Tracking binary black holes

Aug 03, 2011 By Lori Ann White
Chandra X-ray image of the of the galaxy SDSS J171544.05+600835.7, showing plotted X-ray events (left), a digitally-smoothed version of those same events (center), and a model revealing them to be the result of two X-ray sources (right). (Image courtesy Brian Gerke and Greg Madejski.)

Two scientists from the Kavli Institute for Particle Astrophysics and Cosmology have been testing a method to look past the intense radiation pouring out of merging galaxy pairs to see the supermassive black holes at their cores. The researchers want to track how black holes located in the centers of merging galaxies spiral in toward each other until they also join, forming a single even-more-massive black hole. This can give astronomers valuable information about galactic evolution, and about how some black holes – and some galaxies – grew so large so early in the history of the universe. Their research will appear in The Astrophysical Journal Letters.

"The whole issue of two lighter black holes combining into a more has been hypothesized for some time," said Brian Gerke, formerly of KIPAC and now a scientist with the Environmental Energy Technology Division of Lawrence Berkeley National Lab. Confirming – or disproving – that hypothesis, not to mention putting some numbers on how often it happens if it does, is important for determining the way galaxies develop.

"Early on in the universe," added Greg Madejski, Gerke's KIPAC colleague, "we already see very massive black holes. Could that have happened though slow and steady growth? Probably not. Coalescence –" in other words, by merger " – that's how astronomers think it must have happened."

Actually finding instances of such mergers has been difficult due to the nature of the galaxies involved. The black hole pairs Gerke and Madejski look for are at the heart of active galactic nuclei (AGNs), which are responsible for some of the most energetic phenomena in the universe. The energy comes from gas and dust swirling into the black hole at the heart of an AGN; as the matter is sucked in, it's heated enough to give off electromagnetic radiation from all across the spectrum – from warm infrared radiation to highly-energetic X-rays.

If the black holes are still relatively far apart, they can be distinguished by direct observation. But if the merger happened far enough in the past that the resulting galaxy appears – on its face –relatively placid, then the black holes are too close to tell apart using conventional observation methods.

The team, including frequent collaborator Julie Comerford from the University of Texas at Austin and her colleague Dave Pooley along with Gerke and Madejski used a new, efficient three-step method first proposed by Gerke in a 2009 paper to see past the glare of an active nucleus.

Step one: Comb through data from galactic surveys such as the Sloan Digital Sky Survey, looking for certain spectral properties that could indicate dual AGNs.

The team found a galaxy with the catchy name of SDSS J171544.05+600835.7 that outwardly showed little evidence of being the result of a merger, but its spectrum showed some of the right characteristics. A promising test case, but more checking was necessary because similar characteristics can also be caused by jets of gas shooting out in opposite directions from the neighborhood of a single supermassive black hole.

Step two: Take more detailed spectra of candidate galaxies with ground-based telescopes. In this instance, the team started at Lick Observatory on nearby Mount Hamilton and worked their way up to the Keck Observatory on Mauna Kea.

The team determined that the interesting part of the galaxy's spectrum came from two different sources about 1.9 kiloparsecs apart.

Step three: use this evidence to get valuable observing time on NASA's Chandra X-ray Observatory to see if there are two X-ray sources in the same locations as the sources of the emission lines.

In this case, Chandra did indeed see two X-ray sources but, said Gerke, "We don't have quite enough data to say definitely that they're two AGNs." Gerke doesn't sound disappointed. The team has other candidates they're checking, plus additional observations of this object which can settle the matter. "We're still in the stage of 'Just find them. Find as many dual AGNs as you can," Gerke continued.

Madejski looks forward to the day astronomers have identified a substantial population of merging or newly merged AGNs with dual . "That could reveal the unknown signs of coalescing black holes."

Roger Blandford, director of KIPAC, is interested in one theorized sign of coalescing – a burst of gravitational waves, strong enough to be detected by future space-based instruments. "Understanding how common black hole mergers are will help with the design of space observatories to detect them."

The team's method should help speed that day's arrival – especially the use of multiple small, ground-based observatories to pin down good candidates. "Observations with smaller scopes, such as the one we used at Lick, are not expensive," Madejski explained; in other words, it's far easier to get time on them than on one of NASA's Great Observatories. But the results they delivered were good enough to get the team time on one Great Observatory– Chandra.

In fact, said Madejski, "We just learned we got more Chandra observations approved for the next cycle – good news!"

