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Black Holes' Feeding Frenzy

by Hai Fu, IfA Graduate Student

black hole diagram

Top: A gas-rich galaxy collides with a giant galaxy, producing a quasar. Credit: Computer simulation by Joshua Barnes, University of Hawaii. Bottom: Artist's conception of the heart of a quasar, a massive black hole that sucks in a vortex of gas. Hawaii astronomers found that quasars shine because a giant galaxy with a large black hole collides with a gas-rich galaxy that feeds the black hole. Artwork modified with permission from A. Simonnet, Sonoma State University, NASA Education and Public Outreach.

What makes quasars shine? My faculty advisor, Alan Stockton, and I believe we have identified what makes at least some of them shine: the black hole at the center of a massive galaxy with little gas of its own is gobbling up material from a smaller gas-rich galaxy with which it is colliding.

A "quasar" (originally a contraction of "quasi- stellar radio source") looks like a star through an optical telescope, but its large redshift means that it must be very distant and among the most powerful objects in the Universe. Quasars lie at the center of distant giant galaxies, and each consists of a massive black hole surrounded by a vortex of gas. Before the gas falls into the black hole, it spins faster and faster, and its temperature rises until it is hot enough to radiate up to a trillion times the power from the Sun. The question that we tried to answer is, "Where is the gas coming from?"

The merging of two galaxies has long been thought to be an efficient way of driving gas deeply into a galaxy to feed the central black hole, but there was only indirect evidence for such a mechanism until now. Our results provide the first direct observational evidence that the gas from a merger can indeed be driven down to the immediate vicinity of a black hole at the center of a galaxy.

To answer the question, "Where is the gas coming from?" we used ultraviolet spectra of 12 quasars taken by the Faint Object Spectrograph on the Hubble Space Telescope to find out what the gas is made of, as well as images taken on the UH and Canada-France-Hawaii telescopes on Mauna Kea. We found that the gas spiraling into the black hole is almost pure hydrogen and helium, whereas the stars and other material in the surrounding giant galaxy are heavily contaminated by heavier elements such as carbon and oxygen.

This difference implies that the infalling gas has recently come from outside the galaxy, most likely from another galaxy that is merging with the giant one. We also see a chaotic distribution of fast-moving patches of relatively pure hydrogen and helium scattered around the quasar, implying that black holes not only swallow things, but can also expel a large portion of their meal out to thousands of light-years away, likely through an energetic blast that happened millions of years ago.