APOD: A Galaxy Collision in NGC 6745 (2012 Sep 30)

[APOD Robot]

A Galaxy Collision in NGC 6745

Explanation: Galaxies don't normally look like this. NGC 6745 actually shows the results of two galaxies that have been colliding for only hundreds of millions of years. Just off the above digitally sharpened photograph to the lower right is the smaller galaxy, moving away. The larger galaxy, pictured above, used to be a spiral galaxy but now is damaged and appears peculiar. Gravity has distorted the shapes of the galaxies. Although it is likely that no stars in the two galaxies directly collided, the gas, dust, and ambient magnetic fields do interact directly. In fact, a knot of gas pulled off the larger galaxy on the lower right has now begun to form stars. NGC 6745 spans about 80 thousand light-years across and is located about 200 million light-years away.

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[/b]

[Beyond]

Hey, the "knot" is FUN

[Moonlady]

In fact, a knot of gas pulled off the larger galaxy on the lower right has now begun to form stars. NGC 6745 spans about 80 thousand light-years across and is located about 200 million light-years away.

The term collision is here not right...it should be copulation, because the result of joining two galaxies made the new knot of gas, where baby stars are growing

[FLPhotoCatcher]

I've sometimes wondered about the gas molecules or atoms and why they collide so much - are they really so much closer to each other compared to their size than stars? Is there an electro-magnetic force that extends past the electrons that causes more collisions than might be expected?
And another question... The stars that plough through all the gas and dust - are they slowed down by it, or not? I know the light and charged particles keep the gas and dust from actually hitting the star. May the star even be speeded-up as the gas and dust is concentrated in front of the star in a bow wake, and gravity pulls the star towards the gas and dust?

[Ann]

FLPhotoCatcher wrote:
I've sometimes wondered about the gas molecules or atoms and why they collide so much - are they really so much closer to each other compared to their size than stars? Is there an electro-magnetic force that extends past the electrons that causes more collisions than might be expected?

Stars are, basically, "massive point sources" (well, from a "cosmic size" point of view), and they are also very far from one another, typically separated by a few light-years. Gas clouds are extremely spread out and extremely tenuous. But they are also so close that they are "touching" one another in a galaxy. There is no such thing as a perfect vacuum anywhere in a galaxy, meaning that all the seemingly "empty spaces" are filled with a thin, thin gas. Individual gas clouds may move in different directions and collide with each other. When gas in one cloud slams into the gas in another cloud in a collision that may happen over tens, hundreds, thousands or ten thousands of light years, many of the constituent hydrogen atoms are bound to collide with many of the constituent atoms in the other cloud. And even if the atoms don't collide "head on" in such a way that a proton smashes into another proton (which is incredibly unlikely, given the fact that protons repel each other strongly), the atoms may still "collide" so that their electrons are sent into a higher orbit by the extra energy they are imparted by the energy of the collision (whatever the word "collision" exactly implies). When the electrons then fall back into their original state, they will radiate a photon of energy.

Take a look at this image of the Vela Supernova Remnant. When the Vela supernova exploded, perhaps 10,000 years ago or so (I'm too lazy to google it, sorry), a few solar masses of gas was flung rather violently away from the exploding star. The "collision fronts" are still glowing, as the gas slams into the thin ambient gas known as the interstellar medium. In the picture, you can see what looks like a complex network of bubble shapes. The "bubbles" glow at the edges, where the actual collision is taking place. The Vela Supernova Remnant is still expanding.

Ann

[Chris Peterson]

I've sometimes wondered about the gas molecules or atoms and why they collide so much - are they really so much closer to each other compared to their size than stars? Is there an electro-magnetic force that extends past the electrons that causes more collisions than might be expected?

Well, the atoms and molecules don't actually collide, any more than stars do. Or, more precisely, you need to look at what "collide" means in this context, which (also like stars) simply means that two particles interact in a way that allows energy to transfer... which means they are close, but not actually "touching".

Gases are characterized by a parameter called the mean free path, which defines the typical distance between collisions (as described above). For the interstellar medium, the MFP is on the order of a parsec (less for ionized gases). In star forming regions, it is still on the order of astronomical units. What makes the situation different from, say, colliding galaxies, is the number of particles involved. In a galaxy, you have perhaps 10⁹ stars interacting; in an interstellar dust cloud the number of particles exceeds that by many, many orders of magnitude.

