Rotational Properties of 21 Sc Galaxies
Galaxies spin too fast at their edges — most of their mass gives off no light.
If you weigh a spiral galaxy by its starlight, it spins so fast at the edge that it should fly apart. Vera Rubin measured exactly that — and found most of the galaxy is made of something we cannot see.
The idea, unpacked
In the Solar System, the farther a planet is from the Sun, the slower it moves — Neptune dawdles, Mercury races. That is gravity's rule when nearly all the mass sits in the middle: speed should drop off with distance. A galaxy keeps almost all of its light in a bright central disk, so the outer stars ought to behave like outer planets and slow down.
Rubin and Kent Ford measured how fast stars and gas actually orbit at the faint outer edges of spiral galaxies. The speed did not drop. The curves stayed flat — outer stars circling just as fast as inner ones. For that to be true, the galaxy's mass has to keep piling up far beyond where the light ends. There is far more matter than meets the eye, and it does not shine.
Where it came from
By 1980, Vera Rubin had spent years at the Carnegie Institution patiently measuring galaxy after galaxy, using a sensitive image-tube spectrograph her colleague Kent Ford had built. She had chosen a quiet, unfashionable problem — the rotation of spirals — partly to avoid the crowded, competitive fields of the day. With Ford and Norbert Thonnard she published the rotation curves of 21 spiral galaxies, large and small, and they all told the same story: flat curves, no falling off.
The hint was not entirely new. Fritz Zwicky had argued for unseen 'dark matter' in a cluster of galaxies back in 1933, and a few others had glimpsed oddities. But a single odd case is easy to dismiss. Rubin's galaxy-after-galaxy evidence — soon backed by radio measurements of hydrogen gas reaching even farther out — was hard to argue with, and it turned a curiosity into one of the central problems of physics.
Why it mattered
It told us we had been mis-counting the universe. Everything we can see — every star, every glowing cloud — is only a small fraction of what is actually there. The rest, now called dark matter, makes itself known solely through its gravity: it outweighs ordinary matter by roughly five to one, shapes how galaxies form and cluster, and remains, decades later, unidentified. Rubin handed physics a mystery it still has not solved.
A way to picture it
Imagine a carousel where the horses near the rim gallop just as fast as the ones near the centre. If all the weight were in the hub, the outer horses would lag. Their keeping pace tells you the carousel's mass must be spread out all the way to the rim — even though you can only see the bright hub. A galaxy is that carousel, and the spread-out, invisible weight is dark matter.
Where it sits
Henrietta Leavitt and Edwin Hubble (both in this Library) had measured how far away the galaxies are and how the whole universe expands; Rubin measured what they are made of, and found most of it missing from view. Her dark matter joins the cosmic microwave background (Penzias and Wilson) and the bending of light by gravity as independent lines of evidence pointing to the same hidden component — the scaffolding on which all the visible galaxies are hung.
For 21 Sc galaxies whose properties encompass a wide range of radii, masses, and luminosities, we have obtained major axis spectra extending to the faint outer regions, and have deduced rotation curves.
All curves show a fairly rapid velocity rise to V ∼ 125 km s⁻¹ at R ∼ 5 kpc, and a slower rise thereafter.
Most rotation curves are rising slowly even at the farthest measured point.
Neither high nor low luminosity Sc galaxies have falling rotation curves.