Hi,
I am currently working with the TNG100 simulation data. In the particle data (e.g. PartType4), the velocities are in units of km*sqrt(a)/s. Where does the sqrt(a) come from? I would expect either km/s (peculiar velocity) or km*a/s (comoving velocity). If you were to calculate the angular momentum, how would you use the velocity? Would you divide or multiply by sqrt(a) or leave it as is?

Kind regards,
Marloes

Dylan Nelson

6 Dec '23

This is essentially a historical fact, due to how velocities are stored internally in the code. It has been the same since Gadget-2 in 2005. You can see additional details in the Gadget-2 users guide (page 32), and remains the same in AREPO and Gadget-4.

Hi,

I am currently working with the TNG100 simulation data. In the particle data (e.g. PartType4), the velocities are in units of

`km*sqrt(a)/s`

. Where does the`sqrt(a)`

come from? I would expect either`km/s`

(peculiar velocity) or`km*a/s`

(comoving velocity). If you were to calculate the angular momentum, how would you use the velocity? Would you divide or multiply by`sqrt(a)`

or leave it as is?Kind regards,

Marloes

This is essentially a historical fact, due to how velocities are stored internally in the code. It has been the same since Gadget-2 in 2005. You can see additional details in the Gadget-2 users guide (page 32), and remains the same in AREPO and Gadget-4.