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Bonkers
1 / 5 (1) Aug 03, 2011
Surely there is some new Physics available from a consideration of merging black holes. For instance - if they are point-like masses then they would emit G-waves right up to radio frequencies, never mind audio. As they decay into coalescence. This signal should be detectable, it hasn't been detected, why not? Well, how long are the two in this very close orbit? its governed by the energy density of the G-waves they emit, and possibly some tidal forces on surrounding matter - well the energy considerations are so bounded, the loss of angular momentum is another puzzle, if a single coalesced point is reached then where did it go? It looks to me that we cannot support a point-like black hole, it has to have width. In this case at some orbit distance, even if its at the Planck scale, (i.e. a limit on energy density of space) the two collide, and the high-frequency G-waves are stopped. Also, the angular momentum can be conserved.
Bonkers
2 / 5 (2) Aug 03, 2011
We can't see beyond the event horizon, so do we care, can we tell if its a dimensionless point of mass or a small lump? I think so - the merger would be very very slow for points.
Isaacsname
not rated yet Aug 03, 2011
Wouldn't the process be determined by their orientation to each other ? This is quite the brain-bender to think about, but one question I have is how this happens . Do they align like kissing fish, or is the difference in size disgregarded at the start of the process due to random alignments ?
jsdarkdestruction
not rated yet Aug 04, 2011
Hopefully their research goes well, the process of 2 black holes merging is pretty fascinating if you ask me. Cant wait to learn more on it. It is quite the brain bender.
antialias_physorg
3.7 / 5 (3) Aug 04, 2011
For instance - if they are point-like masses then they would emit G-waves right up to radio frequencies, never mind audio.

You do realize that gravity waves, radio waves and audio waves have nothing in common except coincidentally including the word 'wave'? One is the fluctuation of gravity, the next is the change of electromagnetic field strengths and the last is a change in particle density.

This signal should be detectable, it hasn't been detected, why not?

Because we're currently not much in the business of directly detecting gravity waves. There are a number of setups but they can only detect very strong events. Since these black hole mergers are (luckily) very far away their effect on us is, so far, below the detection threshold of our instruments.

The final black hole merger event will probably only be 'visible' as a gravity wave (since everything will happen within the respective event horizons)
eachus
not rated yet Aug 04, 2011
You do realize that gravity waves, radio waves and audio waves have nothing in common except coincidentally including the word 'wave'?


He didn't use the word wave. He spoke about radio frequencies, which are above audio frequencies, but below microwave frequencies.

Because we're currently not much in the business of directly detecting gravity waves. There are a number of setups but they can only detect very strong events. Since these black hole mergers are (luckily) very far away their effect on us is, so far, below the detection threshold of our instruments.


The current gravity wave detectors should be able to detect the merger of supermassive black holes out to several hundred Mps. We haven't seen such an event yet, nor have we seen a merger of smaller black holes in this galaxy, or neutron star mergers, or...

A number of scientists now believe that we are pushing the limits of theory. If we don't see a gravity wave event soon, theory may have to change.
lengould100
not rated yet Aug 04, 2011
A number of scientists now believe that we are pushing the limits of theory. If we don't see a gravity wave event soon, theory may have to change.
I like that. I've never been happy with present concepts of propagation of gravity. Perhaps time to get out of the thinking box.
antialias_physorg
3 / 5 (2) Aug 04, 2011
We haven't seen such an event yet, nor have we seen a merger of smaller black holes in this galaxy, or neutron star mergers, or...

How often do you expect such events to happen? If it's on the order of supernovae then we should get one event every 20 years per galaxy.
Now, I'm no expert but the merger of two large black holes - which are by all accounts less numerous than stars by a factor of several million to one - should therefore be a MUCH rarer event in our galactic neighborhood than a supernova. It's already rare to find two black holes close together. Having them merge while we have looked for this event - what 5 years? - that would be the lottery ticket of the millennium.

Let's keep looking for a few thousand years more and THEN we can reasonably say that gravity waves don't occur (alternatively we need a LOT better detectors to have a go at smaller gravity wave creating events.)
Waterdog
not rated yet Aug 04, 2011
Okay. Rotating black holes are causing frame dragging at near light speed. Both dark matter and regular matter are being caught in this and accelerated. Since dark matter does not by definition interact with electromagnetic forces, the polar jets are composed of normal matter caught up in the rotating charge generated electromagnetic cyclone.

Using this information and estimates of the amount of material being ejected by the jets along with estimates of the total mass being swallowed by the black hole we should be able to come up with an estimate of the ratio of dark matter to normal matter in the vicinity of an active black hole.

Is there a flaw in my reasoning? Can we measure the necessary parameters accurately enough? What are the effects of accelerating dark matter to near light speeds, both relativistic effects and effects on local space. By definition as you approach light speed the mass approaches infinity. Food for thought
antialias_physorg
3 / 5 (2) Aug 04, 2011
Is there a flaw in my reasoning?

Yes. It's circular (you are assuming a certain amount of dark matter near a black hole in order to estimate that same amount)
Waterdog
not rated yet Aug 04, 2011
No I am talking about the amount that crosses the Schwarzschild radius vs the amount caught in the frame dragging of space around the black hole.
Bonkers
1 / 5 (1) Aug 17, 2011
probably silly to come back to this now, you've all run away.
My point was that 2 point masses will be in orbit for a long time, governed by their energy loss - which is G-wave, tidal and frame-drag. These all seem small, the latter may be zero. Therefore this coupled state should last literally ages - increasing teh likelihood of detection.
On another point, I cannot see 2 point masses ever coalescing, but there again black holes do grow by accretion.

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