dendrogram

A dendrogram is a familiar type of organized diagram of a tree (tree being a class of graph, as per mathematics' graph theory, specifically, a graph is a tree if it has interconnections that together span all its nodes, but not forming any loops). Though the term dendrogram is not commonly used, they are very common, e.g., in genealogy, to show the relationships between ancestors or descendants of an individual, and in evolutionary biology, to similarly show the relationships between species.

Hierarchical relationships within astronomical data can be illustrated using dendrograms. They are of current interest as a means to illustrate hierarchies within intensity-mapping data, grouping together the more-intense regions that are close to each other. In this case, for example, two small regions of high intensity spatially residing within a larger region of high-but-somewhat-less intensity would be "leaves" branching from a node indicating the larger region, and so on, hierarchically. Such trees (and dendrograms) can be constructed algorithmically.

In the study of molecular clouds and star formation, hierarchical trees are created from intensity data of a molecular hydrogen tracer such as carbon monoxide, which can be displayed as dendrograms. This can be done for such data mapped in PPV space, providing clues toward theorizing the actual intensity in PPP space, i.e., locating molecular clouds, and their denser parts, such as dense cores. The tree-structured data also offers the possibility of (further) automating analysis, e.g., analyzing large amounts of astronomical survey data into star-formation-relevant information.