Re: lightcone output with a flat backward surface

From: Volker Springel <vspringel_at_MPA-Garching.MPG.DE>
Date: Mon, 9 May 2022 21:52:12 +0200

Dear Yucheng,

Let me perhaps first try to clarify the type=3 lightcone option. This is still a normal lightcone in the sense that all particles you get are at locations where they are overrun by the spherical backwards light-surface with respect to a fiducial observer position. The collection of all these particles in comoving coordinates (out to some maximum redshift) would fill a spherical ball around the fiducial observer. Keeping them all would give you an all-sky redshift survey, keeping only a cone gives you a pencil beam, and cutting everything away except for a disk-like region in comoving coordinates is what type=3 yields. This is mostly interesting for visualization purposes only.

Sure, you could create a fiducial lightcone in which light propagation is treated as parallel in the way you imagine. This could be accomplished by modifying/replacing the functions lightcone_test_for_particle_addition() and lightcone_is_cone_member_basic(). The code to setup the BoxList[] array should also be changed to only enumerate the replicas that are needed in this case. I think such a thing might perhaps be most useful for simulations where a light cone output already stops at high redshift, for example for simulations studying cosmic reionization. If the simulation box covers only a small viewing angle at high z, then the parallel approximation would be natural and could be realized much more efficiently than the spherical lightcone.


> On 15. Apr 2022, at 06:46, Yucheng Zhang <> wrote:
> Dear Volker and Gadget users,
> I have been wondering if it's possible to generate lightcone output with a flat backward surface (i.e. assuming that light travels in parallel to a certain direction instead of a point observer)?
> In the documentation, the type=3 (disk) seems to be a similar option but I'm a bit confused about what's the relation between (a_start, a_end) and the additional thickness parameter.
> The reason this might be useful is that we can then run the simulation in a long box assuming that the lightcone backward surface is moving from one end to the other.
> Then the output lightcone is simply the same box including all the particles with the long side being the redshift.
> Compared to pencil beams, in some discussions where geometry is not very important, this simplification could be more efficient.
> Thanks in advance for any help or suggestions!
> Best regards,
> Yucheng
> --
> Yucheng Zhang
> Ph.D. Candidate | CCPP | Physics | NYU
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Received on 2022-05-09 21:52:13

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