Table of contents:
- Helpful Webpages:
- Link to inclusion in public webpage:
- General Science overview notes:
- Speaker notes:
- Introduction slide / General speaker notes:
- Slide 1: galaxy rotation curves
- Slide 2: observations
Helpful Webpages:
Link to inclusion in public webpage:
General Science overview notes:
Rubin and Ford made careful measurements of Doppler shifts across the disks of several galaxies. They could then calculate the orbital speeds of the stars in different parts of those galaxies.
Because the core region of a spiral galaxy has the highest concentration of visible stars, astronomers assumed that most of the mass and hence gravity of a galaxy would also be concentrated toward its center. In that case, the farther a star is from the center, the slower its expected orbital speed. Similarly, in our solar system, the outer planets move more slowly around the Sun than the inner ones. By observing how the orbital speed of stars depends on their distance from the center of a galaxy, astronomers, in principle, could calculate how the mass is distributed throughout the galaxy.
When Rubin and Ford began making Doppler observations of the orbital speeds in spiral galaxies, they immediately discovered something entirely unexpected. The stars far from the centers of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer in. This was odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit. It followed that there had to be a tremendous amount of unseen matter in the outer regions of galaxies where the visible stars are relatively few. Rubin and Ford went on to study some sixty spiral galaxies and always found the same thing. “What you see in a spiral galaxy,” Rubin concluded, “is not what you get.”
Her calculations showed that galaxies must contain about ten times as much “dark” mass as can be accounted for by the visible stars. In short, at least ninety percent of the mass in galaxies, and therefore in the observable universe, is invisible and unidentified.
Speaker notes:
Introduction slide / General speaker notes:
Researcher's background:
Vera Rubin was born 1928 in Philadelphia, Pennsylvania. Her parents were Jewish immigrants from Eastern Europe, who both worked at Bell Telephone company.
Vera Rubin developed an interest in astronomy at the age of 10 from watching stars from her window. She earned a BSc in astronomy in 1948 at the all-women Vassar College, New York. After being barred from entering Princeton due to her gender, Rubin was accepted to Harvard as a postgraduate, but turned the offer down to attend Cornell with her husband. Rubin graduated with her masters degree in 1951, and obtained her PhD in 1954 from Georgetown University, Washington, D.C.
Throughout her graduate studies, Rubin encountered discouraging sexism and experienced imposter syndrome frequently. However, she routinely broke through barriers and defied stereotypes.
Rubin held academic positions at Montgomery College and Georgetown University, before finally joining the Carnegie Institution of Science in 1965 as a staff member where she remained for the rest of her career.
Synopsis of work:
During her master's, Rubin studied how galaxies move through our universe. She discovered a plane of density which would later be identified as the "supergalactic plane", but her paper was never published and she received harsh pushback. In her PhD, she discovered how galaxies tended to clump together, another controversial idea that when later pursued was proved to be true.
At the Carnegie Institution, Rubin collaborated with Kent Ford, an instrumentalist astronomer who had developed a state-of-the-art spectrometer. Using the spectrometer, Rubin and Ford measured the rotation rates of the nearby Andromeda Galaxy (M31), focusing on Hydrogen-II (HII) regions orbiting at various distances from the galaxy's centre. As they measured out to larger and larger radii, the HII regions seemed to move at the same speed, defying the supposed distribution of mass in the galaxy surmised from starlight observations. This prompted them to test this observation on other galaxies, and they eventually saw the same phenomena in each observation.
The data suggested that a halo of dark matter surrounded each galaxy, creating an even distribution of mass at larger galaxy radii and explained the constant velocities of the orbiting visible matter. Fritz Swicky had theorized the existence of dark matter in 1933 and his work was mostly overlooked, but this discovery provided the first solid evidence of its existence. The nature of dark matter is now one of the most pursued questions in modern astronomy.
Citations and resources:
https://en.wikipedia.org/wiki/Vera_Rubin
https://astronomy.com/news/2016/10/vera-rubin
Societal relevance of work:
The existence of dark matter is now fundamental to the understanding of astrophysics and modern cosmology. Since it's initial discovery by Rubin and Ford, the Planck satellite was able to measure the dark matter content of the universe through the clumping of the cosmic microwave background. In addition, measurements of gravitational lensing by galaxy clusters again confirmed the existence of dark matter. It has been calculated that dark matter accounts for approximately 85% of the matter in the universe.
The true nature of dark matter is still unknown, and remains one of the largest mysteries in modern astronomy. Physicists are searching for dark matter both on earth and in space.
In recognition of her achievements, Rubin was elected to the National Academy of Sciences, and was awarded the National Medal of Science in 1993. Most recently, the Large Synoptic Survey Telescope in Chile was renamed the Vera C. Rubin Observatory in recognition of her contributions to the study of dark matter and her outspoken advocacy for the equal treatment and representation of women in science.
