Equal- and Unequal-mass Mergers of Disk and Elliptical Galaxies with Black Holes

We present binary galaxy merger simulations with varying mass ratios and different progenitor morphologies. The simulations include mergers of gas-rich disks (Sp-Sp), of early-type galaxies and disks (E-Sp, mixed mergers), and mergers of early-type galaxies (E-E, dry mergers). We follow the dynamics of gas, stars and dark matter, and include radiative cooling, star formation and black hole (BH) accretion and the associated feedback processes as in Springel et al. We study the mass assembly of the BHs and discuss technical issues of the implemented model in detail. For Sp-Sp mergers, the peak star formation rate and BH accretion rate decrease and the growth timescales of the central black holes and newly formed stars increase with higher progenitor mass ratios. The peak BH accretion rate typically occurs shortly after the time of BH merging for low progenitor mass ratios (e.g. 3:1 and lower), whereas for higher progenitor mass ratios there is no clear correlation between the peak BH accretion rate and BH merging time. The termination of star formation by BH feedback in disk mergers is significantly less important for higher progenitor mass ratios (e.g. 3:1 and higher). In addition, the inclusion of BH feedback suppresses efficiently star formation in dry E-E mergers and mixed E-Sp mergers. All merger remnants, independent of their progenitors, follow the observed relations between the central BH mass and the stellar velocity dispersion (MBH − σ), the bulge mass (MBH−M∗) and the bulge binding energy (MBH−M∗σ ), with the dominant source of scatter arising from variations in the initial gas mass fraction. The normalizations for all relations and the simulated slope of the MBH−σ and MBH−M∗σ 2 relations are in good agreement with the observations, whereas the simulated slope of the MBH −M∗ relation is slightly steeper compared to the observations. This indicates that the simple BH feedback model self-regulates the mass growth of the BHs in accordance with the observed relations for a very wide range of merger progenitors. Subject headings: galaxies: interaction– galaxies: active galaxies: evolution – galaxies: formation – methods: numerical