I'm trying to convert heliocentric redshifts to galactocentric redshifts (or radial motions). I first tried to find some information about this conversion from the internet, but I didn't find any detailed description of the conversion. So I decided to try to figure it out myself.

I did find out some values for solar vector. One paper said it is 233 km/s to the direction l = 93 deg, b = 2 deg (they also mentioned alternative value of 213 km/s to same direction). My first problem is that I don't know to which coordinate system this refers. At this point, this doesn't matter much, because NED gives all kinds of coordinates for the objects, so I can just try them all, and see which one gives correct results for some known reference values. But I do have to quess that b-value refers to latitude.

Next, I though that the conversion could be done like this:

- calculate the angular distance between the object you're interested and the solar vector direction:

a = sin(SVlat) * sin(OBJlat) + cos(SVlat) * cos(OBJlat) * cos(SVlong - OBJlong)

(SV = solar vector, OBJ = object which redshift is being converted, lat = latitude and long = longitude.)

- calculate how much the solar vector effects to the direction of the object in question:

v_SV->OBJ = v_SV * cos(a)

(v_SV->OBJ = solar vector's velocity component to the object's direction, v_SV = velocity of solar vector.)

- subtract the result from the radial motion of the object:

v_GAL = v_OBJ - v_SV->OBJ

(v_GAL = object's galactocentric radial motion, v_OBJ = heliocentric radial motion of the object.)

When I calculate this with an example object for which I already know the galactocentric and heliocentric radial motions, I don't get correct values with any coordinates given by NED.

What am I doing wrong (in addition to being stupid enough to even try to do this)?

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"Stupidity gets denser in a crowd" - Old Finnish saying. [My website] [Nimblebrain forums]

lat and long coordinates. Picking where you are looking from, especially from earth, involves a ton of formulas. If you use the center of the galaxy as your zero point on a graph, you may simplify your efforts. First assume that everything the distance the sun is from the center of galaxy is rotating at a constant speed. Next find another object approximately the same distance from center but at least 45 degrees angle from GC. Thus forming a triangle with the two other sides 62.5 degrees each. Use this model to plug in all values and see if any math matches either redshift system. If not, then nevermind.

I found what I'm looking for from LEDA, it has suitable equation in the page describing the vgsr parameter. It seems to work.

Thanks anyway for the effort, Fr. Wayne.

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"Stupidity gets denser in a crowd" - Old Finnish saying. [My website] [Nimblebrain forums]

It looks excellent. I'm sure somebody double-checked the math already, so it's a keeper.