumgalactic and intergalactic medium (and thus the available gas), the redshift at which the merging occurred, and so on. This more complex picture may be able to better answer all of the detailed criticisms of the Toomre picture, while leaving the main point intact, that is, that a merger can well change a galaxy totally, and constitutes a new way of making some types of galaxies. □

## E. Athanassoula and Albert Bosma

Marseilles Laboratory for Astrophysics, Marseilles, France.

e-mail: lia@lam.fr; bosma@lam.fr

Published online: 5 July 2019

https://doi.org/10.1038/s41550-019-0822-z

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