How did the odd black holes detected by LIGO form – and can we spot them in the sky?

How did the odd black holes detected by LIGO form – and can we spot them in the sky?
A needle in a haystack? Pan Starrs telescope is scanning billions of galaxies to find the black holes emitting gravitational waves. CC BY-SA

Great scientific discoveries often raise more questions than they answer. Just days after the announcement that gravitational waves from two merging black holes have been detected, astrophysicists are already pondering what this means for our understanding of stars. New studies are already being released and we can expect a flood of creative ideas in the near future.

One of the most surprising things about the discovery is the huge size of the involved which is challenging our understanding of how they form. So how can we find out more? One way is by pinpointing the black holes on the sky so we can try to study them using regular telescopes.

Massive mystery

LIGO, the observatory that detected the , is a so-called laser interferometer. It estimated that the two merging black holes would have masses of about 36 and 29 times that of the sun respectively (described as 36 and 29 "solar masses"), calculated from the frequency of the gravitational waves. But what's so unusual about these masses?

Black holes form after huge supernovae explosions, which can only be produced by massive stars. The masses of the black holes in our own galaxy can be measured by looking at the speed of stars orbiting a black hole. The most in a binary system (a black hole and a companion star orbiting a common centre) in our galaxy is about 10-20 .

How did the odd black holes detected by LIGO form – and can we spot them in the sky?
Multi-wavelength compilation image of Kepler’s supernova remnant, SN 1604. Credit: NASA/wikimedia

This is well explained by our knowledge of stars. The biggest stars are born at about 100 solar masses and end up at around only ten solar masses at their endpoints due to stellar winds blowing out material into space. This means they shouldn't be able to produce the kind of huge black holes that LIGO detected. But there are still big uncertainties about the rate at which this occurs and the influence of a star's spin, the existence of a second star orbiting a common centre (binary stars), and its chemical composition.

So how could the black holes detected by LIGO be so massive? Research has already come out that suggests we can explain that by assuming they come from two collapsing . But the stars that formed them must have had a very different chemical composition to the stars in our own Milky Way, which has a high content of heavy chemical elements like oxygen, sodium, magnesium, silicon, sulphur, iron.

In fact, a paper from the LIGO team and one from two experts on binary stars proposed that they needed to be in small galaxies with very low metal content (we astronomers label all elements heavier than boron a "metal"). That's because, according to atomic physics, low-metal stars lose less mass during their life. So they end up with higher mass than other stars at the end, and form larger black holes.

Sky scanning

The LIGO team could give a rough direction of where on the sky the merger took place, due to the difference in detection time between its two experiments in different parts of the US (0.07 seconds). However this can only be located to about 500 square degrees (an area of 2,000 full moons). Astronomers tried to pinpoint the source by pointing optical, infrared, X-ray and radio telescopes in this area.

However, it wasn't easy. Black hole mergers are not predicted to produce significant electromagnetic radiation such as visible light or X-rays. But there was an intriguing detection of gamma-rays (which are high-energy electromagnetic waves) by the Fermi satellite that lasted just a second and appeared 0.4 seconds after the LIGO signal. However, it is not certain that the two are related, as Fermi can't tell where these gamma rays came from in the sky. The next step is to look for more high-energy emissions coincident in time with future gravitational wave signals to see whether there's a link. There are indeed theories that suggest that two merging holes can produce a gamma-ray burst if they form and merge in a certain way, meaning it is important to keep looking.

We recently scanned the area with the Pan-STARRS telescope and found 56 sources of optical light emissions, but we weren't able to link any of them to the LIGO event. This is not too surprising, it's tough to cover such a large area – including billions of galaxies – fast, deep and with broad wavelength coverage. There are some clever ideas to pick the biggest galaxies using catalogues and focus on observing those. However if these massive black holes have to come from metal-depleted stars, their host galaxies will be small because of where low-metal are found. These galaxies are much more numerous than large ones, and there are too many of them to use this method. Also, many of them are too faint to have been catalogued before.

It will definitely not be an easy observation to make, but what is certain is that we will be looking harder than ever. The world's most powerful telescopes on the ground and in space are all joining the hunt. The journey is just starting and the LIGO discovery is truly inspiring.


Explore further

When black holes meet—inside the cataclysms that cause gravitational waves

This article was originally published on The Conversation. Read the original article.
The Conversation

Citation: How did the odd black holes detected by LIGO form – and can we spot them in the sky? (2016, February 17) retrieved 20 August 2019 from https://phys.org/news/2016-02-odd-black-holes-ligo-sky.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
34 shares

Feedback to editors

User comments

Feb 17, 2016
"Great scientific discoveries often raise more questions than they answer. Just days after the announcement that gravitational waves from two merging black holes have been detected, astrophysicists are already pondering what this means for our understanding of stars."

It is no great discovery; it is as much of an illusion as the "bending" of space-time, which is in turn as illusory and abstraction, that space and time are endowed with tangible physical attributes.

When you start with one big mystery, then it is turtles all the way and you never escape from mystery to knowledge! Please see the comments by "futurehuman" in the Guardian link: https://www.thegu...comments


Feb 17, 2016
A typo: The delay between detectors was 0.007 s, it must be (and was) smaller than the time for a signal at the universal speed limit to pass in between (~10 ms). That is, the signal was actually passing trough the Earth, instead of coming in parallel with the Earth surface at the detectors. (A small likelihood event.)

@Bigbangcon: You are entitled to your own opinion, but not your own facts. We have known for more than a century that spacetime is flexible (as predicted by GR). If it wasn't, you would have written a paper with your contrary evidence and published it.

The last part of your comment is just one big denial of science (which starts with what works, not 'mysteries not in evidence), so I guess why we now know you are spouting pure and toxic bullshit onto innocent pages. Do you know that there are schools where you can actually learn stuff, amazing advances of the last few millenniums!?

Feb 17, 2016
@bschott: Actually you have to be the reverse, and just observe how science works. We have known for 400 years that advances leads to new questions (so new advances).

I'll ask you too: Do you know that there are schools where you can actually learn stuff, amazing advances of the last few millenniums!?

Feb 17, 2016
@ bschott:

"I expect to hear from them any time now."

How dare you defy their "science by proclamation"! Even Albert Einstein did not have much luck!: "Even Einstein doubted his gravitational waves
Albert Einstein's 1936 paper denouncing gravitational waves was rejected by the journal that just published proof of their existence" : Astronomy Magazine http://www.astron...al-waves

More to follow:


Feb 17, 2016
Contd. from above

And doubt expressed by none other than Einstein himself in a letter to his friend Besso: "I consider it quite possible that physics cannot be based on the field concept, i.e., continuous structure. In that case, nothing remains of my entire castle in the air, gravitation theory included, (and of) the rest of modern physics" A. Pais, Subtle is the Lord …" The Science and the Life of Albert Einstein", Oxford University Press, (1982) 467,

Feb 17, 2016
Wonder if LIGO would detect our universe colliding with another?

Feb 17, 2016
Phys1.....you are feeding the trolls.......they are better on your ignore list.....

Feb 17, 2016
Phys1.....you are feeding the trolls.......they are better on your ignore list.....

Only one troll ...
You heard it crank, guidelines oblige me to put you back on ignore. Sayonada.

The moron says. SAYONADA

Sayonara.

Feb 26, 2016
Phys1.....you are feeding the trolls.......they are better on your ignore list.....

Only one troll ...
You heard it crank, guidelines oblige me to put you back on ignore. Sayonada.

The moron says. SAYONADA

Sayonara.

He was just going for the Japanese accent....

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more