Slide 1: galaxy rotation curves
Science details:
Rubin created galaxy rotation curves by measuring the velocity of rotating clouds of partially ionized hydrogen at various radii of the galaxy. These clouds are known as HII regions, and are regions in which star formation has recently taken place. HII regions are found in spiral and irregular galaxies, distributed across the galaxy.
The strongest hydrogen emission line, H-alpha line at 656.3nm, is detectable by telescopes and gives the regions their characteristic red colour. H-alpha emission is produced when an electron transitions from the n=3 to n=2 energy level in hydrogen, and is used to trace ionized hydrogen. After ionization, electrons and protons can recombine and the electron will cascade down to ground state, emitting photons in the process. About 50% of these cascades include the n=3 to n=2 transition, and therefore provides a way to detect hydrogen ionization.
Measuring the doppler shift of these emission lines allowed Rubin to deduce the orbital velocities of different regions of the galaxy.
Citations and resources:
https://en.wikipedia.org/wiki/H-alpha
https://en.wikipedia.org/wiki/H_II_region
https://en.wikipedia.org/wiki/Vera_Rubin
Figures:
Leftmost GIF: illustration of the Doppler effect from an orbiting emission region https://jila.colorado.edu/~ajsh/courses/astr1120_03/text/chapter1/L1S5.html
Bottom right: Sixty seven HII regions seen in the Andromeda Galaxy. Ultraviolet photograph. http://cdsads.u-strasbg.fr/pdf/1970ApJ...159..379R
Top right: Rotational velocities of HII regions in Andromeda, as a function of distance from centre of galaxy. http://cdsads.u-strasbg.fr/pdf/1970ApJ...159..379R
Societal relevance of work:
The existence of dark matter is now fundamental to the understanding of astrophysics and modern cosmology. Since it's initial discovery by Rubin and Ford, the Planck satellite was able to measure the dark matter content of the universe through the clumping of the cosmic microwave background. In addition, measurements of gravitational lensing by galaxy clusters again confirmed the existence of dark matter. It has been calculated that dark matter accounts for approximately 85% of the matter in the universe.
The true nature of dark matter is still unknown, and remains one of the largest mysteries in modern astronomy. Physicists are searching for dark matter both on earth and in space.
In recognition of her achievements, Rubin was elected to the National Academy of Sciences, and was awarded the National Medal of Science in 1993. Most recently, the Large Synoptic Survey Telescope in Chile was renamed the Vera C. Rubin Observatory in recognition of her contributions to the study of dark matter and her outspoken advocacy for the equal treatment and representation of women in science.
Slide 2: observations
Science details:
By observing visible matter in the Andromeda galaxy, it was expected that at high radii the velocity of the orbiting material would slow down, as the mass contained within their orbit would eventually begin to stagnate with larger and larger radii. Rubin and Ford observed that this was not the case, and velocities did not drop as quickly as they expected. This could be explained by the existence of additional 'dark' mass surrounding the galaxy, now known as a dark matter halo.
Citations and resources:
https://en.wikipedia.org/wiki/Vera_Rubin
Figures:
Left: (Left panel) Mass contained within a certain radius to the center of Andromeda. (Right panel) Density of mass at that radius. http://cdsads.u-strasbg.fr/pdf/1970ApJ...159..379R
Right: Rotation curve of the spiral galaxy M33, along with a predicted curve (dashed line) from the distribution of visible matter. Data and model predictions from Corbelli and Salucci 2000. The observations indicate the existence of dark matter, as the higher velocities must be sustained by more mass than accounted for just by visible matter. https://en.wikipedia.org/wiki/Galaxy_rotation_curve#/media/File:Rotation_curve_of_spiral_galaxy_Messier_33_(Triangulum).png
Societal relevance of work:
The existence of dark matter is now fundamental to the understanding of astrophysics and modern cosmology. Since it's initial discovery by Rubin and Ford, the Planck satellite was able to measure the dark matter content of the universe through the clumping of the cosmic microwave background. In addition, measurements of gravitational lensing by galaxy clusters again confirmed the existence of dark matter. It has been calculated that dark matter accounts for approximately 85% of the matter in the universe.
The true nature of dark matter is still unknown, and remains one of the largest mysteries in modern astronomy. Physicists are searching for dark matter both on earth and in space.
In recognition of her achievements, Rubin was elected to the National Academy of Sciences, and was awarded the National Medal of Science in 1993. Most recently, the Large Synoptic Survey Telescope in Chile was renamed the Vera C. Rubin Observatory in recognition of her contributions to the study of dark matter and her outspoken advocacy for the equal treatment and representation of women in science.