### distance calculation and circularities

Moupiya Maji
• 18 Aug '17

Hi, I am interested in calculating the distances between galaxies (to study properties of dwarf galaxies within a certain distance of a host galaxy). When I do this , do I have to take into account of the periodic boundary conditions? For example: dx= x1 - x2

if(dx>boxlength/2) {dx=dx-boxlength} if(dx< (-boxlength/2)) {dx=dx+boxlength}

Also, if I should take care of the boundary conditions, would the above formula be correct? I am concerned as the box seems to run from 0 to L(75000 kpc/h) and not -L/2 to +L/2. Thanks, Moupiya Maji

Dylan Nelson
• 20 Aug '17

Hi Moupiya,

You are right, the box is [0,L] and not [-L/2,+L/2]. And yes your formula is the correct one for a distance vector `dx`, which should then e.g. also be applied to `dy` and `dz` separately.

Moupiya Maji
• 1
• 24 Aug '17

Thanks a lot! I have another question regarding the supplementary catalog containing Stellar Circularities. What does the Snapshot_N/SubfindID field refers to? Is this the index into the Subfind Subhalos properties table? In snapshot 135, there are total 4366546 subhalos; so would a subfind id 739606 refer to the 739606th subfind halo properties?

The description of this catalog says data is available for all subhalos with stellar mass (inside twice the stellar half mass radius) bigger than 10^9 solar mass and that there are 27345 entries for snapshot 135. But if we follow the above procedure than within these subhalos the minimum mass is 10^8.847657 and there are 28982 subhalos within min-max mass range in the entire subhalo catalog. In general I find there are 22561 subhalos with stellar mass (inside twice the stellar half mass radius) bigger than 10^9 solar mass. Are there some other cuts in selecting these 27345 galaxies?

Dylan Nelson
• 25 Aug '17

Hi Moupiya,

You are correct that "a subfind id 739606 refers to the 739606th subfind halo". We use the terms "ID" and "index" fairly interchangeably here.

For the second question, I find that the Illustris-1 z=0 subhalos included in the circularities SubfindID's have a maximum stellar mass of 12.328782 and a minimum stellar mass of 9.000102 in log10. This should be the `SubhaloMassInRadType` field. Maybe check your unit conversion here to solar masses (i.e. missing little `h` factor?).

Moupiya Maji
• 25 Aug '17

Thanks a lot Dylan! I was indeed missing the little h in the mass calculations.

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