Sunday, 15 February 2009
Barred Spirals Lecture at Astrofest by Johan Knapen
(Picture stolen from Pat!)
Galaxy Zoo Get-Togethers are highly recommended - we went to Astrofest again the Saturday before last and I have finally got around to writing up my favourite lecture there. I'm hoping these notes will also lead to some amazing joint Objects of the Day on the forum . . .
Lecture - "Barred spirals: Galaxy evolution in action"
This was a wonderful lecture. Here is Astrofest's page about it. Does Johan Knapen have a website? I couldn't find one, and I would rather like to thank him.
Update, 3rd March 2009: This afternoon I had a big surprise in my inbox: an e-mail from Johan Knapen himself! Thanks so much for writing, and I hope this blog entry is an accurate representation of your lecture. And yes, I will definitely keep writing about galaxies.
Johan's website is here: http://www.iac.es/galeria/jhk
About two thirds of spiral galaxies have a bar - one third a "weak bar", one third a "strong bar". It's logical to assume that a stable orbit of stars in a spiral galaxy would be a round or elliptical (i.e. oval) one - but two thirds of galaxies have unexpectedly straight centres, shaped like this:
NGC 1300, which you might have seen elsewhere! Credit: Spacetelescope.org
The bar influences the galaxy's evolution and dynamics, for example its star formation. Gas inflow often occurs in spiral bars, and spiral arms usually begin at the ends of bars.
On the Hubble Tuning Fork diagram, the number after the E on early-type galaxies refers to the height : width ratio. That is, E0 is perfectly round, while E9 is very long indeed (hence the round/medium/cigar-shaped question in Galaxy Zoo II).
Bars often contain dust lanes leading to their centre. The lane is an outline of gas shocks. (Think of the Cat's Eye nebula with all those "ripple-edges".) Because of this, there is often a ring of star formation. This can be an "inner" or "outer" ring:
In fact, some have "nested bars" - that is, two or three bars in total, with a smaller bar (or two) at not-quite-right angles to the main one.
The shape of the spiral doesn't seem to determine whether it has a bar or not; all spiral types seem to be able to have them. It is more common for a spiral to have a bar than not. Infra-red imaging is good at detecting them where optical has not done so. Blue and ultra-violet light is particularly poor at bar detection.
Why a bar forms is unclear. But any asymmetry in the galaxy seems to end up forming a galaxy bar! A bar is very stable. It has a lot of gravitational strength, so more and more stars join it. Why the disruption in the galaxy occurs in the first place is not yet known. We were shown a video of the formation, which I can't reproduce here, but I went through our barred spirals thread to find some examples. In fact I prefer these to the video, because "0 years" looked like an E0 to me.
The bar itself rotates. It rotates in the shape of a sausage, or caterpillar wheels on a tank. An elliptical bar orbit is no problem. In fact, a star can go in any crazy orbit you like, and stay there for eternity unless something else disrupts it. I couldn't believe the weird shapes we were shown - nor my luck in finding the same exact ones by googling "box orbit"! Here they are, at a website called Dynamical Astronomy Javalab. Elliptical loop orbit, banana orbit, box orbit and fish orbit. Who'd have thought it? I'd love to track some star orbits in elliptical galaxies.
If we add all these complex orbits up, you get what I immediately recognised from Galaxy Zoo as a boxy core. The Hickson 87 Compact Group's edge-on spiral is a good example.
So far, so good. Now, stars are not the only things you get in galaxies. There's gas too. Gas needs to follow the star orbit, as stars outline gravitational potential. The gas loses momentum, collides (as clouds), hence the shock outlines - and therefore it loses energy and angular momentum, so it has to move even further into the centre . . . and so on. For gas, orbiting in a normal circle or oval is all right; but a boxy shape has sharp corners which it cannot suddenly turn as stars can - and that is where the shocks take place. Hence . . . star formation, often in the form of rings. NGC 4314 is an example:
(We were given a reference of Athanassoula 1992. I am still Googling to choose the best site out of rather a lot of heavy-going ones.)
For an example of a weak bar, but a similar starforming ring, we were also shown NGC 7742. (There can be such rings in galaxies with no bars at all. I must admit I can't see a bar here, but I don't have infra-red eyes.)
Such inward-spiralling gas and star formation is also an excellent fuel for an AGN. An AGN is a non-stellar energy source; it must be fed by gas.
This is all "secular evolution", i.e. the internal evolution of the galaxy. For more on secular evolution, check out the Kormendy and Kennicutt model - fortunately it doesn't matter that I didn't have time to copy the whole thing down!
Knapen re-emphasised that infra-red light is best for observing bars (I suspect because of the high amount of gas involved, meaning a lower temperature, but I'm not sure). At a redshift of over 1, bars are hard to study. He also remarked that star formation does not take place in the bar itself, but in the arms and in the centre of the galaxy.
Bars are an interesting phase of galaxy evolution and he hopes they will be studied a lot more; he recommends kinematics and morphology to determine their structure and properties, which I'll let a real physicist explain. S4G, the Spitzer Survey of Stellar Structure in Galaxies, is surveying 2000 galaxies, though someone will need to sort through them all. (Its telescope has warmed up, so it needs to study infra-red.) Could that possibly be a job for us? I think we'd better write in . . .
We finished off with the above beautiful picture of NGC 1672, and with the punchline: "The Universe is full of bars, so never a thirsty moment!"
Potential Objects of the Day
I reckon we can get several, guys. What about:
- General bar evolution
- Boxy bulges and weirdo orbits
- Rings of stars
- The double/triple bars thing.
Any of you guys, please go for it. I've got a thread running which you'll find if you're a regular Object of the Day poster (if you're not yet, but now are inspired to become one, please write to me on the forum!). Alternatively, anyone fancy writing a joint one with me?
Update: Thursday 19th February: Barred Spirals and Barred Evolution.
Galaxy Zoo references that immediately spring to mind:
- The "Best Barred Spirals" topic in the Galaxy Zoo Forum. Well worth a read.
- The "Why Barred Spirals?" thread - ditto.
- Boxy bulges, from the Galaxy Zoo Beta Testing
Further notes and research I did before I found the best stuff:
- A news item Hubblesite - "Barred Spiral Galaxies Are Latecomers to the Universe".
- More on orbits - a PDF file at astrosmo, whatever that is . . .