In 1963, astronomer Maarten Schmidt used the Hale telescope to examine the optical counterpart of the radio source 3C 273. Termed a “quasar”, its red shift of 0.157 indicated that this object must reside a great distance, and with this distance measurement it could be shown that it also must be producing energy at a prodigious rate—with a luminosity 4 trillion times that of the Sun.
Schmidt’s discoveries prepared the way for theories of black holes at the center of galaxies. Through accretion of material from the disks of their host galaxies, black holes have come to be seen as the engines of the new class of objects known as active galactic nuclei (AGN).
MOREEarth orbits a fairly ordinary star, one of roughly 300 billion stars in our cosmic home, the Milky Way Galaxy. The Milky Way is one of roughly 100 billion galaxies in the observable universe. Astronomers believe that all massive galaxies have supermassive black holes at their centers—the Milky Way is no exception—and the black hole at the center of the Milky Way is thought to have a mass of 4 million solar masses. The observations that put us on a path to this understanding were made at Palomar with the Hale Telescope.
In 1963 Caltech astronomers Maarten Schmidt, Jesse Greenstein, and Bev Oke were studying a new class of mysterious objects called quasars. Schmidt measured the first optical spectrum of a quasar known to us as 3C 273 using the prime focus spectrograph on the Hale Telescope. While there were prominent spectral features in the data, they occurred at unexpected wavelengths that did not correspond to known stellar spectral lines. Soon Schmidt realized that the spectral features he had measured were common lines of H, O, and Mg, but were “redshifted” away from their rest wavelengths—indicating a recession speed of nearly 50,000 km/s. This recession speed implied that the source was at great distance (billions of light-years), and the source must be tremendously luminous—trillions of times more luminous than the Sun.
Today we understand quasars like 3C 273 as examples of so-called “active galactic nuclei” that release tremendous energies during the ingestion of matter by supermassive black holes at their centers. Roughly five percent of all galaxies exhibit AGN behavior, and in 1978 Caltech astronomers Wal Sargent and Peter Young using the Hale found dynamical evidence of the very high masses driving these AGN—millions to billions of times the mass of the Sun. Their method measured the range of stellar speeds found in centers of galaxies—so called velocity dispersion. Applying this same method to “normal” galaxies like our own they too showed evidence for massive objects at their centers. Essentially all galaxies have these central black holes, and a census of galaxy properties indicate that central mass is correlated with both velocity dispersion and brightness. Central black holes appear to be important in galaxy formation and evolution.
The center of our galaxy is close enough that we can observe the orbits of individual stars, and the inferred orbits indicate that the central Milky Way black hole has a mass of 4 million solar masses. While impressive, the Milky Way’s central black hole is modest in size compared with some—for example the black hole recently imaged in the core of the giant elliptical galaxy Messier 87 is thought to be roughly a thousand times more massive—approximately 5 billion solar masses. It is clear that the pervasive presence of these black holes is an important part of structure formation in our universe.