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Galaxy encounters — when gravity reshapes
Two galaxies influence each other long before their bright disks touch — their dark-matter halos reach ten times further than the visible light. What happens then is decided by mass ratio, orbit and gas content, not by the distance two patches of light happen to have in a photograph.
Tides: the force difference shapes the form
The gravity of a neighbouring galaxy pulls on everything, but not equally everywhere. The side of a disk facing it is pulled harder than the centre, the far side less. What deforms the disk is not the attraction itself but this difference — the tidal force, the same one that raises two tidal bulges on Earth, one towards the Moon and one away from it. In galaxies two corresponding outgrowths form: a bridge of stars and gas towards the partner and a long tidal tail on the opposite side. Because the tidal force falls off with the cube of the distance, it acts almost only during close passages — and then all the more violently. That such tails really arise from simple gravity was shown by Alar and Jüri Toomre in 1972 with the first computer simulations, which needed only a few hundred test particles to reproduce the observed shapes. Toomre & Toomre (1972): Galactic Bridges and Tails
Why they do not simply miss each other
Two point masses passing each other continue on their orbits — nothing would ever bring them together. But galaxies are extended, and that changes everything. When a galaxy moves through the halo of another it deflects its dark matter and stars and leaves behind a wake of enhanced density that pulls it back and slows it. This dynamical friction costs orbital energy: a wide fly-by becomes a bound orbit, the bound orbit an ever tighter spiral — until the nuclei merge. For partners of roughly equal mass that takes one to two billion years and a few orbits; a small companion can circle its host for many billions of years before it is absorbed. Whether an encounter binds at all depends on the relative velocity: in loose groups like our Local Group, where galaxies meet at one to two hundred kilometres per second, mergers are common. In rich galaxy clusters with velocities above a thousand kilometres per second the members mostly race past one another — the encounter deforms, but it does not bind.
The stages of a merger
A merger of two galaxies of similar mass — a major merger, by convention from a mass ratio of about one to four — follows a recognisable pattern in simulations and observations:
For gas-poor partners the end result is an elliptical galaxy: the ordered rotation of the two disks has been converted into disordered motion, the stars move every which way, the shape is an ellipsoid. This is the main formation channel of the giant ellipticals that populate the centres of galaxy clusters. Their origin often remains visible for a long time — as faint shells and arcs in the outer light, the remnants of the tails that have fallen back. For gas-rich partners, by contrast, a new small disk can form from the returning gas. So the old rule “spirals merge into ellipticals” holds, but with exceptions.
Stars miss, gas collides
In a galaxy collision practically no stars collide. In the solar neighbourhood about four light-years separate two stars — that is some tens of millions of solar diameters. Two galaxies therefore pass through each other like two swarms of gnats; the stars feel only the combined gravitational field, which redirects their orbits. Gas is a different matter: gas clouds are extended and really do collide. They form shock fronts, lose energy, condense and fall to the centre. The result can be a starburst, in which a hundred times as many stars form in a short time as in a quiet disk. The most luminous infrared galaxies in the universe are almost all merging systems whose dust swallows the light of billions of young stars and re-emits it as heat. But a starburst is not an inevitable consequence: it needs gas that is still there and an orbit that drives it to the centre. In the Antennae galaxies NGC 4038/4039 both are the case — their overlap region is among the most productive star factories in the nearby universe. NASA: The Antennae galaxies
AI-generated illustration of a galaxy encounter: two distinguishable nuclei, distorted disks and long tidal tails visualise the effect of gravity. Not a photograph of a specific pair and no prediction for the Milky Way and Andromeda. This image is AI-generated — not a photograph but a visualisation.
Traces of past encounters
Encounters leave scars that can still be read after billions of years. The Milky Way carries several: the stellar stream of the Sagittarius dwarf galaxy, wrapping itself around the whole Galaxy right now; the remains of Gaia-Enceladus in the halo, a galaxy absorbed about ten billion years ago that probably heated the Milky Way's thick disk; and the Magellanic Stream, a band of gas the two Magellanic Clouds trail behind them. M 31, too, is no untouched system. Around it lies a giant stellar stream from a torn-apart companion, and its outskirts are full of shells and arcs. A 2018 study interprets the compact companion M 32 as the surviving core of a once large galaxy that M 31 plundered down to this remnant about two billion years ago — an interpretation that fits the stellar streams, though not the only possible one. For the images this means: the smooth appearance of M 31 hides an eventful history that is simply too faint to see in broadband light. Ibata et al. (2001): The stellar stream around M 31; D'Souza & Bell (2018): M 32 as a remnant
Andromeda: an open ending
M 31 approaches the Milky Way at about 110 kilometres per second, and for a long time a merger in four to five billion years was considered settled. The flaw in that reckoning was the transverse motion: whether two galaxies collide head-on or circle each other is decided by the sideways velocity, and at a distance of 2.5 million light-years that can only be measured with great uncertainty. A study published in 2025 computed the orbit many thousands of times, taking all measurement uncertainties into account — including the perturbations by M 33 and the Large Magellanic Cloud, both of which noticeably alter the orbit. The result: in about half the cases the Milky Way and Andromeda merge within the next ten billion years, in the other half they do not; a merger as early as four to five billion years from now is unlikely, at only a few percent. Should it happen, the outcome would be a single elliptical galaxy, half in jest called “Milkomeda” — and the Sun, by then a red giant itself, would most probably survive the event unharmed on a new orbit. Sawala et al. (2025): No certainty of a Milky Way–Andromeda collision
What one may claim from one's own image
Our Andromeda photograph shows companions and structure, but no future orbit. The small galaxies next to NGC 891 and the “companions” of NGC 7331 are not shown to be partners by their proximity in the image — in the case of NGC 7331 they demonstrably lie about an order of magnitude further away. Interpreting a pair as interacting requires distances, velocities and morphological evidence: bridges, tails, warped disks. An inconspicuous patch of light may remain intriguing without being credited with an unproven encounter. What a photograph can do, however, is show the companions that really are bound — M 32 and M 110 at Andromeda — and hint that things must once have been rougher there.
A few things worth knowing:
- There used to be more collision. In the young universe galaxies stood closer together and merged far more often. A large part of today's galaxy mass came together this way; the quiet present is the exception, not the rule.
- Minor mergers are routine. The Milky Way has absorbed dozens of dwarf galaxies and remained a spiral throughout. Only a major merger destroys the disk.
- Ring galaxies are bull's-eyes. When a small galaxy plunges centrally through a disk, a density wave runs outward and ignites a ring of star formation — the Cartwheel Galaxy is the most famous example.
- Cluster giants feed. The central giant galaxies of large clusters have grown by continually consuming smaller members — “galactic cannibalism” is a serious technical term.
- Black holes merge too. When two galaxies with central black holes come together, the holes sink to the common centre and eventually unite. The gravitational-wave background of such events has been emerging in the timing of pulsars since 2023.
In my own sky
Real companions, real scars — and apparent neighbours that are not:
