Texas astronomer watches as black hole eats a star

By McDonald Observatory

October 23, 2002

AUSTIN, Texas — UT-Austin graduate student Feng Ma didn’t expect to see a black hole gobble up a star when he went out to McDonald Observatory to point a telescope at the next quasar on the list of about 60 he’s studying. But that’s what happened, he realized on later review of his observations of a quasar called TEX 1726+344 with the 2.7-meter Harlan J. Smith Telescope.

Quasars are extremely bright pinpoints of light so distant in space and time that it’s thought we’re seeing them near the beginning of the universe. They are very young galaxies, with giant black holes at their cores. As material spirals around a black hole, it heats up before falling in, giving off massive amounts of radiation.

Astronomers study this radiation by passing it through a slit and spreading it into its component wavelengths, just as light is passed through a prism to create a rainbow. They can tease out which elements are present in the jet streaming out of the galaxy’s core by seeing the patterns of so-called “emission lines” in the quasar’s spectrum. Ma has been studying quasars to see how their emission lines may have changed over the last decade.

But in looking at his spectrum of TEX 1726+344, Ma saw a feature that was not in spectra of the quasar made in 1988 and 1990: an “absorption line.” The presence of this line indicates a cloud of material along our line of sight, that is, in between the quasar’s high-energy jet and Earth. This cloud is absorbing certain wavelengths of light coming from the quasar.

The relative positions of the emission lines and the absorption line on the spectrum show that this cloud is being ejected from the black hole at 6,000 kilometers per second, Ma said. “This leads me to think it’s the signature of a star that’s been ripped apart by the black hole’s gravity,” he said. “Half of the star’s matter fell into the black hole, and the other half was ejected in a gravitational sling-shot. This second half is the fast-moving cloud that caused the absorption line.

“If this interpretation is correct, we could see this feature in the spectrum go away in the next few years. I’d like to keep an eye on this quasar to see what happens,” Ma said.
Ma’s research is published in this month’s issue of Monthly Notices of the Royal Astronomical Society.

TEX 1726+344 was discovered as part of the Texas Radio Survey (1974-1983), led by University of Texas astronomer James Douglas and carried out with the now-defunct Texas Interferometer radio telescope. UT-Austin graduate student Elizabeth Bozyan identified TEX 1726+344 as a quasar in her 1985 doctoral dissertation.

Feng Ma can be reached via email at: feng@astro.as.utexas.edu, or by phone at: 512-471-3644.