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.

Winds of Change ToC

And now for something completely different.

Rather than a science post, this is a quick note to tell you about the upcoming anthology from the Canberra Science Fiction Guild which includes one of my stories. The anthology is called Winds of Change and is edited by Elizabeth Fitzgerald. The table of contents is at the end of this post.

(Yes, I am a bit late with this announcement, but my excuse is I was on holiday, so shhh.)

My story, "Time Capsule" is third, which, I have decided, is a golly good place to be. And despite all my spacey writing on this blog, the story is set entirely on Earth, so make of that what you will.

Winds of Change is going to be launched at Conflux, the annual Canberran science fiction and fantasy convention, on Friday 30 September. I will post more info about getting your hands on a copy (other than if you're at Conflux, of course) when it's available.

The table of contents:

Winds of Change

Edited by Elizabeth Fitzgerald

Stories and authors:

Wraiths by Jason Nahrung

Gravity Express by Naomi Mondello

Time Capsule by Tsana Dolichva

The Tether of Time by Leife Shallcross

Trigger by Zena Shapter

Babel by Robin Shortt

Saint Olivia's Light by Carol Ryles

In Need of Assistance by Chris Andrews

After the Bombs by Adam Tucker

The Horns of Elfland by Crisetta MacLeod

Time Spent by David Coleman

Soul of the Machine by Maxine McArthur

Dream Shadow by Alan Baxter

Giant by Annelise Roberts

Evolution Baby by Lesley Boland

The Princess by Valerie Y.L. Toh

Children of the Ashes by Greg Mellor

By Watcher's Pool by James Goodrum

Turning the Blood by Donna Maree Hanson

Watching by Nicole R Murphy

The Stormchilds by Helen Stubbs

The Fool by Jane Virgo

Dragonfly by Cat Sheely

Stone-singer by Joanna Fay

Habitable Galaxies - Part 1: Galaxy types and origins

Hello blog readers, I'm back :-)

I've talked about habitable planets and habitable parts of our galaxy before. Now I am going to talk about the possible habitability of other galaxies. It's a somewhat lengthy topic so it'll be spread out over a few weeks. Today, some background.

Miscellaneous types

There are two main classes of galaxies: spiral and elliptical. (Also known as late type and early type, but those names are silly as will soon become apparent.) These are called morphological types because they relate to the physical shape of the galaxy. You can also get dwarf galaxies, which are very small—on the scale of galaxies, although they're still billions of times the mass of our sun compares with tens of trillions of suns for galaxy like ours.

Once we get past galaxy shapes there are all sorts of interesting things galaxies can be doing. For example, you can have active (or not active) galaxies, galaxies which are merging, galaxies where stars are actively being born, galaxies filled with old, dying stars, galaxies with high or low metallicities, isolated galaxies and galaxies part of giant groups or clusters. There's a lot of variety, even within each of these (not always mutually exclusive) classifications. You think the two hundred billion* or so stars in our galaxy are a lot? There are eighty billion galaxies just in the observable universe. Which ones, other than carbon copies of our own, can we live in? Let's first have a look at where galaxies come from.

* I use billion here in the American sense to denote 1 000 000 000 = 10⁹ for three reasons: 1. it's most commonly used this way in the scientific community, 2. the word "milliard" (as in the British counting convention) has fallen into disuse and 3. "thousand million" (the replacement for milliard) sounds clunky.

Galactogenesis

As Dougalas Adams said, in the beginning there was nothing, and then it exploded. After that, if we fast forward a bit (which, current theories suggest, the universe did), matter which at this stage consisting mostly of hydrogen with a little bit of helium that was fused in the primordial fires of the big bang, starts to clump together.

(At this point I feel the need to mention that I'm more or less ignoring dark matter because in this context it needlessly complicates things. But it's there too. Just so you know.)

These clouds then collapsed into discs and clumpiness within the discs led to star formation (that is, clumps of hydrogen collapsed in on themselves to make the first stars). So the first galaxies were all disc galaxies, another name (more or less) for spiral galaxies. However, not all clumpy areas are the same size, so you might get a clump of galaxy-sized clumps and then get a bunch of galaxies forming close together. Over time, gravity will pull these galaxies closer together and they will eventually merger (I'll get to mergers shortly). On the other hand, you could have an area of the universe where not much matter ended up and there was only enough stuff to make a few small galaxies.

If you look at our Local Group, there are two big spiral galaxies—ours and Andromeda—a medium-sized galaxy—the Large Magellanic Cloud—and a slew of tiny dwarf galaxies. Some of these dwarf galaxies are in the process of raining down onto the Milky Way or Andromeda. Others are further away and are falling into one of the big galaxies much more slowly. Andromeda and the Milky Way will merge on a time-scale of five or so billion years. Eventually, all the galaxies in the Local Group will merge into one giant Local Structure.

But I'm getting ahead of myself.

So current theory has galaxies being born as discs. Where do the elliptical galaxies, the round or oval balls of stars, come from? The currently popular answer is through mergers.

Mergers

On a cosmic time-scale, small galaxies are constantly falling into (accreting onto) larger galaxies. Every so often two medium-sized or larger galaxies might merge. These events are called major mergers. There are two possible outcomes of major mergers between spiral galaxies:

1. A new, larger, spiral galaxy. There will often also be a period of more vigorous star formation after a major merger like this thanks to the gas and dust in the galaxies being shaken up and combined with more gas from the other galaxy. It is thought this outcome happens when there is enough spare gas in the two progenitor galaxies.
2. An elliptical galaxy. When there isn't enough spare gas in the two merging galaxies, the stars don't rearrange themselves in a nice flat pattern (exactly why is unclear, I think) and we're left with a roundish blob of stars.

There is no question that minor mergers—small galaxies accreting onto larger ones—don't pose much of a threat for life. After all, small galaxies have been accreting onto the Milky Way far longer than we've been around. Whether planets will survive major mergers is another question. In principle, most planets should be more or less OK. Despite the billions or trillions of stars involved, they are actually quite spaced out within galaxies (for example, the nearest stars to our sun are more than four light years away). Therefore, the chances of stars colliding are very small, at least at the sort of distances from the centre of the galaxy where we find ourselves. If the stars don't pass too closely together, their planets will also remain unharmed.

Of course, it's possible that a particular planetary system could be unlucky, but then it could also be unlucky enough to have a supernova explode nearby. In fact, if enough gas nearby is stimulated into forming stars during and after the merger, it could be that a future supernova (from a newly formed massive star) is more of a threat than the actual merger itself.

Finally, will the Earth survive Andromeda and the Milky Way merging? Not exactly, but not because of the actual merger. You see, the sun is scheduled to turn into a red giant before we get to the merger stage, so Earth as we know it will be long gone already.

But wait, there's more

This isn't the full story, of course. Mergers also lead to increased activity in galactic nuclei, but that's a topic I'll be covering next week. Watch this space!

Blog hiatus

Hi all. Unfortunately, I ended up being too busy to write sufficient posts to cover the "I'm on holiday" gap. This means, no new blog posts for a couple of weeks. I'm sure you'll all cope ;-)