Formation of a Spiral Galaxy in a Major Merger

We use numerical simulations to examine the structure of merger remnantsresulting from collisions of gas-rich spiral galaxies. When the gas fraction ofthe progenitors is small, the remnants structurally and kinematically resembleelliptical galaxies, in agreement with earlier work. However, if the progenitordisks are gas-dominated, new types of outcomes are possible. In fact, we showthat a prominent disk may survive in certain cases. To illustrate thisscenario, we analyze an extreme example with progenitor galaxies consisting ofdark matter halos, pure gas disks, and no bulges, as might be appropriate formergers at high redshifts. While rapid star formation triggered by tidaltorques during the merger forms a central, rotating bulge in the remnant, notall the gas is consumed in the burst. The remaining gas cools very quickly andsettles into an extended star-forming disk, yielding an object similar to aspiral galaxy, and not an early type galaxy. This is contrary to the usual viewthat major mergers invariably destroy disks. The morphological evolution ofgalaxies can therefore be more complicated than often assumed, and inparticular, theoretical constraints based on the fragility of spiral disks needto be reevaluated.Comment: submitted to ApJL, 4 figure