Simulations of Galaxy Formation in a Lambda CDM Universe I: Dynamical and Photometric Properties of a Simulated Disk Galaxy

Abstract

We analyze the properties of a disk galaxy simulated with unprecedented numerical resolution in the Lambda CDM cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z~1 which lead to the formation of two distinct dynamical components: a spheroid and a disk. The surface brightness profile is very well approximated by the superposition of an R^{1/4} spheroid and an exponential disk. The surface brightness profile is remarkably similar to that of Sab galaxy UGC615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies ~ 1 mag off the I-band Tully-Fisher relation of late-type spirals, but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. The angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This reflects the dominance of the slowly-rotating, dense spheroidal component. The disk component, on the other hand, has properties rather similar to those of late-type spirals. This suggests that a different form of feedback than adopted in this simulation is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as Lambda CDM remains a challenging, yet so far elusive, proposition.