Halo Lightcone Catalogue

Previously, I created a lightcone from AHF halo catalogues, consisting of 3D positions and redshifts for each host halo, in a box of of $(x,y,z) = (60\times115,115,115)\, {\rm Mpc/h}$.

In this post, I outline my method for including subhalos, and also cutting out a wedge that corresponds to the survey volume.

Subhalos

To find the positions and velocities of each host halo as they cross the light cone, I interpolated their positions between snapshots. However, I did not do the same thing with the subhalos, because I was worried that interpolating positions and velocities for the subhalos might cause them to be off-set from their host halos in an unphysical way.

I have added in the subhalos as follows: assuming that a host halo crossed the light cone between snapshots $j$ and $j+ 1$, I found all the subhalos in that host halo at snapshot $j+1$. I then placed these subhalos in the host, and gave them same offset in position and velocity from their host that they have in snapshot $j+1$.

Note that currently we are taking the properties of the halo at snapshot $j+1$ (i.e. substructure, mass, maximum circular velocity, shape, spin, and whatever we may need). Later we might use some other criteria to decide whether to take properties from $j$ or $j+1$.

Survey volume

First, I converted the comoving $(x,y,z)$ positions of each (sub)halo to an angular position. Following Bernyk et al. 2016, I calculated a distance, RA and Dec for each halo as follows:

$$d = \sqrt{x^2 + y^2 + z^2}$$ $${\rm RA} = \arctan(y/x)$$ $${\rm Dec} = \arcsin(z/d)$$

Since our survey volume is approximately 1 square degree, I only considered (sub)halos with ${\rm RA}<1$ and ${\rm Dec}<1$ degree. Further, I only took halos with $d<60\times115$; this is because the lightcone is not complete for distances larger than this (this doesn’t actually make a difference, because there aren’t halos out that far). Finally, I recentered the survey by rotating the $(x,y,z)$ coordinates by angles $(\psi,\phi,\theta) = (-0.5,0,0.5)$.

Here is a resulting scatter plot of the halo catalogue in the (new) cartesian coordinates (for the $512^3$ simulation):

And a prettier plot, where I plot the mass density:

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