Galaxy rotation, Spiral Galaxies in Collision (20 Apr 2008)

[Mac Coak]

In your photo "Spiral Galaxies in Collision" the galaxies appear to be rotating in different directions. Is this normal?

[Case]

A recent and ongoing study shows that the direction of the arms in spiral galaxies is uniformly distributed in 'all' directions. That is, no regions have been found with concentrations of clockwise or anti-clockwise rotating galaxies. A previous smaller study seemed to indicate that there was such a 'preference'. The larger study seemed to confirm that at first, but when presented with mirrored images, the 'preference' stayed the same. So it seems it was human error: a human viewer would have a preference in seeing a certain rotation when the image wasn't very clear. Careful analysis of the classification of regular and mirrored images shows no preference in either rotation.

The fact that these two have opposite rotation is by chance; another galaxy pair could have matching rotation. These galaxies would have formed independent and far from each other, before they got close to each other.

[Mac Coak]

Thanks, I'm old but new to taking close looks at the heavens.

[orin stepanek]

http://apod.nasa.gov/apod/ap080420.html

Whether or not one is spinning clockwise of counterclockwise depends on the side of the galaxy you are viewing from. Even so these two do have opposite spirals; like two gears meshed together.

Orin

[PassionateBomba]

I think Orin is correct. When determining the rotation direction, It does depend on which side of the galaxy is facing our view.
As an older newbie and very much a novice, Case intrigues me with the studies he has cited. Is the term "spiral" galaxy but a term assigned a particular looking form or shape. A classification. If a galaxy does not fit the criteria of the definition for spiral, then wouldn't it be something else? An "irregular" galaxy perhaps. Then would the studies really be a determination if a galaxy truly is a spiral galaxy?

For my own interest in the APOD. The footnote says, since stars are so far apart, they don't collide during galactic interaction. Does this mean none collide, as an absolute? Surely this isn't right. Of so many, surely some must collide. It also seems unimaginable that as the nucleus of one passes through the other, there is no scar of the occurrence or that the mark is healed so completely and quickly. How could this be?

To combine My interest with that of Mac.
It would seem that interaction of 2 galaxies rotating in opposite directions and striking each other at angles to their galactic plane, would be terrific and violent. I would expect a rainbow of gaseous release which would be visible, viewed with proper filters. Do any such pictures exist?

Future change. Long ago, with clean air, each galaxy might have been viewed as faint stars. Today we see them as galaxies very far away. As the nuclei of these galaxies come closer to alignment, gazers of the future might view them as a single huge galaxy, though they are separate. I think this APOD scenario explains why answers to such questions change as perspective, information, and of course the complex nature of human beings to see things differently, change.

[rigelan]

The galaxies are classified by shape according to our wikipedia friend here:

http://en.wikipedia.org/wiki/Galaxy_mor ... sification

Spirals, Ellipticals, are grouped together. All others are remnants of mergers, and irregular.

[Case]

Is the term "spiral" galaxy but a term assigned a particular looking form or shape. A classification. If a galaxy does not fit the criteria of the definition for spiral, then wouldn't it be something else? An "irregular" galaxy perhaps. Then would the studies really be a determination if a galaxy truly is a spiral galaxy?

Edwin Hubble made a huge contribution to the (visual) classification of galaxies by making a system for it, which is still in use today (with some modifications, such as the inclusion of 'your' irregulars).

The footnote says, since stars are so far apart, they don't collide during galactic interaction.

For comparison: The sun (an average star) has a diameter of 1.392×10^9 m. The nearest star is 4.2 lightyears (39.7×10^15 m) away. That is pretty far: 28.5 million times the size of the sun. So we have a sphere with a diameter of 57 million times the size of the sun with just us in it. A star from a colliding galaxy would have to go through a circle with a surface of π×r², roughly 3000-million-million times the sun, to come closer than any other current star. So, the chances of a star-to-star head-on collision are pretty slim.

It also seems unimaginable that as the nucleus of one passes through the other, there is no scar of the occurrence or that the mark is healed so completely and quickly.

There will be some lasting 'scarring' of the collision event, but that is more in the line of a gravitational pull, which will send stars on a totally different path, distorting the arms of the spiral galaxies. See the clips at Galaxydynamics for some beautiful imagery of how wild such a process can be.

I would expect a rainbow of gaseous release which would be visible, viewed with proper filters.

The space between stars isn't exactly empty. When the gas and dust of galaxies merge, one can expect violent and rapid star formation in huge numbers; I would imagine something like a multitude of Orion Nebulas and Carina Nebulas in every corner of the sky.

[astrolabe]

Hello all,

Just yesterday (04/19) I authored a thread on l last Thursday's APOD on M63. Things never happen in threes with me they always happen invariably in twos. For instance, I was listening to a radio station conduct it's begathon one day in the early '90s and they were thanking one of their supporters who is a bottlled water company. Not 10 seconds later, one of their water delivery trucks came over a rise on the road in front of me! Larger or more important events take longer for the occurance of this duality phenom but occur they do. Factoids are much quicker.......but I digress.

I magine the speed at which galaxies begin converging to the actual event itself is exponential with the rate of speed of the approach increasing over time. Does it not also reason that if one galaxy is rotating one way an the other is opposite that their respective arms would generally be going in the same direction at their merging edges?

If the alternative was occurring, where the two bodies were rotating in the same direction, then the merging edges would be rotating against each other and the speed at which objects were passing each other, along with dust, dark matter etc. would be very great indeed!! Assuming, of course, the the two are roughly in the same plane.

