Preliminary Results of Virtual Reality Mapping of E+A Galaxies and Candidates in Nearby Rich Clusters of Galaxies

The study of nearby rich clusters of galaxies is made more difficult by our two-dimensional perspective on these three-dimensional clusters. While we have some information on the distance of the objects in these clusters from, for example, velocity data, it can be a challenge to understand intuitively the 3D structure of these galaxies from viewing plots and diagrams on 2D images or digital screens. Building a 3D intuition for these data sets can create the opportunity for new scientific insights.

One powerful tool toward this end is virtual reality (VR), which allows for the creation of immersive 3D interactive applications. VR technology has advanced rapidly over the past few years. Commercially available headsets made by companies such as HTC, Valve, and Oculus (a subsidiary of Facebook) can display HD images to each eye and track their positions and orientations in 3D space using either built-in or external sensors. These systems allow the user to view a virtual space in real time and interact with it using controllers or even the user's hands (also tracked by the same sensors). All of this processing and presentation can be done with a system costing less than half as much as a new flagship smartphone. Some headsets like the Oculus Quest can run independently, using built-in processors to run the VR experience, while others such as the Oculus Rift and the HTC Vive connect to desktop computers to provide a more powerful computing platform.

VR is often marketed for use with video games, but this technology also has a place in scientific research. Herein, we describe an application we developed that uses the Unity engine to create visualizations of E+A galaxies in the Leo, Virgo, Hercules, and Coma clusters of galaxies. The Unity engine is a piece of software that facilitates the creation of interactive applications. Unity allows a developer to create virtual objects that can be associated with C# scripts which tell them how to behave, often in complex ways. Unity also handles lighting, object collisions, controller input, basic physics, and other features. For our application, this means that Unity can handle taking input from VR controllers, using that input to make a change to the virtual environment (for instance moving a plot around or opening a menu), and displaying the result through the headset back to the user.

E+A galaxies are a type of post-starburst galaxy in which all star formation has been recently (within ∼1 Gyr) quenched. Using the Sloan Digital Sky Survey Data Release 16 and the Extended Virgo Cluster Catalogue (Kim et al. 2014) the Y/Dim collaboration has compiled a catalog of E+A galaxies and candidates (Falcone et al. 2021) following the prescription described in Greene et al. (2021). Figure 1 shows the view through an Oculus Rift headset of our interactive visualizations of the Coma and Virgo Clusters. These two-dimensional images unfortunately do not capture the true experience of studying through VR, but give some sense of the visualization capabilities of the software. Detailed analysis of the three-dimensional distributions of the E+A's with respect to the other galaxies in the clusters are ongoing.

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In this note, we report two interesting features that have been observed, which can be seen in Figure 1. The first is a "nose" of low velocity objects in the R.A.–decl. center of the Virgo cluster. With the VR application, we can clearly see the nose from any angle where velocity is not along the line of sight. The second feature is a string of eight E+A galaxy candidates near the center of the Coma Cluster (Ostling et al. 2021). These candidates are notable because they appear to lie within a galaxy filament identified by Malavasi et al. (2020) that runs eastward out from the cluster. This set of E+A candidates and their co-location within the filament would be difficult to discern from a standard 2D view of a crowded field. Using our VR application, we are able both to see the location of these galaxies and to highlight the filament of which they appear to be a part. The latter was accomplished by drawing translucent red cylinders across the filament's known location. Finding the location for these cylinders took some effort; while we knew their location in R.A.–decl.–Velocity space, we needed to find their location on Unity's internal coordinate space in order to draw them as virtual objects (the two coordinate systems being related by a linear isomorphism). To determine where in Unity's coordinate space to draw the cylinders, we temporarily placed test galaxies at the R.A., decl., and velocity where the cylinders' endpoints should be, recorded where in Unity's coordinate space the test galaxies were located, and then calculated the center, length, and rotation of the cylinders necessary to position them properly onto those endpoints.

The VR visualizations described above are available at https://github.com/aggliu/VR-Galaxies-Viewer (Liu 2021a).

A set of test interactives for this project using Unity web player that can be viewed without a VR headset are available at https://aggliu.github.io/GalaxyInteractives (Liu 2021b). This VR application is currently being further used to study other parts of the Y/Dim data set (e.g., Quayum et al. 2021; Wurmser et al. 2021) and continues to be updated for use in a wide variety of astrophysical applications.

We thank the other members of the Y/Dim Collaboration for valuable discussions, assistance with data processing, and testing of the VR systems: Julia Falcone, Spencer Greenfield, Maya Joyce, Nicole Kerrison, Mariarosa Marinelli, William Ostling, Rafid Quayum, Rosemary Williams, and Serena Wurmser. This work was supported by the Alfred P. Sloan Foundation via the SDSS-IV Faculty and Student Team (FaST) initiative, ARC Agreement SSP483, and by NSF grants AST-1852355, 1852360, 1460939, and 1460860 to the American Museum of Natural History and CUNY College of Staten Island.