HYDRODYNAMICS OF HIGH-REDSHIFT GALAXY COLLISIONS: FROM GAS-RICH DISKS TO DISPERSION-DOMINATED MERGERS AND COMPACT SPHEROIDS

Abstract

Disk galaxies at high redshift (z~2) are characterized by high fractions of\ncold gas, strong turbulence, and giant star-forming clumps. Major mergers of\ndisk galaxies at high redshift should then generally involve such turbulent\nclumpy disks. Merger simulations, however, model the ISM as a stable,\nhomogeneous, and thermally pressurized medium. We present the first merger\nsimulations with high fractions of cold, turbulent, and clumpy gas. We discuss\nthe major new features of these models compared to models where the gas is\nartificially stabilized and warmed. Gas turbulence, which is already strong in\nhigh-redshift disks, is further enhanced in mergers. Some phases are\ndispersion-dominated, with most of the gas kinetic energy in the form of\nvelocity dispersion and very chaotic velocity fields, unlike merger models\nusing a thermally stabilized gas. These mergers can reach very high star\nformation rates, and have multi-component gas spectra consistent with\nSubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas\nare also characterized by highly dissipative gas collapse to the center of\nmass, with the stellar component following in a global contraction. The final\ngalaxies are early-type with relatively small radii and high Sersic indices,\nlike high-redshift compact spheroids. The mass fraction in a disk component\nthat survives or re-forms after a merger is severely reduced compared to models\nwith stabilized gas, and the formation of a massive disk component would\nrequire significant accretion of external baryons afterwards. Mergers thus\nappear to destroy extended disks even when the gas fraction is high, and this\nlends further support to smooth infall as the main formation mechanism for\nmassive disk galaxies.\n