Stephen Skory  committed a5999e4

Adding a post about Rockstar.

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File content/attachments/HOP_halo0002_0.png

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File content/post/Rockstar.rst

+The Rockstar Halo Finder in yt
+Over the last few weeks, Matt Turk, Christopher Moody, and Stephen Skory
+have been working to improve the integration of the Rockstar halo finder in yt.
+Rockstar was written primarily by Peter Behroozi and has a main website
+`here <>`_.
+Linked there is the source and the most current edition of the method paper
+which includes a timing and scaling study.
+Rockstar is a six dimensional halo finder, meaning that it considers both
+particle position and momentum when locating dark matter halos.
+It is also capable of locating bound substructure in halos and producing
+a detailed merger tree.
+As of this writing its main deficit is that it cannot
+handle simulations with varying particle mass.
+This means that in simulations
+that include star particles, the star particles must be excluded for the
+purposes of halo finding.
+Also, Rockstar cannot analyze "zoom-in" or "nested" simulations.
+Here is a brief list of the main improvements:
+  * Improved the Cython Rockstar wrapper to allow multiple reader tasks.
+    For large datasets on parallel disk systems using multiple reader tasks
+    can significantly speed up the overall analysis.
+  * It is now possible to load Rockstar halos off disk (for post analysis)
+    that behave like all the other halo objects available in yt (HOP, FOF).
+    It is possible to access the halo particle data, and the particle data
+    is only loaded on demand.
+  * Additionally, Rockstar halos have attached to them supplementary
+    information pulled directly from the Rockstar output.
+    In particular, if there is merger tree information, it is stored there.
+  * Rockstar should now build automatically with the rest of yt when the
+    ```` is used.
+  * Rockstar can work "inline" with Enzo, meaning that it can locate halos
+    from a running Enzo cosmology simulation without first staging the data
+    to disk.
+The full documentation on how to run Rockstar is available
+`here <>`_.
+Examples of Substructure Location
+One of the compelling features of Rockstar is the ability to identify bound
+substructure of halos.
+Below are two images showing the halos identified by HOP and Rockstar
+over-plotted on a projection of gas density.
+Note that the circles mean different things in the two cases.
+In the case of HOP, the circles show the radius from the center of mass to the
+*most distant particle*,
+while for Rockstar it is from the center of mass to the *calculated virial
+Paying attention to the central region of the halo, notice how Rockstar
+identifies the small in-falling subhalos that HOP doesn't. This is not
+surprising because HOP is not designed to detect substructure.
+.. attachment-image:: HOP_halo0002_0.png
+   :width: 1605
+   :height: 1500
+   :scale: 33 %
+   :alt: HOP Output
+   :align: center
+.. attachment-image:: ROCK_halo0006_0.png
+   :width: 1605
+   :height: 1500
+   :scale: 33 %
+   :alt: Rockstar Output
+   :align: center
+In the Rockstar image, the halos on the periphery are not encircled due to the
+way the image was prepared.