Studies in interacting galaxies a thesis submitted for the degree of Doctor of Philosophy in the faculty of science, Bangalore University, Bangalore P. M. S. Namboodiri [Ph.D Thesis]

Thesis Supervisors R. K. Kochhar and B. C. Chandrasekhara

Doctor of Philosophy Indian Institute of Astrophysics, Bangalore 1989

The discovery of photographs of Palomar Sky revealed that galaxies are peculiar galaxies from the Survey Programme conclusively
not island universes evolving slowly in isolation but they interact in various ways with their environment which consists of satellites, neighbouring galaxies and large intergalactic clouds. Many objects appeared in pairs, groups and clusters. Binary galaxies represent an important feature of the extra-galactic universe. The optical observations of the powerful radio source Cygnus A showed that it is an unusual system consisting of two galaxies. Numerical simulations carried out by Toomre & Toomre (1972)
convincingly demonstrated that tidal forces play an important role in the dynamics of interacting galaxies. They could
reproduce the features of four well-known interacting systems Arp 295, M 51 + NGC 5195 NGC 4676 and NGC 4038/39.
Gravitational interaction between galaxie~ could profoundly alter their morphological features. Sometimes the outer parts of a galaxy pair are distorted into long bridges and tails. The presence of multiple nuclei in irregular galaxies enhances the importance of tidal interaction. Recent observations using advanced techniques point out the importance of star formation in interacting galaxies. Observations also suggest that many compact elliptical galaxies formed as a result of gravitational interaction. The study of interacting galaxies is, therefore , vital in the sense that it may give clues to the origin and evolution of
galaxies. In the present work, we have used numerical simulations to study various processes associated with
colliding galaxies. It is commonly assumed that tidal encounters produce most damage'in the outer layers of a galaxy
while leaving the inner part relatively unaffected. The material escaping from the outer parts may form intergalactic
filaments and tails. The restricted three-body treatment has been found useful in this investigation. This method has been
used to study the formation of bridges and tails in interacting galaxies. The important parameters in a pair of
galaxies undergoing collision are the impact parameter, the velocity of the encounter and the mass of the galaxies.
Various combinations of these collision parameters have been used to study the tidal distortions produced in the outer
parts of a disk galaxy due the passage of point mass perturber. The perturber is moving in the plane of the test
galaxy along a parabolic orbit. The sense of rotation of the disk is the same as the direction of motion of the perturber.
It is shown that in grazing collisions, well-developed bridges and tails are formed when the two galaxies have comparable
mass. The test galaxy remained unaffected when the perturber s significantly less massive than the test system while a
massive perturber produced considerable damage to the test galaxy. It is also shown that similar types of bridges and tails are formed if the masses and distances are chosen in a particular way.
The restricted three-body treatment gives a quali tati ve picture of the distortions caused in the outer parts of a galaxy. A quanti tati ve study of the interactions require the use of self-consistent N-body simulations. The tidal effects on a spherical galaxy due to a massive companion is studied using Aarseth's N-body 2 code. It is flexible, makes no assumption about the geometry of the system and is easy to adapt to a variety of applications. The spherical galaxy consists of 250 particles and the perturber is a point
mass. The distance of closest approach is chosen such that collisions does not lead to a merger. A wide range of density
ratios and eccentricities of the relative orbit have been considered. The disruption of the test galaxy has been
observed when the numerical value of the fractional change in the energy is greater than two and in this case the test
galaxy loses more than 40 % of its mass. Considerable amount of orbital angular momentum is transferred to the test galaxy
during the course of an encounter. A large fraction of the transferred angular momentum is carried away by the escaping
particles. The changes in the energy and angular momentum show smooth variation with time in the case of unbound orbit
encounters and irregular variation in bound orbit cases. The half-mass radius of the test galaxy remains almost the same as
that of" initial test galaxy. For a constant pericentric distance, increasing the density ratio decreases the tidal effects. The dependence of tidal effects on eccentricity of the relative orbit has also been estimated. N-body simulation
results are compared with those of earlier workers and also with estimates obtained from analytical computations.
Throughout the thesis, sections, figures, and tables are numbered sequentially. Equations within a chapter are also numbered sequentially. The references are given at the end of the thesis.