Instead, Rubin et al measured rotation curves that became roughly *flat* out past the point where the density of luminous matter dropped off. And stuff at the edge of the galaxy was orbiting too fast given the matter they could account for.

The 1980 Astrophysical Journal paper by Rubin, Ford, and Thonnard is usually cited in textbooks discussing this phenomenon. Look at those rotation curves!

https://ui.adsabs.harvard.edu/abs/1980ApJ...238..471R/abstract

Image: Astrophysical Journal, Vol. 238, p. 471-487 (1980)

The key point is that once you're out past most of the luminous stuff – the matter that makes up what you think of as the galaxy – orbital speeds should start to fall off. (In the simple case we started with this is called "Keplerian falloff.”)

We expect to see this falloff in the orbital velocity of gas and other matter loitering out past the edge of the galaxy.

But this isn't what Vera Rubin and her collaborators found!

Most of the galaxies we observe are not spherical. But a spiral galaxy (like our Milky Way) should have a qualitatively similar behavior.

Roughly speaking, v increases with distance from the center, then at some point it begins to drop off.

Here's the Newtonian gravity rotation curve for a disk-shaped galaxy.

A "rotation curve" is a plot with orbital speed on the vertical axis and distance from the center of the galaxy on the horizontal.

For the scenario I just described, you would expect it to look something like the plot below:

M(r) increases as you move out from the center of the galaxy. The density of luminous matter – literally the stuff you can see – may be more or less constant out to r, in which case M(r) ~ r³. Then you would expect orbital speeds within the galaxy to grow linearly with r.

The galaxy has a finite size, so past some distance R_L there is no more luminous matter. For r > R_L the mass M(r) no longer changes. Since it stays the same, but r gets bigger, the orbital speed now decreases like v ~1/ √r.

To keep things simple, imagine a spherical galaxy.

According to Newton, a star at distance r from the center of the galaxy should orbit with speed that depends on r and the amount of galactic mass M(r) out to distance r.

In the formula below, G is Newton's constant, 6.67 x 10⁻¹¹ m³ kg⁻¹ s⁻².

Suppose you measure orbital velocities of stars and gas circling the center of a galaxy.

If you know how matter is distributed throughout the galaxy, you can use plain old Newtonian gravity to get an idea of how fast things *should* be moving, and compare that with your observations.

Vera Rubin joined the faculty of the Carnegie Institute in 1964.

In the early 1970s she began the work for which she is most famous: Optical spectra of spiral galaxies that extended far enough from the galactic nucleus to provide support for flat rotation curves.

So what is a rotation curve, what does it mean to be "flat," and what does that have to do with dark matter?

Again, the work was not well-received.

But Rubin was ahead of her time, and the investigation of large-scale structure would soon become one of the major themes of astronomy!

For example, observations in the 1980s by Margaret Geller and collaborators, showing galaxies clustered along delicate filaments that outlined vast and nearly empty voids.

What Geller described as looking like a “kitchen sink full of soap suds.”

Figure: de Lapparent, Geller, Huchra (APJ, 302:L1-L5)

After completing her PhD in 1954, Rubin presented these results at the AAS meeting.

One advisor, William Shaw, offered to present the results for her, because she was pregnant. But only if he could present them in *his* name.

https://www.astronomy.com/science/how-vera-rubin-confirmed-dark-matter/

After that, Rubin pursued her PhD at Georgetown, where she was supervised by George Gamow. She studied variations in the spatial distribution of galaxies – work that would explain the phenomena she had observed in her Master's Thesis.

Though no one understood it at the time, Rubin had identified the Supergalactic Plane.

So Rubin studied astronomy at Vassar College, graduating in 1948. After that, she followed her husband to Cornell where she completed a Master's Degree in 1951.

For her Master's Thesis, Vera Rubin showed that some galaxies and clusters seemed to deviate from their local Hubble flow. This wasn't taken very seriously by her colleagues. In her words, it was "believed by very few astronomers."

Around that same time, a young Vera Rubin was looking up at the night sky in wonder, memorizing the paths of meteors and pondering the stars.

In interviews, she would often say “I just couldn’t look at the sky without wondering how anyone could do anything but study the stars.”

The modern case for dark matter began with Fritz Zwicky in the 1930s.

His observations of galaxies in the Coma Cluster suggested there simply wasn't enough luminous matter present to gravitationally hold everything together. Hence, there must be matter there that you can't see!

Astronomer Vera Rubin was born #OTD in 1928.

Her work on galactic rotation curves became one of the main pieces of evidence for the existence of dark matter, and she deserved a Nobel Prize for it.

Image: Vassar College / Emilio Segrè Visual Archives

Someone left their copy of the 2023 Old Farmers Almanac in the little free library, which sort of feels like giving up.

[Camera cuts to a shot of Oppenheimer at the Trinity site, surrounded by colleagues. Enrico Fermi is beatboxing.]

My name’s Oppenheimer and I’m here to say
“Now I am become death” in a major way

@han_of_the_east lol

@Kiloku the othersie

Michael Collins took this photo #OTD in 1969, with Earth hanging in the background as Apollo 11’s Lunar Module Eagle rejoined Command Module Columbia.

Except for Michael Collins, every human being born on or before July 21, 1969 is in the frame of this photo!

Image: NASA