To accomplish the goal of enabling the broad availability of merger histories for galaxies, the project has set out a number of specific tasks.
1. Development of data processing methods
To extract information from a galaxy with shells, quantitative information is needed both on the host galaxy and on the shell system.
First, it is necessary to identify which galaxies do possess stellar shells and then the positions of these shells need to be precisely measured. On a small scale, it is possible to identify shell galaxies and then measure their positions simply by eye, or at least by semi-automated methods under human supervision, as it is the case for essentially all current research in the field – but for such a huge dataset as expected from LSST, an automated procedure is needed for both tasks
To infer the merger time, it is necessary to compare observed distribution of shell radii with their modeled evolution. To model the evolution, we need to derive the gravitational potential of the host galaxy from the measured light profile of the visible matter, using various well-proven tools such as IRAF and GALFIT, but in configurations specifically optimized for this specific purpose. After assuming an appropriate relation between the visible light and the absolute amount of present mass (including the dark matter), the host gravitational potential can be finally extracted.
2. Merger time calculations
The gravitational potentials serve as an input for algorithms computing the evolution of shell radii for the respective host galaxy. The lower limit of the merger age is then straightforwardly given by the radius of the outermost observed shell in the galaxy. Further constraints come from the number and spacing of the shells. The application of existing methods to a specific galaxy requires human insight when comparing the predicted shell systems with the observed ones. The main goal of this task is thus to find optimal methods of automated application of the calculated models to real data.
3. Application of the newly developed tools to current survey data
Based on a complete distance- and magnitude-limited sample of galaxies drawn from the 2MASS catalog, we select a few thousand galaxies as the basic sample. We inspect the images visually obtain several hundreds of shell galaxies, which we then use as for the training of machine-learning techniques. We also process the Legacy Surveys data for the selected galaxies using the aforementioned methods, working towards a fully automated method of creating a catalogue. Finally, using the methods of merger time calculations, we create a catalog of several hundreds of galaxies with estimated times of mergers.
4. Study of correlations between mergers and transient events
We employ the newly created catalog of merger histories to explore the impact of galaxy merger history on other astrophysical phenomena happening in these galaxies. We identify suitable streams and catalogues of various kinds of transient events in galaxies – short-lived, dramatic events, such us supernova explosions or gamma-ray bursts. We cross-correlate the events from these sources with the catalogue of galactic merger events and investigate the possible links between merger history and rates of transient events. This will help to form hypotheses that can be confirmed or disproved by upcoming LSST data that will enable larger statistics on both the galaxy mergers and the variable and transient objects and events.
Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor REA can be held responsible for them.