Spiral galaxy or Elliptical galaxy Samenness

In message , David Knisely
writes

Oh, I imagine 300 to 800 years would probably be enough, although with
extended baseline radio interferometry (and suitable reference radio
"noisy" quasars along or close to a visual line close to that of M31's
nucleus), this number range could easily be cut in half.
Clear skies to you.

That gives a whole new meaning to very long baseline interferometry!
Like starting a series of observations in Cassini's time. Thanks.
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In message , David Knisely
writes
Jonathan Silverlight wrote:

Isn't the point that the transverse velocity is unknown?

Yes, it is, but unfortunately, that isn't what many of the quotations
from press releases said. More than a few used the words "will
collide", instead of "may collide". Again, the idea of a possible
"head-on" collision gets the most attention (and maybe the most bucks
depending on the researchers). The most likely scenario is that M31
and our Milky Way galaxy will pass each other, perhaps becoming
slightly distorted due to tidal interactions, as M51 and its large
companion galaxy NGC 5195 are doing. It is just too soon to tell.

How big would it have to be before we can be certain that M31 will
miss us

Oh, I imagine 300 to 800 years would probably be enough, although with
extended baseline radio interferometry (and suitable reference radio
"noisy" quasars along or close to a visual line close to that of M31's
nucleus), this number range could easily be cut in half.
Clear skies to you.

Typing "transverse velocity of M31" into Google gives two hits, one of
which assumes it's zero http://arxiv.org/pdf/astro-ph/0210255 and the
other http://arxiv.org/pdf/astro-ph/0010480 says the velocities of
nearby galaxies are on the order of 100 km s-1, large enough to be
detected and tested by the proposed SIM and GAIA satellite missions.

RADIAL AND TRANSVERSE VELOCITIES OF NEARBY GALAXIES
P. J. E. Peebles 1, S. D. Phelps 1, Edward. J. Shaya 2,3, and R. Brent
Tully 4
1 Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544
2 Raytheon ITSS, Goddard SFC, Greenbelt MD 20771
3 U. of Maryland, Physics Dept., College Park MD, 20743
4 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822
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"Jonathan Silverlight" wrote...
in message ...

Typing "transverse velocity of M31" into Google gives two hits, one of
which assumes it's zero http://arxiv.org/pdf/astro-ph/0210255 and the
other http://arxiv.org/pdf/astro-ph/0010480 says the velocities of
nearby galaxies are on the order of 100 km s-1, large enough to be
detected and tested by the proposed SIM and GAIA satellite missions.

RADIAL AND TRANSVERSE VELOCITIES OF NEARBY GALAXIES
P. J. E. Peebles 1, S. D. Phelps 1, Edward. J. Shaya 2,3, and R. Brent
Tully 4
1 Joseph Henry Laboratories, Princeton University, Princeton, NJ 08544
2 Raytheon ITSS, Goddard SFC, Greenbelt MD 20771
3 U. of Maryland, Physics Dept., College Park MD, 20743
4 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822
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'Lo Jonathan --

This is really great stuff! In the first link you gave...

"There are 15 probable companion galaxies to M31.
Their galactic coordinates, projected position from the
center of M31 and line of velocity are listed in Table 3.
The observed radial velocities contain contributions
from the Galactic rotation and from the relative velocity
between the Galaxy and M31. Assuming that the
transverse velocity of M31 with respect to the Galaxy is
zero, we remove these contributions and list the
corrected velocities in the frame of M31 in Table 3."

To me this talks about M31's transverse velocity with
respect to any of its given companion galaxies. It is
NOT assuming M31 transverse velocity to be zero with
respect to our Milky Way. Or am i misreading it?

Also, it would be interesting to hear more detail about
how you deduced that SIM and GAIA will be able to
detect/measure Andromeda's transverse velocity!

This could really shed some light on the dynamics of
our Local Group!

happy days and...
starry starry nights!

--
A Universe of Beauty
Our Hubble often shows,
Let's not forget our duty
To pitch the way it Glows!

Paine Ellsworth

In message
, Painius
writes

This is really great stuff! In the first link you gave...

"There are 15 probable companion galaxies to M31.
Their galactic coordinates, projected position from the
center of M31 and line of velocity are listed in Table 3.
The observed radial velocities contain contributions
from the Galactic rotation and from the relative velocity
between the Galaxy and M31. Assuming that the
transverse velocity of M31 with respect to the Galaxy is
zero, we remove these contributions and list the
corrected velocities in the frame of M31 in Table 3."

To me this talks about M31's transverse velocity with
respect to any of its given companion galaxies. It is
NOT assuming M31 transverse velocity to be zero with
respect to our Milky Way. Or am i misreading it?

I think so. The other paper is only doing modelling, but they refer to
the galactocentric velocity of M31. Since we know the rotation speed of
our galaxy, I'm guessing that it simplifies the calculation if you
assume it's zero.
The sum of the line-of-sight velocities in table 3 is quite close to
zero (13 km/s) so there's no obvious bias, as there might be if the
transverse velocity is quite high.


Also, it would be interesting to hear more detail about
how you deduced that SIM and GAIA will be able to
detect/measure Andromeda's transverse velocity!

Evans et al. said that, not me, and I have no idea. I was hoping one of
the experts here would tell us. My guess is still that you can measure
the apparent movement of the centre of M31 relative to quasars, or even
stars in the halo of our galaxy.
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