Andromeda Galaxy

[purintjp]

As I understand it, the galaxies moving towards us are in our local

galactic cluster. Since Andromeda is moving towards us at the current

time I was wondering if using the shrinking universe analogy

that is used to speculate that most galaxies that are moving away

from us at one time were much closer to us at creation would

suggest that Andromeda was farther away from us at our creation times.

It would seem that this would have to be true or Andromeda would have already collided with us during expansion.

Would the distances between now and then jibe with the rate at which

it is coming towards us without being too far away at creation to

be affected by gravitational forces ? It would seem that it has been

coming towards us for an awfully long time to still be 1 million light years

away.

[Martin]

As I understand it, the galaxies moving towards us are in our local galactic cluster...

 

That's more or less right. If the distance to a galaxy is decreasing then chances are it is in some sense a "neighbor". Either in our local group, or in a neighboring cluster like the Virgo cluster. People talk about somewhat larger groupings called superclusters.

 

Galaxies do have random motions which could be towards or away or in any direction. You have to get fairly far away before the expansion effect completely dominates that random motion.

====

 

Now you are asking about inferring back in time---where was Andromeda relative to us?

 

I don't know of that kind of computer modeling being done and I don't know that it could be done with any reliability at present, because I don't think we know Andromeda's trajectory accurately enough.

 

All I can do is discuss possibilities with you. You know two objects can orbit each other indefinitely, getting closer and then farther and then closer and then farther, without ever colliding.

 

Two stars can do this---it is just a case of elliptical orbit.

 

Then after many orbits something could cause one of them to VEER and they may then collide. The random flyby of a third star could destabilize the orbit of the first two. I think you can picture that.

 

People do make computer models of the solar system and run them backwards in time as well as forwards.

 

But I never heard of someone modeling our local group of galaxies. It is complicated, I think there are some 10-20 large-ish things plus a lot of little bitties. I don't think we have observed these things enough to really know the speed and direction of each one. I don't see how we could expect to make a good model and run it back in time.

 

As far as I know, Milky and Andromeda have been orbiting each other, both members of this local group, for as long as they've existed.

 

Now the distance between M and A is closing, which by itself does not mean they will collide (a comet can approach the sun without hitting the sun). But some people predict collision on this pass, and that seems quite reasonable to expect. I've heard that a lot. But that doesn't mean that M and A haven't been orbiting each other in the past without colliding.

 

I have to pass, therefore. I don't have any hard information. Maybe DH or somebody else has some.

[purintjp]

Thank you Martin for your always concise explanations and time.

I hadn't considered the possiblility or them orbiting each other.

That being said I did some quick calculations and using a speed

of 130 km/s being the total approach speed giving a speed of

about .0004*c they would only have traveled about 4,000,000 lt/yrs

in 10 billion years. I know that's a rough estimate but doesn't seem

to give enough time for many orbits. Am I missing something ?

[Martin]

My hat is off to you, P. I did not stop to do a calculation. I am just on the point of going out and can't check your arithmetic, but assuming that is right then there seems time for only something on the order of one orbit, if that.

I will double check when I get back. My guess now is that you are not missing anything. Can't be more specific.

[louis wu]

The Milky Way is indeed expected to collide with Andromeda sometime soon (within a billion years or so), transforming our aesthetically pleasing spiral galaxy into a mishmash elliptical one. As tangential velocities are difficult to measure for galaxies there is some doubt over the event.

 

Incidentally our whole galactic supercluster is on a journey to and presumed collision course with something called 'The Great Attractor'. The Great Attractor is at least 250 million light years away and may be 500 million or more light years away. Details on this huge gravitational anomaly are scarce as it lies in the galactic plane so we cannot get a good view of it. Events could prove very interesting when we get there, though the Sun will have burned out long before.

[Martin]

As tangential velocities are difficult to measure for galaxies there is some doubt over the event.

...

