# Perpendicular peculiar velocity

1. Nov 23, 2014

### sketos

Hello,

I have data from a simulation of galaxies with various properties and I want to retrieve the peculiar velocity. If we want to relate with real galaxy surveys, what we can measure from real data is the radial peculiar velocity.

My question is, since we have a simulation i imagine we can measure the perpendicular velocity, but how?? Is there an equation which relates the perpendicular velocity of galaxies with other properties (i.e. the radial velocity)

P.S : to calculate the radial velocity I had the redshift of each galaxy with and without peculiar velocities.

Cheers

2. Nov 23, 2014

### Simon Bridge

... depend son the data in the simulation.
The peculiar velocity is not usually related to the radial... but to the angular velocity.

... which you cannot do because you do not know how to get the peculiar velocity.

3. Nov 23, 2014

### sketos

I don't think we are on the same page here. Since we are talking about mock catalogues of galaxies produced in simulation we have knowledge of individual velocity of the galaxies.

For example as I mentioned above we have the cosmological redshift of each Galaxy which corresponds to the velocity due to cosmic flow and I also have the observational redshift which corresponds to the velocity due to cosmic flow + radial component of the galaxy's peculiar velocity.

In other words I can actually measure the radial component of the peculiar velocity and my question remains... Is there a way to calculate the perpendicular peculiar velocity of each Galaxy and how?

The angular velocity has nothing to do with it.

4. Nov 24, 2014

### Simon Bridge

In that case you need to start by defining "peculiar velocity".
You simulation data gives you a 3D velocity vector wrt something right?
The use of "radial component" usually implies there are angular components.

5. Nov 24, 2014

### sketos

unfortunately i dont have such a vector in the data i received ( that would make my life easier ). I have distances, angular coordinates, observational and cosmological redshift and various other properties (i.e. luminocities, SFR, etc). I am not sure if i can measure the tangential component of the peculiar velocity ( so in a sense my question is quite arbitrary ).

Is there another way to compute, since i dont have a 3D velocity vector?

6. Nov 24, 2014

### Simon Bridge

Do you have angular components data with time?
Maybe none of the galaxies has significant enough angular displacements?
But start with an 3xplicit definition of what you want to find.

7. Nov 24, 2014

### sketos

My final goal is to compute the pairwise peculiar velocity dispersion. Normally in real galaxy surveys, one has to model the pairwise velocity dispersion with the two-point correlation function but since i have simulated data should be able to compute it directly.

If you define the pairwise velocity as: u12(r)=u1(x)-u2(x+r) then the pairwise velocity dispersion should be
σ12(r)=<(u12(r)-<u12(r)>)2/3>1/2

I thought to take as u2=ur2+up2, where ur and up are the radial and perpendicular components of the peculiar velocity (for each galaxy).

That would be straightforward if i had the orthogonal components of peculiar velocity for every galaxy . I don't have time varying data, it's just a mock catalog of galaxies. ( i know i am not giving you much to work it through but this is what i know so far )

8. Nov 24, 2014

### Chronos

You cannot derive a Doppler shift from motion perpendicular to the line of sight. So, I'm curious if you are asking if it can be inferred.