* Astronomy

Astronomers have spotted two colossal black holes that appear to be orbiting each other 100 times closer than any previously seen. But they are still too far apart to produce large ripples in the fabric of space-time – suggesting some black holes may "stall" before spiralling towards each other and merging.

When two galaxies collide, the black holes at their cores are thought to fall towards the centre of the resulting larger galaxy. These black holes then go into orbit around each other and should eventually merge.

"But no one has ever found a really clear example of a binary black hole before" - David Merritt, astrophysicist at the Rochester Institute of Technology in New York, US.

A "wide" binary system was found with black holes within 3000 light years of each other in 2002, and earlier in April, another was announced at a separation of 28,000 light years.

Now, astronomers led by Cristina Rodriguez at the University of New Mexico in Albuquerque, US, have found what appear to be two black holes within 24 light years of each other. The pair was identified using the Very Long Baseline Array, a system of 10 radio dishes scattered from Hawaii to the Caribbean Sea. The VLBA boasts the sharpest resolution of any telescope in existence.
The objects are buried within a blob-shaped galaxy called 0402+379, which lies about 470 million light years from Earth. Both appear to be "active" black holes that are devouring their surroundings.

The team says spectral observations taken with the Hobby-Eberly Telescope in Texas confirm the objects are indeed orbiting each other and have a combined mass of 150 million Suns.
But Merritt, who is not on the team, cautions that one black hole may actually lie farther from the other than expected. That is because the spectrum attributed to the orbital movement of the second black hole might simply be due to gas sloshing around the first.

"I think it's likely they are actually close, but it's difficult to make the case airtight" - David Merritt.

If the black holes are indeed just 24 light years away from each other, they could help researchers understand the black hole merger process. At extremely close distances – perhaps 1000 times closer than this pair – black holes are thought to be inexorably drawn together as they lose angular momentum by radiating gravitational waves that ripple space-time.
But in order to get to such tight orbits, the behemoths must lose energy some other way. Researchers think they may get rid of angular momentum by flinging nearby stars and gas outwards at high speed.

"If that happens enough, you could shrink their orbit. But there are big uncertainties – how many stars are there that find their way close to the binary system, and how much gas is down there?"- David Merritt

"They have to lose angular momentum, but we don't know if that's an easy thing to do or a hard thing to do. Do they just get stalled – or do they never get together?" - Gregory Taylor, Team member at the University of New Mexico.

This pair certainly appears to have become stalled. That is because the black holes are still too far apart to begin emitting gravitational waves – even though their host galaxies probably merged millions or billions of years ago.

"It wouldn't prove all binaries stall like this, but it shows there's one case where it seems to be happening" - Gregory Taylor.

Source New Scientist

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Breaking All the Rules: The Compact Symmetric Object 0402+379

H. L. Maness (Grinnell College), G. B. Taylor, R. T. Zavala (NRAO, Socorro), A. B. Peck (CfA, SMA Project), L. K. Pollack (UC, Berkeley)

We present results of multi-frequency VLBA observations of the compact symmetric object (CSO) 0402+379. The parsec-scale morphology of 0402+379 allows us to confirm it as a Compact Symmetric Object (CSO), while VLA data clearly show the presence of kiloparsec-scale structure. Thus, 0402+379 is only the second known CSO to possess large scale structure. Another puzzling morphological characteristic found from our observations is the presence of two central, compact, flat-spectrum components, which we identify as possible active nuclei. We also present the discovery of neutral hydrogen absorption along the southern hotspot of 0402+379 with a central velocity ~ 1000 km s-1 greater than the systemic velocity. Multi-epoch observations from the VLA archive, the Caltech-Jodrell Bank Survey, and the VLBA Calibrator Survey allow us to further analyse these anomalous features. Results of this analysis reveal significant motion in the northern hotspot, as well as appreciable variability in both of the core candidates. We consider the possibility that 0402+379 was formed during a recent merger. In this case, the two candidate cores could be interpreted as binary supermassive black holes that have not yet coalesced, whereas the large-scale radio emission could be attributed to interactions directly linked to the merger or to previous activity associated with one of the cores.

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