A Pipeline for Constructing a Catalog of Multi-method Models of Interacting Galaxies

A PIPELINE FOR CONSTRUCTING A CATALOG OF MULTI-METHOD MODELS OF INTERACTING GALAXIES Anthony Holincheck, PhD George Mason University, 2013 Dissertation Director: Dr. John Wallin Galaxies represent a fundamental unit of matter for describing the large-scale structure of the universe. One of the major processes affecting the formation and evolution of galaxies are mutual interactions. These interactions can including gravitational tidal distortion, mass transfer, and even mergers. In any hierarchical model, mergers are the key mechanism in galaxy formation and evolution. Computer simulations of interacting galaxies have evolved in the last four decades from simple restricted three-body algorithms to full n-body gravity models. These codes often included sophisticated physical mechanisms such as gas dynamics, supernova feedback, and central blackholes. As the level of complexity, and perhaps realism, increases so does the amount of computational resources needed. These advanced simulations are often used in parameter studies of interactions. They are usually only employed in an ad hoc fashion to recreate the dynamical history of specific sets of interacting galaxies. These specific models are often created with only a few dozen or at most few hundred sets of simulation parameters being attempted. This dissertation presents a prototype pipeline for modeling specific pairs of interacting galaxies in bulk. The process begins with a simple image of the current disturbed morphology and an estimate of distance to the system and mass of the galaxies. With the use of an updated restricted three-body simulation code and the help of Citizen Scientists, the pipeline is able to sample hundreds of thousands of points in parameter space for each system. Through the use of a convenient interface and innovative scoring algorithm, the pipeline aids researchers in identifying the best set of simulation parameters. This dissertation demonstrates a successful recreation of the disturbed morphologies of 62 pairs of interacting galaxies. The pipeline also provides for examining the level of convergence and uniqueness of the dynamical properties of each system. By creating a population of models for actual systems, the current research is able to compare simulation-based and observational values on a larger scale than previous efforts. Several potential relationships between star formation rate and dynamical time since closest approach are presented. Chapter 1: Introduction In terms of the large-scale structure of the universe, galaxies represent a fundamental unit of matter. Galaxies tend to be found in groups called clusters, which in turn form a hierarchy of super-clusters tracing the distribution of matter in the universe. Understanding the formation of galaxies and the manner in which they evolve over their lifetimes is important for addressing a number of important questions (Springel et al., 2005). These include what role inflation may have played in the early universe, what is dark matter, and how will the universe as a whole evolve. One of the major processes affecting the formation and evolution of galaxies are mutual interactions. These interactions can including gravitational tidal distortion, mass transfer, and even mergers. In any hierarchical model mergers are the key mechanism in galaxy formation and evolution. The various interaction effects lead to what one important review (Struck, 2006b) summarizes as five major themes to be studied: the generation of tidal morphologies, induced nuclear activity, induced star formation, the role of collisions in galaxy evolution, and the dependence of these effects on the clustering environment. This dissertation will focus mainly on the generation of tidal morphologies and address in part some aspects of induced star formation. When constructing models of specific systems, achieving a match to the tidal morphology is a necessary prerequisite for obtaining an accurate model. While it might be possible to have matched the shape correctly but have unrealistic predictions related to other characteristics such as star formation rate, it is unrealistic to claim an accurate model for a specific system without a match to the morphology. Galaxy interactions take place on timescales of a billion years or more. Even though we are able to look back through time to earlier epochs and see galaxies at many stages of interaction, we cannot hope to observe any particular system for more than just a single 1 instant in time. Because of this static view provided by observations, researchers have turned to simulations of interacting systems. Previous progress in developing detailed models of specific, observed systems has been ad hoc. The time to model individual systems has been quoted as taking as much as six months (Hammer et al., 2009). Since 2000, a number of researchers have developed semiautomated methods for trying to speed the process, (Theis and