To know how galaxies rotate

Rotation is an inherent property of all celestial bodies — small and large. No wonder, newer and newer techniques were applied to extract this information quantitatively. As expected, each new entry into the list unfolded the mysteries of stars, planets and all other bodies. Our own galaxy, the Milky Way, is no exception. Its rotation attracted the attention of Vera Rubin (1928 – 2016), an American astronomer, almost 50 years ago. Does it rotate like a ceiling fan? Or resemble the planets revolving around the Sun?

The rotation needs to be inferred by resolving the direction of motion of individual stars as seen from Earth. The stars have a specific direction as measured from Earth; this is a vector sum of the velocities in the radial and transverse directions. This exercise was the basis to establish the revolution of the Sun around the centre of the galaxy. Very soon, the revolution of other stars could also be traced. A perfect case for application of the universal law of gravitation, people thought, and derived the mass of the galaxy.

We assume that stars and interstellar matter (represented by the ionised hydrogen clouds called H II regions) orbit the centre of a galaxy obeying the law of gravitation. The H II regions offered a better means to measure; their motion could be read out by the Doppler shifts of the hydrogen emission lines. It appeared that the stars and the interstellar medium (ISM) followed the same revolutionary motion.

Varied velocities
However, there were many discrepancies that were believed to have originated by limitations on observational techniques. A glance at any picture of a galaxy immediately brings to our mind that the central region is dense and most of the matter is concentrated at the centre. We expect to see that as a consequence of the revolution, the velocities of the individual components should decrease gradually towards the outer regions.

Vera studied the variation of the velocities of matter with distance from the centres of several galaxies. Her measurements were of fundamental importance and served as empirical evidence. Assuming that we are seeing all the matter in the galaxy, we cannot account for the centripetal acceleration observed. She found that the velocities of the clouds did not decrease with increasing distance from the galactic centre, and in some cases even increased a little.

This is in striking contrast to the decrease in velocity with radius predicted by Keplerian motion, which would occur if all the mass of the galaxy were concentrated in the centre of the galaxy. In this case, one has the result that the velocity decreases inversely as the square root of the distance from centre. In a paper, she wrote, “Most galaxies exhibit rising rotational velocities at the last measured velocity; only for the largest galaxies are the rotation curves flat. Thus, the smallest Sc’s (that is, the lowest luminosity) exhibit the same lack of a Keplerian velocity decrease at large R as do the large luminosity spirals. This form of rotation curve implies that the mass is not centrally condensed but that significant mass is located at large R. The mass is not converging to a limiting mass at the edge of the optical image. The conclusion is inescapable that non-luminous matter exists beyond the optical galaxy.”

The idea of ‘dark matter’ was thus expressed in very clear terms more than three decades ago by Vera. This is the first ever conclusive evidence pointing to the presence of dark matter in galaxies. That laid to rest all the previous hypotheses based on theoretical considerations. Her first observations were reported in 1970 based on the Andromeda galaxy. During the next 15 years, Vera and her collaborators proved that it is a general feature of all galaxies, after studying 60 spiral galaxies.

Clear observational evidence
Thus, astrophysicists have now agreed upon the fact that there is matter outside the bulge which has not been observed but exerts a gravitational force and ‘raises’ the velocities of the clouds above that expected from the known distribution of visible matter. There were many counter arguments. But the observations made were unambiguous; the luminous stars were representing only a small fraction of much larger mass they were made out of. The shapes of the galaxies had to be revised from a central bulge with spiral arms to a massive halo of ‘dark matter’ enveloping the entire galaxy.

Vera’s work on dark matter helped many to pursue research both in observations and theory. During her career, Vera discovered a bizarre galaxy, NGC 4550, where half the stars were rotating one way and the other in the opposite direction — a clear indication of merger of galaxies. Vera’s work was the first clear, observational evidence for dark matter as an important and general phenomenon in galaxies other than our own, and brought the subject of dark matter to the forefront of astrophysical research. She wrote, “We have peered into a new world; and have seen that it is more mysterious and more complex than we had imagined. Still more mysteries remain hidden. Their discovery awaits the adventurous scientists of the future.”

(The writer is director, Jawaharlal Nehru Planetarium, Bengaluru)