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HubbleSite Astronomers Unveil Growing Black Holes in Colliding Galaxies

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Space Telescope Science



Some of the Hubble Space Telescope's most stunning images reveal galaxies in distress. Many of them are in the throes of a gravitational encounter with another galaxy. The photos show perfect pinwheel patterns stretched and pulled into irregular shapes. Streamers of gas and dust flow from galaxies into space. And in this chaos, batches of young, blue stars glow like tree lights, fueled by the dust and gas kicked up by the galactic encounter. For some galaxies, the powerful meeting with a passing galaxy will eventually end in mergers.

But hidden from view deep inside the dusty cores of these merging galaxies is the slow dance of their supermassive black holes toward an eventual union. Visible light cannot penetrate these shrouded central regions. X-ray data, however, have detected the black-hole courtship. And now astronomers analyzing near-infrared images from the sharp-eyed Hubble Space Telescope and the W. M. Keck Observatory in Hawaii are offering the best view yet of close pairs of black holes as they move slowly toward each other.

The study is the largest survey of the cores of nearby galaxies in near-infrared light. The Hubble observations represent over 20 years' worth of snapshots from its vast archive. The survey targeted galaxies residing an average distance of 330 million light-years from Earth.

The census helps astronomers confirm computer simulations showing that black holes grow faster during the last 10 million to 20 million years of the galactic merger. The Hubble and Keck Observatory images captured close-up views of this final stage, when the bulked-up black holes are only about 3,000 light-years apart — a near-embrace in cosmic terms. The study shows that galaxy encounters are important for astronomers' understanding of how black holes became so monstrously big.

These monster black holes also unleash powerful energy in the form of gravitational waves, the kind of ripples in space-time that were just recently detected by ground-breaking experiments. The images also provide a close-up preview of a phenomenon that must have been more common in the early universe, when galaxy mergers were more frequent.

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