Feb 28 2020

Astronomers Detect Largest Explosion Ever

Published by under Astronomy
Comments: 0

We can quibble about whether or not the Big Bang should be considered an explosion, or whether it happened “in the universe.” It was the expansion of spacetime that is the universe. In any case, astronomers have detected what they think is the biggest explosion (at least discovered so far) since the big bang –  in the Ophiuchus galaxy cluster, 390 million light years from us. The explosion is essentially a bubble with a diameter the size of 15 Milky Way galaxies – about 1.5 million light years across. That’s five time bigger than the previous record holder.

Astronomers first suspected something was going on when they discovered a big X-ray bubble. They report:

It was discovered in the Chandra X-ray image by Werner and collaborators, who considered a possibility of it being a boundary of an AGN-inflated bubble located outside the core, but discounted this possibility because it required much too powerful an AGN outburst.

An AGN is an active galactic nuclei – more on that below. So they initially discounted it because it was too big, but they then followed up with radio observation, and found an identical radio bubble, confirming that this was a real fossil of an ancient explosion, centered around an AGN. So what’s going on here?

Well, astronomers are not sure. The do not know exactly what may have caused some a massively energetic event. But let’s give some background on AGNs – these are supermassive black holes (SMBH) in the centers of galaxies. Most galaxies have them, including our own. But some supermassive black holes are more super massive than others – getting up to billions of solar masses. More importantly to their activity, some of the black holes are feeding, which means that gas and dust are actively swirling around the event horizon forming an accretion disc and then plunging into the incredible gravity well of the black hole. All that gravity represents an unimaginable amount of energy, and when that gas and dust falls in it swirls around at relativistic speeds – near the speed of light.

All this gas and dust also gets really not, millions of Kelvin. The heated gas emits radiation in the radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands. Further still, because it is rotating so fast, any charged particles will generate a massive magnetic field, which then shoots out jets of gamma rays in opposite directions at the poles. So there is a disc of hot material in the plane of the SMBH and jets of gamma rays at the poles. There are various types of AGNs, but astronomers think they are all the same phenomenon as described here, just seen from different angles and under different conditions (whether or not they are shining through a cloud of dust, for example).

AGNs are the most energetic things in the universe. So it makes sense that when we find the most energetic explosion in the universe, an AGN is involved, and in this case it is. What we don’t know is exactly what happened to that SMBH to cause such a bubble of energy. Essentially – stay tuned. Astronomers will be turning larger radio telescope arrays toward Ophiuchus to gather more information.

One side question I had was – what is the implication of all this for life? Did this explosion wipe out all life in the Ophiuchus galaxy cluster? Even a single galaxy being sterilized of all life is a colossal tragedy. This was a slow-motion explosion, happening over hundreds of millions of years, but for each planet affected it would have been an instantaneous death from above as the wave-front of EM radiation hit them. I don’t know if this is the case, and I have not seen any speculation about this, so I’ll have to ask an astronomer at some point. But I am very concerned for any Ophiuchusians.

This lead me to another question – what are the implications for life from AGNs in general? Here I did find a paper directly addressing this issue. Harvard University scientists Manasvi Lingam, Idan Ginsburg, and Shmuel Bialy wrote a paper arguing that the net effect of AGNs is positive for life, which I found initially surprising. The “kill zone” for all that radiation is only about 100 light years. Our Milky Way is about 100,000 light years across, so this is a tiny radius at the center. Also, between about 100-150 light year away, the UV light from the AGN would actually be a benefit to life, providing energy for photosynthesis and perhaps jumpstarting life through increased chemical reactions. Still, this is a tiny radius compared to the galaxy. Most of the galaxy (and I am assuming this means not in the direct line-of-sight of the gamma ray beams shooting from the poles) would be unaffected by the AGN at the center.  So mostly neutral to life, but perhaps a small net positive. 

However, there was one implication of this that intrigued me further – what about rogue planets? A rogue planet is one that has been ejected from its stellar system and now roams interstellar space, without a parent star. Estimates as to how many rogue planets there are vary widely, but up to as many as 100,000 per star in our galaxy (planets down to the size of Pluto). Think about that – there are about 250 billion stars in our galaxy, so that would be 2.5 x 10^16 rogue planets. Is it possible that any of these rogue planets harbor life? How could that work?

Some planets may still be hot from the collisions that formed them and the decay of radioactive materials in their crust. Dig down far enough in the Earth and things get really hot fast. So many of these rogue planets may have enough heat in certain zones to keep water liquid, and chemical processes can support life without light. Some rogues planets may also be gas giants like Jupiter, with large moons. Perhaps such a moon could be warmed by tidal forces or magnetic fields and again have liquid water and chemical processes for life. It’s even possible that there is more life on rogue planets than planets around stars. Dark life may be the rule, and we are the exception.

But the paper on AGNs and life raises another possibility – in that 50 light year or so radius zone, UV light from the AGN may be bright enough to support photosynthesis. Rogue planets in that zone, and there would be a lot of them, could therefore harbor photosynthesizing life. The galaxy center would therefore act like their sun.

The technique of microlensing, based on the general relativity effects of mass on light, has been used to detect exoplanets. So far 14 planets bound to a parent star have been found using microlensing, and 10 rogue planets. This is not enough of a survey to generate any statistics, and we have been mostly looking around other stars. If we do a thorough survey of even a small patch of sky for rogue exoplanets using microlensing, we may get a better estimate of their number.

Finally, I think this is all an untapped resource for science fiction. I am not aware of any science fiction story that takes place on a rogue planet (I’m sure there is some out there, given the amount of literature, but none has come across my radar). If you know of any, let me know in the comments.

The big picture is that the universe keeps getting weirder the more we study it. Our notion of what is “typical” is based on our extremely limited example of one planet in one solar system in one part of one galaxy. But there is a lot of strange and cool stuff out there.

No responses yet