View Full Version : Are the M51 and M101 groups receding from us?

2008-Dec-03, 04:44 PM
I am currently looking to find M51 and M101.

I have found M81 and M82 and understand that due to mutual gravitational attraction their M81 Group may not be moving away (receding) from us (the Milky Way), although the Virgo cluster galaxies are certainly receding from us.

I would be very grateful for any advice - I know that the RV and red-shift data on the SEDS website and Wikipedia can sometimes be misleading to amateurs.


Edmonton, AB, Canada

2008-Dec-03, 05:10 PM
For such nearby objects, one correction that will be important is for our own motion with respect to the center of the Milky Way (and further to our best estimate of the Local Group's center of mass. Of these, the Milky Way correction is better determined (not a surprise).

A quick way to find these is to look up a galaxy in NED (the NASA Extragalactic Database, ned.caltech.edu). For M81, this shows a heliocentric redshift z=-0.000113, which expressed as a Doppler velocity is -34 km/s (in approach). (Heliocentric redshifts are in the Sun's reference frame, considered as in linear motion, with the Earth's orbital component removed). Scrolling down in the NED listing you come to V (Galactocentric GSR) standing for Galactic Standard of Rest, our best guess of the center-of-mass reference frame of the Milky Way after taking away the Sun's orbital motion. This is +73 km/s for M81. Similarly, there is a (less well-determined) estimate of the correction for the Local Group center of mass frame, which is +108 km/s. We do expect the masses of such adjacent groups to affect the local Hubble expansion, but the net redshifts are still positive between the Local Group and M81.

2008-Dec-03, 10:13 PM
Thanks very much for such a comprehensive answer. The NED link is most useful.

I would re-cap what I have learnt from this thread, seds.org and ned.caltech.edu as follows:

1) A measure of the velocity of a galaxy would depend on the point of reference (Sun, galaxy centre, group centre etc) of the observor as well as the object's activity within its own group - a galaxy might thus have a negative red-shift and still be receding from us.

2) Members of nearer groups beyond the Local Group such as M81 can still gravitationally interact with the Local Group so any current recession on their part may not necessarily be long-term.

3) There is a clearer pattern of recession away from us in somewhat further groups such as M51 group, M101 group etc.

4) The attractive effect of massive of groups such as the Virgo cluster counteracts the general trend of recession.