In this Master's thesis, I have studied the properties of disk galaxy merger remnants formed in numerical simulations. I examined the star formation histories, black hole accretion, and the evolution of the total black hole mass during a 3 Gyr period in nine disk galaxy mergers with initially different galaxy progenitor masses, black hole masses and gas mass fractions. The simulations used in my analysis were performed by Johansson et al. (2009a) using the TreeSPH-code GADGET-2.
My primary aim was to study the termination of star formation in the simulated mergers but I also examined the maximum rates of star formation and black hole accretion in these mergers and calculated the evolution of the galaxy colors during the merging process. I found the progenitor gas mass fraction to be the dominating effect in determining the star formation rate, black hole accretion rate, and the amount of black hole mass growth. A higher gas mass fraction systematically resulted in a higher star formation rate, black hole accretion rate, and larger final black hole mass.
In the evolution of the galaxy colors, the dominating effects were the progenitor mass ratio and the progenitor gas mass fraction. Mergers with a higher gas mass fraction systematically resulted in redder remnants than corresponding mergers with a lower gas fraction. Mergers with a lower progenitor mass ratio resulted in redder remnants than mergers with the same black hole mass and gas fraction but a higher progenitor mass ratio. The redness of the low mass ratio mergers is due to the efficient black hole feedback which heats cold star-forming gas and drives it out of the galaxy.
The color evolution of the simulated merger sample indicates that gas-rich disk galaxy mergers can, indeed, form red and dead remnants. This is in agreement with the merger scenario for early- type galaxy formation which states that star-forming gas-rich disk galaxies can result in red and non-star-forming ellipticals through merging.
