redshift to z value

[nytecam]

Hi - I'm trying to work out the 'distance' of a galaxy group but only know individual galaxy radial velocities eg RV +6214 [range +5624 to +7409] .  Is there a site that converts RV to z value with minimal fuss eg no maths ! TIA 

[ollypenrice]

http://www.convert-me.com/en/convert/length/auredshift.html

Any good? Giving the result in kilometres is a bit quaint though. Megaparsecs might have been less cumbersome! I haven't tested this site, by the way.

Olly

[robin_astro]

Hi Maurice,

The full formula can be found here (equation 9)

http://ned.ipac.caltech.edu/level5/Hogg/Hogg3.html

but for small velocities like this a simple non relativistic calculation is close enough and is simple to calculate as ~ RV(km/s)/300000  

Cheers

Robin

[nytecam]

http://www.convert-me.com/en/convert/length/auredshift.html

Any good? Giving the result in kilometres is a bit quaint though. Megaparsecs might have been less cumbersome! I haven't tested this site, by the way.

Olly

Thanks Dolly (can't stop auto correct!) But this site formula just shifts the decimal point 3places to left eg RV 6000 = z6.000 which can't be right:-)

[nytecam]

Hi Maurice,

The full formula can be found here (equation 9)

http://ned.ipac.caltech.edu/level5/Hogg/Hogg3.html

but for small velocities like this a simple non relativistic calculation is close enough and is simple to calculate as ~ RV(km/s)/300000  

Cheers

Robin

Thank Robin - at what range does relativistic effects become significant for the simple rule? 1BLY?

[George Jones]

Hi - I'm trying to work out the 'distance' of a galaxy group but only know individual galaxy radial velocities eg RV +6214 [range +5624 to +7409] .

RV in what units?

Unfortunately, in cosmology, several different definitions of distance and speed are used. These different definitions give the same results for small cosmological distances, but differ at large distances. It is not that one is correct and the others are wrong, it is just that they are different.

I might have a go tomorrow at creating a table and a graph for this.

The full formula can be found here (equation 9)

http://ned.ipac.caltech.edu/level5/Hogg/Hogg3.html

but for small velocities like this a simple non relativistic calculation is close enough and is simple to calculate as ~ RV(km/s)/300000  

Equation (9) is a formula for special relativity that is being used in the context of general relativity. There is some justification for doing this, but it is somewhat nonstandard.