Galaxy collisions are foundational events in the Universe. They happen when two systems mingle stars in a cosmic dance. They also cause spectacular mergers of supermassive black holes. The result is one very changed galaxy and a singular, ultra-massive black hole.<\/p>\n
<\/p>\n These colossal events are a major force in the evolution of galaxies. It\u2019s how smaller galaxies combine to form ever-larger ones. Such mergers have been going on since the earliest epochs of cosmic time. Galaxy mergers continue today. Our Milky Way continues to gobble up smaller ones and it will collide with the Andromeda Galaxy in a few billion years. When that happens, both galaxies\u2019 supermassive black holes could also merge.<\/p>\n We don\u2019t see the whole process from start to finish because it takes millions of years to complete. Yet, that doesn\u2019t stop astronomers from looking for\u2014and finding\u2014evidence of galaxy and supermassive black-hole collisions. The latest discovery used the Hubble Space Telescope (HST) to spot three bright, visible light \u201chot spots\u201d deep inside a pair of colliding galaxies. These targets lie relatively close to us\u2014only about 800 million light-years away. Astronomers followed up with Chandra observations and radio data from the Karl G. Jansky Very Large Array.<\/p>\n Typically, galaxies with bright cores, called \u201cactive galactic nuclei\u201d (AGN for short), exist very far away. They\u2019re often seen earlier in cosmic time. The chance to study a galaxy and a pair of supermassive black holes in a collision in the \u201cmodern\u201d nearby Universe is a good time to study the mechanics of such an event.<\/p>\n The discovery of a future cosmic collision came when HST\u2019s Advanced Camera for Surveys spotted three optical diffraction spikes in the heart of a colliding galaxy called MCG-03-34-64. Two of those \u201chot spots\u201d appear very close together\u2014only about 300 light-years apart. They trace the presence of oxygen gas in the core. It\u2019s being ionized by something very energetic and the hot spots surprised the astronomers. (The third hot spot isn\u2019t well understood.) \u201cWe were not expecting to see something like this,\u201d said Anna Trindade Falc\u00e3o of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts. \u201cThis view is not a common occurrence in the nearby Universe, and told us there\u2019s something else going on inside the galaxy.\u201d<\/p>\n Falc\u00e3o and her colleagues wanted to know what was going on to cause those bright spots. So, they used the Chandra X-ray observatory to focus on the action. \u201cWhen we looked at MCG-03-34-64 in the X-ray band, we saw two separated, powerful sources of high-energy emission coincident with the bright optical points of light seen with Hubble. We put these pieces together and concluded that we were likely looking at two closely spaced supermassive black holes,\u201d said Falc\u00e3o.<\/p>\n The team also found observations of these objects in archival radio telescope data. Those powerful radio emissions proved that the pair of black holes exists and are edging closer together. \u201cWhen you see bright light in optical, X-rays, and radio wavelengths, a lot of things can be ruled out, leaving the conclusion these can only be explained as close black holes,\u201d noted Falc\u00e3o. When you put all the pieces together it gives you the picture of the AGN duo.\u201d<\/p>\n These central supermassive black holes will collide in perhaps a hundred million years. Each is at the core of a single galaxy. As those galaxies draw ever closer together, the black holes in their hearts will start to interact. Eventually, they\u2019ll merge in a powerful event, emitting gravitational waves as part of the process.<\/p>\n Astronomers suggest (via simulations and observations) that mergers of galaxies with supermassive black holes trigger a lot of activity. As the collisions proceed, interstellar gas flows toward the galactic centers. It also gets compressed in other regions and both activities trigger bursts of star formation. Some gas also accretes onto those central supermassive black holes, causing increased emissions as material spirals through the accretion disk.<\/p>\n These mergers happen continually in the Universe. Models of galaxy evolution, coupled with observational evidence suggest that many AGNs at the hearts of galaxies experience mergers. Colliding supermassive black hole pairs within those AGNs also suggest that those black holes grow through mergers.<\/p>\n Understanding the merger of close-together AGNs such as the ones seen in MCG MCG-03-34-64 offers a unique window into the final stages of what astronomers call \u201cSMBH binary coalescence\u201d. Such events are and will continue to be a major way to measure the effects of these mergers. They\u2019ll offer a rich field of study using observatories sensitive to light across the spectrum, as well as future gravitational wave detectors. <\/p>\n Those detections will require advanced versions of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which made its first detections only a few years ago. Supermassive black hole merger-induced gravitational waves will be the target of future instruments such as LISA (short for Laser Interferometer Space Antenna). It will deploy three space-based detectors millions of miles apart to capture the long-wavelength gravitational waves emitted when black hole behemoths like the ones in MCG-03-34-64 collide. Since those mergers occur throughout the Universe, it\u2019ll be a rich field of study that contributes greatly to our understanding of galaxy mergers as part of cosmic evolution.<\/p>\n NASA\u2019s Hubble, Chandra Find Supermassive Black Hole DuoSpotting Incipient Supermassive Black Hole Collisions<\/h3>\n
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The Upcoming Collision<\/h3>\n
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Supermassive Black Hole Collisions and Future Detections<\/h3>\n
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For More Information<\/h4>\n
Resolving a Candidate Dual Active Galactic Nucleus with ~100 pc Separation in MCG-03-34-64<\/p>\nLike this:<\/h3>\n