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Lecture 25
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Computing in Astronomy

Matthew Turk

Spring 2018

Lecture 25

Last Time

  • ODEs
  • Cython
  • Not Quite Calling External Functions

This Time

  • Calling External Functions
  • Halo Finding
    • Friends-of-friends
    • Union Find and Disjoint Sets
    • Parallelism

Calling External Functions

Cython can be used to call external C functions. To do this:

cdef extern from "something.h":
    cdef void hello_world()

This declares the function, much like in a .h file. You can declare these separately in .pxd files.

You must also link against the appropriate file or library.

Halo Finding

Think of topographic maps. We draw these by identifying individual "height" values and linking them between regions.

Halo finding in astrophysical simulations typically follows one of a handful of methods:

  • Simple Friends-of-Friends
  • Density-based estimates (i.e., eliminating saddlepoints)
  • Phase-space (6D) linking with iterative unbinding
  • Folding methods (ORIGAMI)

Friends-of-Friends

How do we identify structures?

Identify a linking length $L$ that we use to connect sets.

Union Find Algorithm

How do we do this when accumulating particles in a halo? We can use a Disjoint Set and a Union Find process.

  • For each particle, assign a unique ID. This is its set.
  • Look within the linking length radius. If any particle is part of the

We can call this a process of assigning "Contours" (or "Sets") and ensuring that they are flattened.

We will build this.