My question? In the latter senario, would more visible photons be generated along with frictional heat signatures result?

[Chris Peterson]

For my own interest in the APOD. The footnote says, since stars are so far apart, they don't collide during galactic interaction. Does this mean none collide, as an absolute? Surely this isn't right. Of so many, surely some must collide.

No. Even at the core of a galaxy, the volume of empty space is immensely greater than the volume of stars. The probability of a collision is very, very small.

It also seems unimaginable that as the nucleus of one passes through the other, there is no scar of the occurrence or that the mark is healed so completely and quickly. How could this be?

During the time the galaxies are merging, clouds of gas and dust from each intersect. This produces shock waves and heating. In addition, whole regions from each galaxy are launched into different orbits, distorting the structures. These effects could certainly be considered long lasting scars.

To combine My interest with that of Mac.
It would seem that interaction of 2 galaxies rotating in opposite directions and striking each other at angles to their galactic plane, would be terrific and violent. I would expect a rainbow of gaseous release which would be visible, viewed with proper filters. Do any such pictures exist?

Many images of colliding galaxies show hot hydrogen regions that result from the collisions. I'm not sure the relative directions of rotation are all that important, however. You need to consider the rotational speed with the speed of convergence. If the latter is much greater, the additive or subtractive effects of rotational components won't matter much.

[astrolabe]

Hello Chris,

Many thanks. Your response to PassionateBomba answered my questions as well.

[bystander]

The galaxies are classified by shape according to our wikipedia friend here:

http://en.wikipedia.org/wiki/Galaxy_mor ... sification

Spirals, Ellipticals, are grouped together. All others are remnants of mergers, and irregular.

I understand the difference between ellipticals and spirals, even irregulars. I don't agree irregulars are necessarily the result of mergers (SMC & LMC). What I question is the difference between lenticular and spiral. So many galaxies I've seen classified as lenticular appear to be spirals to me.

Spiral: M104: Sombrero Galaxy

Lenticular: NGC 5866: Spindle Galaxy

[Chris Peterson]

I don't agree irregulars are necessarily the result of mergers (SMC & LMC).

Irregular galaxies may not "necessarily" result from mergers. That is simply the process that produces most of them. It's possible that there are other causes as well. The LMC, for instance, which is an irregular galaxy showing a subtle barred structure, is probably a spiral that has been distorted by gravitational interactions with the Milky Way, and maybe the SMC as well. There may not have been an actual collision.

What I question is the difference between lenticular and spiral. So many galaxies I've seen classified as lenticular appear to be spirals to me.

Well, galaxies are classified on a continuum, and the divisions between types are arbitrary. There are bound to be borderline cases that some would see as spiral, and some lenticular. It doesn't really matter. Personally, I'd say the two examples you give are both very solid members of their assigned categories, though. M104 is very obviously spiral, and NGC5866 (to my eyes) shows not a hint of spiral structure.

[bystander]

Irregular galaxies may not "necessarily" result from mergers. That is simply the process that produces most of them. It's possible that there are other causes as well. The LMC, for instance, which is an irregular galaxy showing a subtle barred structure, is probably a spiral that has been distorted by gravitational interactions with the Milky Way, and maybe the SMC as well. There may not have been an actual collision.

Ok, I can agree with that. Distorted and stripped of mass, they can even be considered as merger remnants (as rigelan said).

Well, galaxies are classified on a continuum, and the divisions between types are arbitrary. There are bound to be borderline cases that some would see as spiral, and some lenticular. It doesn't really matter. Personally, I'd say the two examples you give are both very solid members of their assigned categories, though. M104 is very obviously spiral, and NGC5866 (to my eyes) shows not a hint of spiral structure.

I agree M104 is obviously a spiral, what I can't see is why NGC5866 is obviously not, especially since it edge on. I've seen others of questionable distinction (particularly S0B's), but can't come up with examples right now.

APOD: 2006 January 15 - The Sombrero Galaxy from HST - M104
APOD: 2006 June 12 - Edge On Galaxy NGC 5866 - Spindle Galaxy

[bystander]

I found a couple of questionable classifications:

APOD: 2002 April 8 - NGC 2787: A Barred Lenticular Galaxy
Wikipedia: NGC 2787
S0Br? Looks "spiral" to me.

APOD: 2006 April 25 - M82 Galaxy with a Supergalactic Wind
Wikipedia: Messier 82
Questionably classified as "irregular". More like spiral or "lenticular".

[Gordon]

Although it has been stated that stars 'usually do not collide' when galaxies do, I don't recall ever reading about what DOES happen when they do? Undoubtedly this happens, right?

[JohnD]

Gordon,
When stars in a galaxy are about four light years apart, there is an (insert suitable Hitchers Guide to the Galaxy adjective) amount of space in which to miss each other.
This has been wonederfully modelled, for instance at: http://uk.youtube.com/watch?v=m4bMMOyj2xw, which looks remarkably like the recent APOD! Look for the blue, star forming regions!

Much more likely is that binary stars, that orbit each other, will eventually loose enough energy to crash together, as in this simulation of two neutron stars: http://uk.youtube.com/watch?v=qPyGvq0AVUQ

John

[rigelan]

Irregular galaxies may not "necessarily" result from mergers. That is simply the process that produces most of them. It's possible that there are other causes as well. The LMC, for instance, which is an irregular galaxy showing a subtle barred structure, is probably a spiral that has been distorted by gravitational interactions with the Milky Way, and maybe the SMC as well. There may not have been an actual collision.

That's what I meant to say anyway. They result from some anomolous "external" (As in a second galaxy, or dark matter or something) gravitational force.