And another question... The stars that plough through all the gas and dust - are they slowed down by it, or not? I know the light and charged particles keep the gas and dust from actually hitting the star. May the star even be speeded-up as the gas and dust is concentrated in front of the star in a bow wake, and gravity pulls the star towards the gas and dust?

I don't think the material has a significant dynamical effect on stars passing through it. It's simply too tenuous.

[Joseph_Erner]

Just my opinion, but I think this would be a better picture if it also showed the smaller colliding galaxy. Is that picture available anywhere?

[geckzilla]

Just my opinion, but I think this would be a better picture if it also showed the smaller colliding galaxy. Is that picture available anywhere?

From what I understand, about a third of the smaller galaxy is in the lower right of the frame. Here's an overall view: http://www.astrosurf.com/mcianci/ngc6745.html

Classic Hubble. Images are taken for their scientific value and the aesthetic value is considered later only for the poor processors left to deal with the terrible composition. This one isn't that bad, though. There's a lot of them with big blank bars of missing data straight across the middle or entire quarters of galaxies missing.

[bystander]

Just my opinion, but I think this would be a better picture if it also showed the smaller colliding galaxy. Is that picture available anywhere?

http://cseligman.com/text/atlas/ngc67.htm
Scroll down to NGC 6745, PGC 200361 and PGC 200362

[i]Credit: Courtney Seligman[/i]

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[Boomer12k]

To Ann....Vela SN Remnant distance 11,000 - 12,300....so you are in the "ball park"...

Reminds me of a Binary Star system where the companion star siphons off gas and material from the other star...

I wonder if the INTERACTION causes central material to be so perturbed as to "fall into" the larger galaxy's Central Black Hole....as it warps on the structure quite a bit. From the picture I don't see it...but then it may just not have happened yet...

(Really trying to phrase that correctly for Chris... )

It seems like a very destructive thing....which ends up being a VERY CREATIVE THING....We got here through several Super Nova explosions....and we turned out fairly fair....(Astronomically we are sitting pretty, if not our Earthly political situations)

:---[===] *

[Boomer12k]

Oh....And we should call it ....THE LEAF GALAXY....

:---[===] *

[ta152h0]

assuming there is two objects involved known as " black holes " which would eventually merge, maybe , and pull this peculiar shape back to what is known as a spiral galaxy .........maybe. Which brings the question when two singularities touch, what happens ? I have often wondered what the surface looks like, at the atomic level where a steel beam meets air ?

[Chris Peterson]

assuming there is two objects involved known as " black holes " which would eventually merge, maybe , and pull this peculiar shape back to what is known as a spiral galaxy .........maybe. Which brings the question when two singularities touch, what happens ? I have often wondered what the surface looks like, at the atomic level where a steel beam meets air ?

I don't think severely disrupted galaxies become spirals. Spirals are an early stage in galaxy evolution. Once most of the gas and dust has become stars (as we see in all evolved galaxies) a collision will result in stars in all different inclinations... with nothing to bring them back into a plane. So an elliptical galaxy will be the result.

There are numerical simulations showing the result of two black holes colliding (not their singularities, however, since no physics describes singularities). The black holes merge after a weird little dance that results in a very briefly distorted event horizon, and then a single black hole with the combined mass of the parents.

[Ann]

assuming there is two objects involved known as " black holes " which would eventually merge, maybe , and pull this peculiar shape back to what is known as a spiral galaxy .........maybe. Which brings the question when two singularities touch, what happens ? I have often wondered what the surface looks like, at the atomic level where a steel beam meets air ?

Well, you can watch a simulation of merging black holes here.

At the atomic level the atoms, too, get smashed, so that the electrons are force-fed into the protons and the resulting neutrons are then squeezed into nothingness as far as their volume goes... right, Chris?

Ann

[FLPhotoCatcher]

And another question... The stars that plough through all the gas and dust - are they slowed down by it, or not? I know the light and charged particles keep the gas and dust from actually hitting the star. May the star even be speeded-up as the gas and dust is concentrated in front of the star in a bow wake, and gravity pulls the star towards the gas and dust?

Chris said:
I don't think the material has a significant dynamical effect on stars passing through it. It's simply too tenuous.

What if the gas was more dense, but still held at bay from the star as a bow wave? There would surely be a significant amount of mass in front of the star, and, however little, the star would be pulled towards it. If so, it would be a startling sight - a star accelerates more if there is a bunch of gas to plow through compared to if there is little gas to plow through. A question about the first law of thermodynamics springs to mind.

