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Svemir
2007-Jan-24, 09:38 AM
Hi, this question is inspired by the thread of Wilson on ATM (dualisam or whatever- he mixes expansion of spacetime with the kinetic movement of local group of galaxies).
Here we are, Andromeda and Milky way are rushing to each other at the relative speed of 300km/s, which means a collision will occur 3 billion years in the future giving the distance of 2 billion lightyears.
I did not self check out if the numbers fit, but let's say they do.
(taken from 2 differnet sites in a few seconds)
Question:
5 billion years ago these two galaxies were at the distances where/when the gravity was
{5 billion x (NUMBER_OF_SEC_IN_A_YEAR) [SEC] X 300 [SEC)KM/SEC]} SQUARED weaker.
Which force made them on the collision course?

Amber Robot
2007-Jan-24, 02:52 PM
You're assuming they were going at constant speed for those 5 billion years, right?

Tobin Dax
2007-Jan-24, 03:58 PM
You're assuming they were going at constant speed for those 5 billion years, right?

And along the same path they're on now.

Tim Thompson
2007-Jan-24, 04:41 PM
5 billion years ago these two galaxies were at the distances where/when the gravity was
{5 billion x (NUMBER_OF_SEC_IN_A_YEAR) [SEC] X 300 [SEC)KM/SEC]} SQUARED weaker.
Which force made them on the collision course?
This is a false assumption. Milky Way (http://www.seds.org/messier/more/mw.html) & M31 (http://www.seds.org/messier/m/m031.html) were nothing like that far apart. Both galaxies are members of the local group (http://www.seds.org/messier/more/local.html) of galaxies, all gravitationally bound together. Aside from the radial (along the line of sight) velocity of 300 km/sec (see below), there is a considerable but as yet unmeasured transverse (perpendicular to the line of sight) velocity component (see, however, Loeb, et al., 2005 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ApJ...633..894L&amp;db_key=AST&amp;d ata_type=HTML&amp;format=&amp;high=4366fa465123900)). Neither the radial nor transverse velocities can be treated as constant. It just happens that the current heliocentric radial velocity of M31 is 300 km/sec. The galaxies are actually closing on each other at closer to 100 km/sec.

The two galaxies have probably never been more than (just a wild guess) about twice as far apart as they are now (lest they be no longer gravitationally bound). They are milling about in the group, bound together by gravity. And gravity is the force that pulls them together.

Kaptain K
2007-Jan-25, 07:52 AM
Also note that the transverse component of the velocity is unknown and is probably not zero.

BigDon
2007-Jan-25, 10:20 AM
Kaptain, what makes the transverse component of the velocity not knowable at the present time?

Nereid
2007-Jan-25, 12:53 PM
Kaptain, what makes the transverse component of the velocity not knowable at the present time?M31 is too far away for us to measure any proper motions (= the component of the velocity transverse to the radial) in the several hundred to thousand km/sec range yet, using any current technique.

See, however, the Loeb et al. paper that Tim Thompson references; here is the abstract:
A major uncertainty in the dynamical history of the Local Group of galaxies originates from the unknown transverse speed of the Andromeda galaxy (M31) relative to the Milky Way. We show that the recent VLBA measurement of the proper motion of Andromeda's satellite, M33, severely constrains the possible values of M31's proper motion. The condition that M33's stellar disk has not been tidally disrupted by either M31 or the Milky Way over the past 10 billion years favors a proper motion amplitude of 100+/-20 km s-1 for M31 with the quadrant of a negative velocity component along right ascension and a positive component along declination strongly ruled out. This inference can be tested by future astrometric measurements with SIM, Gaia, or the SKA. Our results imply that the dark halos of Andromeda and the Milky Way will pass through each other within the next 5-10 billion years.

Kaptain K
2007-Jan-25, 01:17 PM
Thanks Nereid for answering this question.

BTW, from the abstract you quoted:

...Andromeda's satellite, M33...
Since when is M33 (the Pinwheel galaxy in Triangulum) a "satellite of M31". I thought that it was one of the three major members of the local group! M33 is 3/4 million light years from M31 so I would say "loosely bound pair" rather than "satellite". Also, M33 is approaching the MW at 24 km/sec, as compared to M31's 100 km/sec. So even if it is "bound', it is becoming even less so as time goes on.

Tobin Dax
2007-Jan-25, 01:31 PM
Thanks Nereid for answering this question.

BTW, from the abstract you quoted: [ ...Andromeda's satellite, M33...]

Since when is M33 (the Pinwheel galaxy in Triangulum) a "satellite of M31". I thought that it was one of the three major members of the local group! M33 is 3/4 million light years from M31 so I would say "loosely bound pair" rather than "satellite". Also, M33 is approaching the MW at 24 km/sec, as compared to M31's 100 km/sec. So even if it is "bound', it is becoming even less so as time goes on.

What you said. M32 and NGC 205 are much closer to the definition of satellites I'd use. I'll have to look at that later when I'm in the office and can get the to ApJ article.

Svemir
2007-Jan-26, 11:40 AM