One Size Doesn't Fit All

photo: Black Holes Shed Light on Galaxy Formation

June 5, 2000

This comparison of the hearts of four elliptical galaxies shows that the more massive a galaxy's central bulge of stars, the heftier its black hole. The galaxies are part of a census of 30 galaxies conducted by astronomers using NASA's Hubble Space Telescope. Black holes are dense, compact objects possessing such strong gravitational forces that not even light can escape them.

The column of black-and-white pictures at left, taken by ground-based telescopes, shows the galaxies. The inset boxes define the central regions of stars. Close-up images of these regions, as seen by Hubble's Wide Field and Planetary Camera 2, are in the middle column. The column at right lists the masses of the black holes and illustrates the respective diameters of the event horizons. An event horizon defines a black hole's boundary. Any material that crosses that boundary becomes ensnared in a black hole's grasp and cannot escape. The event horizons cannot be seen in the Hubble images because they are 25 million times smaller than the scale of the pictures.

Astronomers determined the mass of each black hole by measuring the motion of stars swirling around it: the closer the stars approach the black hole, the faster their velocity. Only through observations with Hubble's superior vision could astronomers probe to the core of the galaxy where these effects are easily measured. They discovered a remarkable new correlation between a black hole's mass and the average speed of the stars in a galaxy's central bulge. The faster the stars are moving, the more massive the black hole. This information suggests that the galaxy and the black hole grew simultaneously.Astronomers are concluding that monstrous black holes weren't simply born big but instead grew on a measured diet of gas and stars controlled by their host galaxies in the early formative years of the universe. These results, gleaned from a NASA Hubble Space Telescope census of more than 30 galaxies, are painting a broad picture of a galaxy's evolution and its long and intimate relationship with its central giant black hole. Though much more analysis remains, an initial look at Hubble evidence favors the idea that titanic black holes did not precede a galaxy's birth but instead co-evolved with the galaxy by trapping a surprisingly exact percentage of the mass of the central hub of stars and gas in a galaxy.


Black holes cannot be seen, so how did the Hubble telescope obtain these results?

Hubble's "black hole hunter," the Space Telescope Imaging Spectrograph (STIS), precisely measures the speed of gas and stars around a black hole. This measurement provides clues for the existence of a black hole. In this black hole census, astronomers determined the mass of each black hole by measuring the motion of stars swirling around it: the closer the stars approach the black hole, the faster their velocity. Only through observations with Hubble's superior vision could astronomers probe to the core of the galaxy where these effects are easily measured. They discovered a remarkable new correlation between a black hole's mass and the average speed of the stars in the galaxy's central bulge. The faster the stars are moving, the more massive the black hole. This information suggests that the galaxy and the black hole grew simultaneously.


What do these results mean to astronomers?

The bottom line is that the final mass of a black hole is not primordial; it is determined during the galaxy formation process. Black holes in small galaxies went relatively undernourished, weighing in at a mere few million solar masses. But black holes in the centers of giant galaxies, some tipping the scale at over one billion solar masses, were so engorged with infalling gas that they once blazed as quasars, the brightest objects in the cosmos.

The results also explain why galaxies with small central bulges of stars, like our Milky Way, have diminutive black holes of a few million solar masses, while giant elliptical galaxies house billion-solar-mass black holes, some still smoldering from their days as quasars.