 

Exactly, I think that's a good point to bring out. Using dopplershift we can measure the radial velocity (of approach) very accurately. But any "sideways" drift is almost impossible to measure.

 

I remember some years back seeing a computer animated movie simulating the collision of the two galaxies and it was spectacular. Flinging different parts every whichway. Passing thru or around and then falling back together etc. And after quite a lot of milling around, we settled down into an elliptical galaxy (not as pretty as a spiral, you rightly note.)

 

But the computer simulation was made based on, I believe, some arbitrary assumption about the sideways component. The overall scenario is plausible but there is indeed "some doubt" as to details of the expected merger.

 

Incidentally our whole galactic supercluster is on a journey to and presumed collision course with something called 'The Great Attractor'.

 

I haven't kept up with the latest on estimated motions. My memory is that our galaxy (and indeed the Local Group we belong to) is going 500-600 km/s in the general direction of Hydra-Centaurus (the direction of the presumed G.A. concentration of mass). And the nearest really big cluster is the Virgo cluster, also estimated to be on roughly the same course.

 

The Local Group could be seen as a kind of minor satellite of the Virgo cluster and so we can think in terms of a Virgo supercluster, that we belong to (consisting of the Virgo cluster and surrounding smaller stuff like our Local Group.

 

So I guess the conjecture was that the Virgo supercluster was collectively going 500-600 km/s in the Hydra-Centaurus direction.

 

Could you review for us how we can tell? How are these velocities measured/estimated?

 

It is an interesting topic, I think.

 

The estimates I recall seeing were of velocities with respect to the microwave background rest frame. Is that your impression too?

 

I would say that the only velocities of this sort that we can measure reliably are the solar system's relative to CMB, and the solar system's relative to galactic center. Then doing a vector subtraction one gets the velocity of the Milky galactic center relative to CMB.

 

So Milky's velocity relative to the microwave background has been determined with pretty good precision.

 

But how to go from there to an estimate of the Virgo cluster's speed and direction? It seems bound to involve a lot more guesswork.

 

BTW I enjoyed Larry Niven's Ringworld---the characters especially Nessus associated with your namesake.

 

I hope you'll post some source material on these neighborhood velocities. I've lost track of whatever old links, and there has probably been some more recent stuff.

[purintjp]

Good stuff guys!!

I hadn't ever heard about the "Great Attractor". Can someone provide a

well-respected link to learn about it.

 

Also, I was wondering how far away would we have to be from

Andromeda before gravity would be overcome and the expansion of

space would take over. (Approx.- I'm assuming that no other gravitational force would interfere)

[Martin]

Good stuff guys!!

I hadn't ever heard about the "Great Attractor". Can someone provide a

well-respected link to learn about it.

 

well, I started a Great Attractor thread. It has some links. We heard a lot about the Great Attractor back in the 1980s or 1990s. Later some people seem to have found that it was not as massive as initially supposed and there is another large concentration of galaxies out beyond it that may be more important. I'm not sure about this. Anyway one does not hear so much about G.A. nowadays.

 

In any case it was never imagined that we are getting nearer to G.A. Even back in the 1980s it was realized by the people who detected it that the distance from us to the G.A. is increasing.

 

 

Also, I was wondering how far away would we have to be from

Andromeda before gravity would be overcome and the expansion of

space would take over. (Approx.- I'm assuming that no other gravitational force would interfere)

 

That's a really good question! Are you familiar with the idea of motion relative to the microwave background? That would be a convenient way to address this. Here's a way to get a handle. The expansion of distances (between points that are not moving relative to background) is given by the Hubble rate

 

71 km/s for a distance of one Megaparsec.

 

That means that if two observers are not moving relative to background and they are separated by a distance of one Megaparsec, then the distance that separates them is growing by 71 km/s.

 

Now Andromeda is approaching at some speed that is on the order of that, for simplicity say it is exactly 71 km/s.

 

Maybe it is 50 and maybe 100 but this doesn't matter, call it 71.

