Black Holes Shed Light on Galaxy Formation

1. 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.

2. 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.