Is the Andromeda Galaxy going to collide with the Milky Way?

[tony873004]

Is the Andromeda Galaxy going to collide with the Milky Way?

I've heard that the Andromeda Galaxy is going to collide with the Milky Way someday. I understand how its radial velocity relative with respect to the Milky Way is determined through its blueshift, but how do we know its tangental velocity? Without knowing that, how can we conclude that a collision is inevitable, rather than a hyperbolic pass or even a closed orbit of the two galaxies around each other, but without actually colliding.

 

[Chronos]

Astronomers are pretty much convinced a collision is inevitable. Current estimates indicate this will occur in about 3 billion years. To call it a collision is somewhat an exaggeration. Very few solid bodies will actually hit each other. Be more like two very diffuse swarms of gnats flying through each other. Deep space astronomers will be annoyed since Andromeda will cover most of the sky for quite awhile. See here for details:
http://www.cita.utoronto.ca/~dubinski/tflops/

 

[Phobos]

tony - You're right that there is some uncertainty in the tangential velocity. There's a chance it could "miss", but like Chronos said, most astronomers are predicting a "hit" (unclear if direct or glancing).

I put "hit" and "miss" in quotes, because either way, the two galaxies will be dramatically altered due to gravitational interactions.

 

[marcus]

... but how do we know its tangental velocity? ...

this question has not yet been addressed in this thread.
it is a good question.
all star-jocks (chronos, phobos, labguy,...) know that tangential velocity is measured by watching something over time and recording change in position

somebody must know if any tangential velocity has been detected for Andromeda. my guess is that it probably has not. which means that it must necessarily be fairly small. one maybe does not know it but nevertheless one knows that it is not big (because one has not detected it)

that is just my guess: that it is not going sideways enough to make a clean miss.

I think it's interesting and i hope you keep asking till you get something definite

 

[turbo]

that is just my guess: that it is not going sideways enough to make a clean miss.

I think it's interesting and i hope you keep asking till you get something definite

Darn, Marcus! Are you going to keep bugging him for the next couple of billion years, until he can predict a miss? That's a pretty ambitious program!

 

[tony873004]

Thanks for your responses. I wouldn't imagine that something as distant as Andromeda could have its tangential velocity measured by watching it over a long period of time. We've only had the technology to look at spectra for a couple hundred years. That's not much over the billions of years timespan predicted before the collision.

I've worked some numbers, based on each galaxy being about 1 trillion solar masses and 130,000 ly in diameter, 2.4 million ly apart, radial velocity of 118 km/s

Collision meaning that the mimimum separation will be less than their combined radii. This ignores any tidal effects that might cause the galaxies to stretch towards each other.

If tangental velocity is between 0 - 40 km/s then a collision will result.
If tangental velocity is between 40 97 km/s then the galaxies are orbiting each other in elliptical orbits.
If the tangental velocity is greater than 97 km/s then the galaxies will make a hyperbolic pass of each other.

So I guess the big question is still how well we know the tangental speed if we know it at all.

 

[marcus]

Darn, Marcus! Are you going to keep bugging him for the next couple of billion years, until he can predict a miss? That's a pretty ambitious program!

heh heh
You are right.
I have done some rough calculation and it seems to me that Andra could be rushing sideways just as fast as she is rushing towards us and that we could still not detect that sideways motion even by watching closely for a hundred years.

It would just not show up as a measurable angle against the background of more distant (presumably more stationary) objects.

And so how can they exclude the possibility that she is going to make a clean miss?

She could be going twice as fast sideways as she is coming towards us and we wouldn't know. Or would we, can you confirm any of this? I did only roughest estimates.

my handbook says she is 2.2 million LY away, or 670 kpc. you may have more recent data on the distance and on the two masses

 

[marcus]

...

I've worked some numbers, based on each galaxy being about 1 trillion solar masses and 130,000 ly in diameter, 2.4 million ly apart, radial velocity of 118 km/s
...
...
If the tangental velocity is greater than 97 km/s then the galaxies will make a hyperbolic pass of each other.
...

tony, I was glad to see someone else had calculated. I got some roughly comparable numbers.

I also used 1 trillion solar for each
I used around 2.2 million LY apart, not signif diff. from your 2.4.
I got something like they would clear if tangential is 120 km/second
which is roughly like your 97 km/s

I didnt find the minimum like you did---aparently 97.
I just found one speed that I felt would make them clear. that 120 km/s speed is 4E-4 of speed of light and I checked to see if it could reasonably be expected to be detectable in 100 years. Seemed to me it couldnt.

So maybe some expert will come down and tell us we're wrong but I don't see how they can say theyre going to collide.


except for this: where would the transvelocity come from? We know where the other velocity comes from----we are all falling towards Virgo and together with Virgo we are all falling towards Hydra----and andromeda is falling towards Milky. All we Local Group galaxies are falling towards an attractor plus Milky has been attracting Andromeda all this time. So you would EXPECT Andromeda to be drifting towards us. but I don't think you would expect Andromeda to have a large sideways velocity because where would it have come from? I know. this is pretty vague.

 

[Chronos]

You could get a rough idea by checking the redshift differential between opposing arms. I'll have to check to see if there is any published data.

 

[Labguy]

The distance to M31 was revised several years ago to 2.9 million ly. From HST Cepheid observations:
http://www.daviddarling.info/encyclopedia/A/Andromeda.html