 

A parsec is 3.26 lightyears. A Megaparsec is 3.26 million lightyears.

 

To oversimplify let's pretend Milky is stationary and Andromeda is moving towards us at 71.

 

Then if you kept everything the same and put Andromeda 3.26 million lightyears away, the expansion of the distance would just cancel Andromeda's random motion.

 

I'm not saying that gravity would be overcome, only that Andromeda's motion toward us would be overcome. (If it were magically repositioned that far away from us, everything else the same.) I haven't completely answered your question, but maybe this gives a rough idea. Some quick-and-dirty handle on it.

 

To be sure, double the distance. Imagine putting Andromeda 6 million lightyears away. That would surely break it loose. It would continue drifting away forever. Goodbye Andromeda.

 

Someone else may be willing to make a more detailed analysis, using estimates of the masses of the two galaxies including the surrounding clouds of dark matter and all that. But my impression is that we do not have reliable estimates of the relevant masses. The mass figures have changed over the years. Look up mass of Milky on google and see what you get. I've seen 400 billion solar and 1000 billion solar. I've seen Milky estimated more massive than Andromeda and less massive. Mass estimates of galaxies involve guesswork. But the actual observed speed, from Doppler, is more directly measurable.

Edited February 1, 2010 by Martin

[purintjp]

Thank you Martin for the reply. I had not heard of the measuring to

the CMB until this very thread.

 

What you replied goes to the very heart of my original thread post.

If you assumed that the two galaxies are 12 billion years old and ran

the scenario backwards the distance between the galaxies would

have been 2.5M lt/yrs + 5M lt/yrs to equal 7.5 M lt/yrs. I assume

that the Hubble constant would have been different 12 Billion yrs. ago

but the math is probably beyond me. Seems like there might be a chink

in the armor of something.

[Martin]

Seems like there might be a chink

in the armor of something.

 

There is a chink! But the chink is a flaw in your argument (not in mainstream cosmology). You assume that Andromeda has a constant velocity towards us for all that time. No reason to assume that.

It might have been moving away from us part of that time and then later started closing in. You really can't estimate what the distance was 12 billion years ago, merely by assuming constant velocity.

 

The structure-formation people do this kind of thing with elaborate computer models where they simulate the formation of clusters of galaxies. To do it properly takes a lot of numbercrunching.

 

What I gave you was a hypothetical example, because you asked how far A would have to be. If you magically transported Andromeda today to 6 million lightyears away and kept everything else the same (ie same velocity relative to background, still pointed at us)...

 

I didn't suggest you try to extrapolate back into the past, that's a computer modeling job.

[purintjp]

So based on what you are telling me, it is certain that given what we

currently know about expansion and cluster formation that we can

probably be sure that at some point in time Andromeda was nudged

into our path (or vice-versa) or that the velocity was altered at some point ?

If so, could it have been collisions that it had with other smaller galaxies ?

[Martin]

... we can

probably be sure that at some point in time Andromeda was nudged

into our path (or vice-versa) or that the velocity was altered at some point ?

...

 

That is reasonable. The velocity in any case would have been changing if only because of the gravitational attraction of the two galaxies!

 

But also the velocity could have been affected by the fact that the Hubble rate has changed greatly over the past 8-10 billion years! It used to be much larger than the 71 it is today. It is still declining (but partly because of the dark energy effect the decline is slowing---as I recall H is supposed to level out around 65 or so.)

 

In order to go further with this you need to crunch some numbers.

 

Why don't you do this problem, pur?

Forget about expansion. Assume Milky is one trillion solar masses. Assume you release a brick at a distance of 3 million light years and it starts falling toward Milky. How long does it take to get to within 2.5 million light years?

Very very roughly. Get it within a factor of two of the correct answer and you are OK.

That's about the simplest problem in this general area that I can think of. If you can't tackle it then there is no way you can usefully pursue this stuff, and we can end our discussion.