Habitable Galaxies - Part 2: Active galaxies

This is part two in a series of posts about habitable galaxies. Post 1, covering types of galaxies and galaxy mergers, is here and this earlier post on the (most) habitable areas of our own galaxy is also relevant.

A major attribute of galaxies is whether or not they are active. There are a few different things active can mean—actively star-forming, for example—but what I want to focus on today is whether they have active nuclei.

What's in a nucleus?

At the centre of our galaxy and most other large galaxies, there is supermassive black hole. I have briefly mentioned black holes in the past and I will eventually get around to writing a dedicated post on them. Honest. What you need to know to understand their role in galactic nuclei is as follows:

• They are very small and very dense.
• The supermassive part means that they range from around a hundred thousand times to the mass of the sun to billions of solar masses. The Milky Way's central black hole was calculated in 2008 (by this group) to be about 4.1 million times the mass of the sun. In kilograms that's about 8 x 10³⁶ or an 8 followed by thirty-six zeroes.
• As their name suggests, supermassive black holes are very massive. What massive really means (in any physics context, not just with regards to black holes) is that they exert a strong gravitational force.

Before you ask, we don't really know where these come from—there are theories, but no single one is yet the most accepted—but we do know that they must form early on in a galaxy's life (possibly even before the stars form, depending on which theory you subscribe to) and their evolution is closely tied with the host galaxy's.

Other things that can be found in the centres of galaxies include stars, dust and gas. Although following the orbits of stars in the centre of our own galaxy is what convinced us there was a supermassive black hole there (nothing else could be so massive and so small), most of what those stars do is simply orbit. (Yes, it is possible for one to fall into the black hole and yes, that would be very interesting and would generate a lot of energy but from what we've observed, the Milky Way's central stars seem to be in stable orbits. If you are interested in reading a (short and fairly uncomplicated) paper about S2, the star closest to our supermassive black hole, you can find it here.

When there is gas or dust in the vicinity of the black hole, it will tend to spiral inwards until it eventually passes the event horizon*. As this occurs, huge amounts of energy are released, outshining all the stars in the galaxy. This is what is called an active galactic nucleus. It is also, more or less, what causes quasars, the most distant objects we observe (because they're so bright we can see them very far away, you see).

Here is a nice NASA / ESA Hubble Space Telescope picture of jets coming off the nearby AGN, M87.

 * The point of no return.

Active life?

So the next question, the crux of this post, is can we have life in a galaxy with an AGN? The short answer is maybe. Of course, we have no concrete proof either way. Sagittarius A* (yes, that asterisk is part of the name), our central black hole, is not currently active** and we have even less evidence for life in other galaxies than we do for life on other planets within the Milky Way. The slightly longer answer is, it depends. There is evidence to suggest that the Milky Way was active in the past few million years and since there is still life on Earth, we can suppose that an AGN doesn't necessarily sterilise a galaxy.

Some months ago, some colleagues and I got into a discussion regarding whether a really bright AGN (even one unrealistically bright for the size of our galaxy and Sag A*) could wipe out life. We came to the conclusion that it would only do so if you were standing close enough to it. From memory, we estimated that if a planet somehow managed to find itself*** in an orbit a parsec from the active black hole, the black hole would through about as much light at it as the sun does. However, AGN emit much harder radiation than stars, meaning that a larger proportion of the energy would be at X-ray and gamma ray frequencies, both unfavourable to life. If we put the planet where Earth is, then even with nothing blocking the way we don't have a very high increase in dangerous radiation.

However, something is blocking the way: dust. As far as we know, dust near the black hole is requisite for turning on an AGN. But even ignoring that, there are many clumps of dust in the disc of the Milky Way. So many that we are unable to see through it all if we look along the disc. (Schlegel et al surveyed the dust in the galaxy and came up with this map. White bits have more dust, black bits have less.) In essence, as well as making it hard for us to notice supernovae near the centre of the galaxy, this would help shield us from AGN light. I wouldn't be surprised if we didn't immediately notice the AGN. Of course, closer in to the centre of the galaxy you have more problems and it starts to depend more on exact placement. Also, the dust actually only shields visible and UV light, so once you get too close the more concentrated X-rays and gamma rays become more of a problem.

On the other hand, a planet is less likely to form in the path of an AGN jet, simply because there are fewer stars in that direction. If it did, however, it would definitely not survive the experience.

Elliptical galaxies have significantly less gas in them (some might say no gas, but there would have to be some in the centre for the AGN to turn on, not to mention dust created by dying stars).  This would mean less shielding, making the AGN more noticeable. The bigger barrier to surviving the experience, however, would be the fact that elliptical galaxies are larger with with more massive central black hole which would generate a more energetic AGN (with more detrimental radiation). The final point to consider is that theories suggest AGN in elliptical galaxies are turned on thanks to dust being stirred up (into the black hole) from a merger. So the merger event could have some impact (see last week's post) on continued habitability. The dearth of dust also means that new planets would not be able to form in an elliptical galaxy.

So to summarise, an AGN wouldn't necessarily sterilise a galaxy, but might kill off life that had set up too close to the centre. Depending on an inhabited planet's placement in a galaxy, an AGN might not have a very large effect on daily life. There are a few additional problems for elliptical galaxies, but again, so long as the planet isn't too close to the centre (and its sun doesn't migrate to the centre of the galaxy too quickly), there's no reason for life to automatically be extinguished. Score 2 for extragalactic life.

Next time: the habitability of galaxies in different environments in the universe.


** Probably.
*** I personally really don't think a planet would be able to form in that region thanks to the density of stars and subsequent gravitational forces. I don't have any hard evidence to support this, but to me it makes sense.