Kohle, 2001), (Wahde and Donner, 2001), (Barnes and Hibbard, 2009), (Smith et al., 2010). These methods have seen success in matching artificial systems, but their application to real sets of interacting galaxies usually requires detailed observation data beyond a simple image as well as customized fitness functions. This dissertation contributes to the study of interacting galaxies by presenting a set of software tools and processes that were used to develop models for 62 pairs of interacting galaxies. These tools incorporate the input of a large number of Citizen Scientist volunteers who helped search for and review candidate models for particular systems. As part of the research done for this dissertation, a website called Merger Zoo (Holincheck et al., 2010b) was launched to enable the contributions of volunteers from the Zooniverse community. The specific models developed with the help of the volunteers, the tools for processing and analyzing the results, and the set of evaluated results will all contribute to future work in this field. 1.1 A Short History of the Study of Interacting Galaxies There are several excellent reviews in the literature discussing the history, observational evidence, and an overview of simulation techniques for studying interacting galaxies. Three of them are Barnes and Hernquist (1992), Struck (1999), and Struck (2006b). Noting the growth and diversification in the field since the early 1970s, the authors of each paper make note of how it is not practical to give a comprehensive overview in a single volume. In the early part of the 20th century, astronomers came to realize that many of the nebulous objects they had been observing were actually galaxies of similar significance to 2 the Milky Way. With a universe full of “island universes” to catalog, Hubble proposed a classification scheme where galaxies were categorized based on their morphologies. His scheme, with modifications, is still the most widely used system. To illustrate his categories, Hubble famously proposed a tuning fork diagram with elliptical galaxies on the handle, two types of spirals on the forks, and lenticular galaxies at or near the intersection. If the fork is oriented horizontally, the handle is on the left with barred spirals on the bottom fork and non-barred spirals on the top fork. Hubble suggested a possible evolutionary sequence where galaxies would become more organized as they transitioned from ellipticals to lentinculars and then into spirals of increasingly loose spiral arms. This speculated order of galaxy types resulted in astronomers referring to ellipticals as early-type galaxies and spirals as late-type ones. This particular evolutionary path has been discredited, but yet the terminology remains. Galaxies that were unsymmetrical were called irregular galaxies by Hubble. The first type of irregulars have a rough or clumpy appearance. The second type have a smooth appearance. De Vaucouleurs added refinements such as intermediate categories on the prongs of the tuning fork and a third prong for weakly barred systems. Equipped with this classification system, astronomers of the earlyto mid-20th century began amassing catalogs of galaxies and their Hubble-types. Among the important realizations to come from this effort where that galaxies tended to exist in large clusters, and that not all galaxies could be assigned an exact Hubble type. The second realization, that some galaxies had peculiar, though not necessarily irregular, shapes led some to speculate as to the origins of these morphologies. The fact that galaxies were often found in dense clusters provided a possible explanation: that these isolated, island universes may on occasion pass close to one another and interact. These interactions may be simple close passages, collisions, or even captures resulting in mergers. One of the first to speculate that tidal forces caused peculiar morphologies was Zwicky in the 1950s. At the time his theory was considered unlikely because collisions between galaxies were thought to be improbable(Struck, 2006b) and that gravity alone would not 3 be able to cause such narrow features (Barnes and Hernquist, 1992). In the 1960s, working with the data collected from the Palomar sky survey, Arp collected images of over 300 peculiar galaxies. He published this atlas of objects with his attempt at a categorization scheme. Several attempts in the 1960s and 1970s were successful in demonstrating that tidal distortions cause by gravity could produce the thin tails and bridges seen in interacting galaxies. The researchers used a simple model for pairs of interacting galaxies. Each galaxy was treated as a point mass and is surrounded by a disk of mass-less test particles. The particles in each disk are influenced by the gravity of both galaxies. These models are referred to by several names, but will be called restricted three-body methods in this work. The results from these models showed that the more dramatic features of interacting galaxies are produced