M.Sc Student Shaked Yaniv Parameters of the Local Group of Galaxies Using N-Body Simulation with MOND Department of Physics Professor Adi Nusser

Abstract

The Local Group (LG) of galaxies contains 46 galaxies within a distance of 5 Mpc from our Galaxy.  We are provided with measurements of redshift, luminosity and angular position of each galaxy. The distances are known for some of the galaxies, but are only estimated for the others using dynamics models. The transverse velocities of the galaxies are, also, not directly observed.

Given these measurements, we model the back-in-time behavior of the LG using N-Body simulations, in which galaxies are represented by point particles. The initial conditions for these simulations are from 12 sets of possible solutions of the Least Action method. The masses of the galaxies are estimated using a global value Mass to light ratio (M/L).

We assume that at early times the galaxies have been close to one another. When we run the simulation back in time, we determine the lookback time when galaxies were nearest to each other. We identify this time as the age of the universe, and tune the M/L ratio in order to get a cosmic age between 14-15 Gyrs.    We use two kinds of N-Body simulations. One uses Newton's Law, and the other uses MOND (Modified Newtonian Dynamics) as an alternative theory of gravity.

In Newtonian dynamics we obtain M / L = 75 • MΘ / LΘ with a cosmic age of 14.4 Gyrs, and in MOND we obtain M / L = 0.5 • MΘ / LΘ with a cosmic age of 11.7 Gyrs. A cosmic age of 15 Gyrs is obtained in MOND only for M/L as low as  0.0001 • MΘ / LΘ, which is unreasonable value. We, therefore, demonstrate that if we decrease a0 in MOND by two orders, to be 8•10-11[cm/sec2] we get a cosmic age of 14.3 Gyrs from the simulation. The 12 different initial conditions we use from the Least Action method give approximately the same results.