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Adaptive optics reveals galaxy collisions

27 Jun 2007

Two black holes have been pinpointed at the center of a collision of two galaxies thanks to adaptive optics at the Keck Observatory.

Astronomers in California used adaptive optics (AO) to pinpoint the exact location of a pair of supermassive black holes at the center of a collision of two galaxies. The study of the galaxy merger, known as NGC 6240, could help scientists understand more about the evolution and properties of the galaxy at large.

“People had observed this pair of colliding galaxies at different wavelengths and seen what they thought were black holes, but it’s been very hard to make sense of how the observations at various wavelengths correspond to each other,” Claire Max, an astronomer at Lawrence Livermore National Laboratory, told optics.org. “The AO results enabled us to tie it all together, so now we can see the hot dust in the infrared, the stars in the visible and infrared, and the X-rays and radio emissions coming from right around the black holes.”

Previous observations made at NASA’s Chandra X-Ray Observatory and by the Hubble Space Telescope show that there were two black holes in the core of NGC 6240. However, in the infrared light used in the Keck AO observations, the black holes are more distinct.

The telescope detects near infrared light at wavelengths of 2.12 and 3.8 microns respectively with a spatial resolution of 0.06 arc sec. “We are not ‘seeing’ the black hole directly,” commented Max. “Rather, we are imaging infrared light that is emitted by hot dust surrounding each black hole.”

“Because of this spatial resolution, we were able to see much more detail within the complex nuclear regions of NGC 6240,” continued Max. “This additional resolution was necessary in order to figure out which of the many features were the black holes themselves.”

The next step for the astronomers is to analyze the spectra in the regions close to the two black holes. “Our goal is to map out the velocity field of this galaxy merger to determine the kinematics of the collision process between the two galaxies,” concluded Max. “From the spatial distribution of velocities we can measure the mass of each black hole.”

Collisions between galaxies are believed to be a major determinant of galaxy evolution. “When two galaxies collide, their central black holes merge as the galaxies themselves merge,” explained Max. “It is this ‘co-evolution’ of galaxies and their central black holes that is responsible for the observed correlations between central black hole mass and properties of the galaxy at large. By studying nearby colliding galaxies we have a close-in laboratory to understand the merger process in detail.”

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