Hello,
The NASA/IPAC Extragalactic Database gives two velocities for each galaxy: The velocity (helio) and velocity (CMB).
I need just to know, if possible, the difference between these two velocities.
Thanks in advance.
Dear Melati Rabia
I don't know for sure - because I don't use the database - but I suppose that the velocity (helios) describes the velocity of galaxies in relation to the solar system as point of reference (the observer).
The velocity in relation to the CMBR is different because the CMBR dipole (Doppler effect) can be used to determine the velocity of the solar system in relation to the universe itself as a rest frame. Actually we can state that the CMB velocity is the absolute velocity of our solar system, inclusive the direction of the motion of the solar system (but we are orbiting the centre of the Milky Way). This makes it possible to transform the velocity (helios) into an "absolute" velocity in relation to the CMBR.
Anyway, to know the real velocity and direction of motion of a galaxy we have to measure the local CMBR and determine the dipole and that's impossible. So I assume that the velocity (CMB) is just a calculated transformation from the velocity (helios) with the local CMBR dipole of the solar system as point of reference.
With kind regards, Sydney
Hello dear colleague
Effectively, the velocity (helio) is the radial velocity or line-of-sight velocity. It denotes the speed with which the object moves away from the Earth (or approaches it, for a negative radial velocity).
Thank you so much for your answer.
Best regards
There are three basic kinds of galactic movements. Galaxies in a cluster orbit the cluster, often there is a dominant plane to their rotation. Intergalactic movement of stars in a galaxy is mostly attributed to dark matter, which I think is a wrong hypotheses. The movement of galaxies relative to us is called perspective movement which related to the Hubble constant. The movement of galaxies relative to the most distant galaxies is called their proper movement.
Independent galaxies outside a cluster move with the field that surrounds them. This can be called field flow, like the dark flow. This I believe is also the dominant force within galaxies that determines stellar speeds rather than dark matter.. Gravity can only slow down or control stellar speeds within a galaxy, not increase their velocities.
Dear honorable Forrest Noble,
Thank you very much for your response. I have other questions for you, if possible.
1/ Is the trajectory of the proper galaxies movement known?
2/ In a group of galaxies such as the M87 group for example, do the galaxies orbit around the center of the group?
3/ The CMB velocity of a galaxy, represents the velocity of its gravitation around the cluster ?, its proper velocity ?, or the velocity of its movement related to the Hubble constantt?.
Best regards
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