Monday, 23 March 2009

Grazie Half65 for ARP 142

I'm insanely excited today over a 16 by 16 pixel picture: a favicon from Half65! It is the beautiful ARP 142 galaxy system, the individual galaxies known as NGC 2936 and NGC 2937.

Credit: SDSS. To zoom in and out and explore around, look at Navigate.

This was the first really "interesting" galaxy I found at Galaxy Zoo No. 1. Ah, the old days - I was living in Brighton, classifying in the computer room when I should have been revising for my chemistry exams. (Actually I did much better than I expected - I thoroughly recommend reading 1 page of notes, doing some classifying to relax the brain and let it sink in, and then reading another page of notes, rather than cramming it all in at once!) The forum which changed my life had not yet been born. Fortunately, a temporary blog was set up by a kind enthusiast, Salmon Chase, and it was one of the first things I posted. My friend Gill's husband named it "the penguin galaxy" and it seemed to catch on!

Of course, I didn't know anything about galaxies at the time, or how to search SDSS, and it was a while before I found out it was called ARP 142. What can we tell about it just from looking at it?

The "egg" the "bird's head" is "looking at" is an elliptical galaxy; the "bird's head" itself is a wonderful merger, with dark dust lanes and bursts of star formation at the top towards the "beak". My impression is that it has been dragged round anticlockwise in a quarter-circle. It's quite peculiar that the elliptical appears so unaffected, but perhaps its stars are closely bound enough not to be so disrupted - or perhaps there's something I'm missing. I checked their redshifts, to see if perhaps the elliptical was further away and looks bigger than it is. It isn't. It's closer to us: z=0.023, while the merger is 0.024. Behind the star at 1 o'clock is a small irregular galaxy known as UGC 5130. Apart from that, it seems to be a relatively empty area.

We can also look at the spectrum. This is an emission spectrum (drat - I haven't written about those yet, though I covered absorption spectra and their history this week*):

What exactly does an emission spectra mean for a galaxy? It means it is very energetic. It probably indicates star formation. Well, we can see that from the blue splodges anyway. And why should that be?

When a hot light source - an energetic photon - hits an electron in an atom, the electron leaps up to higher orbit. Perhaps the next-highest (think of the Earth leaping up to Mars's orbit), or several orbits higher (Earth leaping into Saturn or Neptune's orbit). It has to be of a very, very specific energy - a particular wavelength of light - just the right amount of energy to boost the photon to a specific place. Too high or low and the photon will be sent on its way. But even then, atoms and subatomic particles don't like to be in a higher energy state than they have to be, so eventually the atom falls back down again, emitting that photon again. But since the photon's gone off in any old direction rather than the one it was originally heading in - towards us - that wavelength of light is missing in the spectrum.

That's a hot light source and a cool atom. What about a cool light source and a hot atom? That's where emission spectra come in. That's when the electron was already up in a higher-than-necessary orbit - "in an excited state", as we say - and shoves the photon off in our direction. And that's when the spectrum shows huge peaks.

In this case, the highest peak is of H-alpha, or Hα. That means the electrons are falling from the third orbit to the second, rather like Mercury falling from Earth's orbit down to Venus's. There is also a bit of H-beta (from the 4th to the 2nd) and an even littler bit of H-gamma (from the 5th to the 2nd). That clump of star formation the camera is focussed on in this case is not as violent as some galaxies. The peas, for example, have high OIII emissions - oxygen with two electrons knocked off. I'll write about them in another blog post!

All the above I was able to get out of my eyes and SDSS. Doubtless somebody smarter than I currently am could get a lot more, such as the exact data for the colour filters . . .

The galaxy system is from the ARP catalogue: the Atlas of Peculiar Galaxies, drawn up by Halton Arp. Its number is between 102 and 145, into which category fall the elliptical and elliptical-like galaxy types (of which there's a lovely catalogue here).

ARP 142 is in Hydra. There's a fantastic picture you can zoom right in on on this site, in which it looks more like a bird than ever! That site describes it as "emanating", though is not specific on what is being emanated. You can also find it on skyfactory.

A paper has been written by McCain et al on "ARP 142: Another interacting galaxy with very large internal motions". (As again is clear visually, I think!) Intriguingly, the Hα is not coming off the elliptical, but off the merger - which was once, according to their research, a spiral which is gradually being stripped by the elliptical - not quite the same as my quarter-circle impression above! They state that there is a massive "velocity difference", but I'm not clear on whether that means of the galaxy's movement, or whether the Hα is moving at that speed. Evidently that means ionisation is taking place, but shock waves are unimportant. (Recall they are very important in barred spirals and are often a trigger for star formation in spiral arms . . .) Looking at it visually backs up this conclusion; blue galaxies are full of star formation while ARP only has the odd burst of it here and there.

There's just so much to learn!

P.S. Happy Mother's Day.

P.P.S. Grazie Half for correcting my spelling :D

P.P.P.S. *But I did later. Here is the story of emission spectra, and a wonderful story it is too.

1 comment:

Half65 said...

I like the Penguin.
It's very familiar to me.
Glad to be useful.
Great post Alice.