Ion debris around Andromeda Galaxy
A study in 2015 measured metallic absorption lines in the light from distant quasars, passing through space around the M31 galaxy. This enabled getting the velocity of these ions relative to Earth.
They call this region the CircumGalactic Medium, or CGM.
Ions near M31's core are moving a little faster toward Earth than most of those in M31's CGM.
M31's velocity of -300 km/s is calculated from calcium ions when measuring the spectrum of the M31 galactic bulge. This M31 velocity is only motion of ions strictly in the line of sight.
These ions in the M31 CGM are measured at -150 km/s to either -280 or -306 km/s. The description is unclear whether the species and densities had apparent patterns in their distributions.
These ions, when far from M31, could be moving at an angle to our line of sight to M31.
In that context, without doing the trigonometry calculations, these velocities seem as expected.
There are 2 things I learned from this study:
1) there is an area called the CGM.
2) I read this line:
We also indicate the position of the Local Group barycenter with the green cross.
I did not know this position has been identified. Is this point in 3-D?
That barycenter statement requires questions. Does the word barycenter mean just a center of gravity, or a COG for elliptical orbits in a system in motion?
It is impossible to verify elliptical orbits in our Local Group, around a barycenter, while COG can apply to stationary objects. We have never measured the proper velocity in 3-dimensions of the large galaxies in our Local Group, including our Milky Way.
They could use the M31 calcium ion velocity as the velocity of the entire M31 galaxy, which is an invalid assumption. Unfortunately, that is in only 1 specific direction (line of sight to Earth), and not in 3 dimensions as it must be.
How did they precisely measure the mass of all the individual galaxies in the Local Group?
It is impossible to count their stars precisely, for various reasons.
That green cross indicates no range of its uncertainty.
I am surprised we know all the masses and distances with such precision.
Is that certainty appropriate?