ytree: A Python package for analyzing merger trees

Britton D. Smith, Meagan Lang

Research output: Contribution to journalArticlepeer-review


The formation of cosmological structure is dominated, especially on large scales, by the force ofgravity. In the early Universe, matter is distributed homogeneously, with only small fluctuationsabout the average density. Overdense regions undergo gravitational collapse to form boundstructures, called halos, which will host galaxies within them. Halos grow via accretion ofthe surrounding material and by merging with other halos. This process of merging to formincreasingly massive halos is naturally conceptualized as an inverted tree, where small branchesconnect up to continually larger ones, leading eventually to a trunk.One of the main products of cosmological simulations is a series of catalogs of halos within thesimulated volume at different epochs. Halos within successive epochs can be linked togetherto create merger trees that describe a halo’s growth history. An example of such a mergertree is shown in Figure 1. A variety of algorithms and software packages exist for both haloidentification and merger tree calculation, resulting in a plethora of different data formats thatare non-trivial to load back into memory. A range of negative consequences arise from thissituation, including the difficulty of comparing methods or scientific results and users beinglocked into less than ideal workflows.
Original languageEnglish
JournalJournal of Open Source Software
Publication statusPublished - 18 Dec 2019


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