[Chris Peterson]

At the atomic level the atoms, too, get smashed, so that the electrons are force-fed into the protons and the resulting neutrons are then squeezed into nothingness as far as their volume goes... right, Chris?

Their volume... presumably. But their angular momentum, charge, and mass persist as... something.

[Chris Peterson]

What if the gas was more dense, but still held at bay from the star as a bow wave? There would surely be a significant amount of mass in front of the star, and, however little, the star would be pulled towards it. If so, it would be a startling sight - a star accelerates more if there is a bunch of gas to plow through compared to if there is little gas to plow through. A question about the first law of thermodynamics springs to mind.

Sure, if the star has a concentrated mass on one direction, it will experience a gravitational force. But gravity is a weak force, and the mass is very widely distributed. So like I said, I think the dynamical effects are insignificant.

[FLPhotoCatcher]

Sure, if the star has a concentrated mass on one direction, it will experience a gravitational force. But gravity is a weak force, and the mass is very widely distributed. So like I said, I think the dynamical effects are insignificant.

I don't mean, looking at it by eye, that it would LOOK significant. In theory though, it could be significant. Take things to the extreme. Imagine a wall of gas 1 million light years thick. Then send two identical stars toward it at an identical speed - say a few dozen km/sec. In front of star #1 would be an empty tunnel 3 light years in diameter. Star #2 would have to make its own path. It would be very counter-intuitive if star #2 made it through the gas a few weeks or even seconds sooner.

[rstevenson]

I don't mean, looking at it by eye, that it would LOOK significant. In theory though, it could be significant. Take things to the extreme. Imagine a wall of gas 1 million light years thick. Then send two identical stars toward it at an identical speed - say a few dozen km/sec. In front of star #1 would be an empty tunnel 3 light years in diameter. Star #2 would have to make its own path. It would be very counter-intuitive if star #2 made it through the gas a few weeks or even seconds sooner.

When I'm puzzling out something like this I try to reduce it to a simpler form to clarify my thinking. So let's try to visualize the situation this way...

Imagine a steel ball moving through space towards a sheet of soft gelatin. In the first case, imagine a hole in the gelatin large enough for the ball to pass through without touching. The ball will be gravitationally accelerated slightly as it approaches the mass of the gelatin, will pass through unscathed, and will then slow down as the gelatin pulls back on it gravitationally. Net effect: zero.

Now imagine the same situation but without the hole. The ball would again be gravitationally accelerated by the gelatin as it approached. The ball would then hit the gelatin, slowing down but passing through it, while gaining a little gelatinous mass in the process, and would then slow down as the gelatin pulls back on it gravitationally. Net effect: a heavier ball moving slower than before.

(Yes, in case 2 the ball is accelerated and decelerated a tiny bit more than in case 1, because there's a little bit more gelatin mass to pull on it. But the difference cancels out when you consider both approaching and leaving the vicinity of the gelatin. In other words, it isn't just the gelatin directly in front of the ball that pulls on it gravitationally; it's all of the gelatin.)

The situation you imagined, of a star approaching a wall of gas, is exactly analagous. In the first instance, the hole would allow the star to pass through unimpeded, but the star would still accelerate slightly as it approached the wall and then slow down after it passed through -- both changes in speed due to the gravitational attraction of the gas. In the second instance the same would happen, with the addition of the star slowing down a little as it passes through the gas, while some of the gas gets added to the mass of the star.

Does that help?

Rob

[Chris Peterson]

I don't mean, looking at it by eye, that it would LOOK significant. In theory though, it could be significant. Take things to the extreme. Imagine a wall of gas 1 million light years thick. Then send two identical stars toward it at an identical speed - say a few dozen km/sec. In front of star #1 would be an empty tunnel 3 light years in diameter. Star #2 would have to make its own path. It would be very counter-intuitive if star #2 made it through the gas a few weeks or even seconds sooner.

Any dynamical effects will be caused by gravity alone, and will be tiny. The gas is far too tenuous to offer any resistance to passage. In fact, the gas will never get close to the star that is passing through, since the solar wind from that star will disperse it for many astronomical units around.

[rstevenson]

So in a sense, Chris, the star is ensuring that only case 1 of my examples above will happen? It's making its own hole through the gas and dust, thus gaining no mass and losing no momentum.

That's the trouble with analogies -- they don't always work.

Rob

[Ann]

Behold Vela X-1, whose bow shock doesn't slow it down!

Ann