Also, just adding to what Nigel said: the spiral arms are not so much the result of spin as a region with a high density of stars most likely due to density waves; density waves could have a number of causes, for example, an interaction with a nearby companion galaxy (which is quite likely for M51).

But whatever; they’re pretty!

@19 Nigel: Why, half a WITCH of course!!!

I don’t believe there’s such a thing as a dumb question.

What’s the difference between a duck?

L.

Please [#5 Ian] offer him and his team a very big thankyou from me. Love the WISE work! One of my favourite missions and very much appreciated.

Incidentally, I watched the launch of WISE – over the internet on NASA-TVwhich was one of the first one’s I viewed like that so remember it well and have followed it closely since. I’m still hoping the WISE data will lead to a nearby brown dwarf or two turning up perhaps even one closer than Proxima Centauri. Don’t suppose you can give us here any hints or scoops at all can you?

@11. Joel :

@ Egaeus (6) – NGC 6822, aka Barnard’s Galaxy, is an awkward one. First, it’s a very small galaxy. It definitely has a central bar, and there is what appears to be a small and vague single arm, more obvious in visible light. .. [SNIP] .. It’s usually classed as a barred irregular, but that’s an odd sort of group on the edges of spirals. It’s also very, very similar in appearance to the Large Magellanic Cloud, which may have originally been more spirally before getting distorted by the Milky Way. NGC 6822 is part of the local group, but I’m not sure how close it is to any other larger Galaxies which may or may not have affected it.

Informative comment – thanks.

The LMC definitely has spiral structure of sorts in my book & looks like a barred spiral albeit a small & disrupted one to me.

@2. PS :

I have two questions about these pics from a minimally educated though highly inquisitive mind:

1) The spiral shapes are due to a “spin” and we believe that the spin is generated by super massive black holes in the center of these galaxies. Correct? So my question is, how visible is the spin in these views? That is, how long would it take for one of these to complete a rotation? Obviously the further out, the longer the duration, but how observable is it?

Are you asking here can we detect the rotation of these galaxies? Not in our lifetimes I think given the timescales involved.

2) In layman’s terms, how would you describe the reasons why two spirals would look so dramatically different? For example 628 vs 1398. Not the direction of the spin, but why one looks so “clean” while the other looks more scattered. Does it relate to how long it’s been spinning? The positioning of the matter when it was caught up in the spin?

Almost all galaxies have their own unique appearances and various factors create them incl. formation history ie. how the ga scloud that formed its first stars fell together, subsequent gravitational encounters with other galaxies, mergers and the intergalactic environment around them esp. for those in dense clusters and superclusters.

These are probably questions that would have been answered in a 6th grade science class, but I was too foolish then to know what I was missing when I goofed off in that class.

Heck, we’re all human and as such fallible beings – and we all have to start somewhere. Curiousity is good as is further reading on subjects you’re curious about – I’d recommend almost anything by Tim Ferris, Carl Sagan and Isaac Asimov – and never be afraid to ask and seek information. I don’t believe there’s such a thing as a dumb question. (Well, okay maybe very, very rarely! )

Remember : Some of us like answering questions too!

http://en.wikipedia.org/wiki/Atlas_of_Peculiar_Galaxies

&

http://apod.nasa.gov/apod/ap110421.html

Arp 273 on A.P.O.D.

&

http://apod.nasa.gov/apod/ap110226.html

Shell galaxies in Pisces, again via APOD for just a handful of rather different looking not-so-spiral “spiral” galaxies.

@ 3. Sam H : Huh? (Puzzled.)

PAH’s ain’t oils!

Really not sure what you’re getting at there at all, sorry. Perhaps that’s just me?

@ 5. Ian : Ned Wright, the Principal Investigator on WISE, is an acquaintance of mine.M

Please offer him and this team a very big thankyou from me. Love the WISE work! One of my favourite missions and very much appreciated.

It’s also very, very similar in appearance to the Large Magellanic Cloud, which may have originally been more spirally before getting distorted by the Milky Way. NGC 6822 is part of the local group, but I’m not sure how close it is to any other larger Galaxies which may or may not have affected it.

http://apod.nasa.gov/apod/ap080619.html

The images here are in the IR only.

The spiral shapes are due to a “spin” and we believe that the spin is generated by super massive black holes in the center of these galaxies. Correct?

In essence . . . no.

The spin of a galaxy is a consequence of the residual angular momentum of the gas cloud that collapsed to form the galaxy in the first place, aeons ago.

Thus, all objects in a galaxy orbit the galaxy’s centre of mass. In many galaxies, the objects all rotate in the same direction and plane, but there are also many galaxies for which this is not so. IIUC, most eliptical galaxies have different populations of stars with different orbital planes and directions. This is (IIUC again) thought to be because elliptical galaxies often arise through the merging of two or more smaller galaxies.

However, the spiral arms of a spiral galaxy are not physical objects. They are regions where the stars happen to be closer together, and thus where the galaxy appears to be brighter. And while a galaxy’s spiral arms contain stars, they do not contain a constant population of stars. Stars leave the spiral arms and join the spiral arms as those stars orbit the galaxy’s centre of mass. However, because the spiral arms are denser collections of stars within the galaxy, they have a stronger gravitational influence than the regions between the spiral arms. Therefore, stars speed up as they approach a spiral arm, and slow down before they leave it. Therefore, the stars tend to spend more of their time in a spiral arm than between spiral arms. Thus, the spiral arms are relatively stable structures.

So my question is, how visible is the spin in these views?

The orbital motion of individual objects varies.

I have no idea if the spiral arms themselves rotate about the galaxy’s centre – or, more precisely, present the illusion of rotation.

That is, how long would it take for one of these to complete a rotation? Obviously the further out, the longer the duration, but how observable is it?

The spiral arms – if they rotate at all – would rotate with the same angular velocity at all distances from the galactic centre. Otherwise, they would rapidly cease to be visible.

The individual stars within spiral arms do revolve about the galaxy’s centre of mass at different angular velocities, and these velocities are indeed greater near the centre than farther out. This is measureable in two ways. First, as proper motion of the stars, i.e. if you take photographs of that galaxy at intervals of – say – 10 years, you should be able to detect changes in the position of the stars relative to distant objects such as background galaxies. This is, I think, a rather unusual and not very accurate measurement, because the motion will be very small on a human timescale unless the stars are moving with enormous velocities. Second, where a galaxy is not “face-on” from our point of view, we can use the redshift or blueshift of stars on opposite sides of the galaxy to get a far more accurate measure of the motion of those stars towards or away from us (I think this is termed radial velocity). The blue shift and red shift of the light from those stars arises because as the stars orbit the galactic centre of amss, those on one side of the galaxy will be moving towards us relative to that centre, while those on the opposite side of the galaxy will be moving away. This motion causes Doppler shifts in the spectra of those stars that we can measure with good precision.

Strange…the delicate tendrils of gas thousands of light years long, which may contain the ingredients for life to form on countless worlds, is now involved in the mass slaughter of whole, wondrously complex ecosystems of that same life here on Earth. The freak wonders of evolution on a beautiful planet have been wiped out by an element that may have been a catalyst in the very dawn of the rare tapestry created by the same process. Even more ironic – this toxin was actually once alive itself.

Now what could that mean, if anything at all (aside from “oil bureaucracies really suck“)?

1) The spiral shapes are due to a “spin” and we believe that the spin is generated by super massive black holes in the center of these galaxies. Correct? So my question is, how visible is the spin in these views? That is, how long would it take for one of these to complete a rotation? Obviously the further out, the longer the duration, but how observable is it?

2) In layman’s terms, how would you describe the reasons why two spirals would look so dramatically different? For example 628 vs 1398. Not the direction of the spin, but why one looks so “clean” while the other looks more scattered. Does it relate to how long it’s been spinning? The positioning of the matter when it was caught up in the spin?

These are probably questions that would have been answered in a 6th grade science class, but I was too foolish then to know what I was missing when I goofed off in that class.

On the WISE one the left arm is nearly vertical, on the NOAO one it’s tilted a bit to the right. If you rotate the WISE one to the right then it WOULD be aligned with the NOAO one, sure, but the opposite of that is to the left, counter-clockwise

(I guess I might be wrong since I know zilch about anything but yesterday I really had to put effort into understanding the coriolis effect reading your book (I never got it correct in school – always knowing that northern hemisphere low pressure systems rotated counter-clockwise but never knowing WHY the winds from the zero meridian went east – but NOW I got it – so I hope I am not mixing clockwise and counter-clockwise up when it comes to image-rotation ^^;)