Simulations of Galaxy Formation in a Λ Cold Dark Matter Universe. II. The Fine Structure of Simulated Galactic Disks

We present a detailed analysis of the dynamical properties of a simulateddisk galaxy assembled hierarchically in the Lambda CDM cosmogony. At z=0, twodistinct dynamical components are identified on the basis of the orbitalparameters of stars in the galaxy: a slowly rotating, centrally concentratedspheroid and a disk-like component largely supported by rotation. Thesecomponents are also recognized in the surface brightness profile of the galaxy,which can be very well approximated by the superposition of an R^{1/4} spheroidand an exponential disk. However, neither does the dynamically-identifiedspheroid follow de Vaucouleurs' law nor is the disk purely exponential, aresultwhich calls for caution when estimating the importance of the disk fromtraditional photometric decomposition techniques. The disk may be furtherdecomposed into a thin, dynamically cold component with stars on nearlycircularorbits and a hotter, thicker component with orbital parameterstransitional between the thin disk and the spheroid. The spheroid is old, andhas essentiallyno stars younger than the time elapsed since the last majoraccretion event ~8 Gyr ago. The majority of thin disk stars, form after themerging activity is over, although a significant fraction ~15% of thin-diskstars are old enough to predate the last major merger event. This unexpectedpopulation of old disk stars consists mainly of the tidal debris of satelliteswhose orbital plane was coincident with the disk and whose orbits werecircularized by dynamical friction prior to full disruption. Our resultshighlight the role of satellite accretion events in shaping the disk and thespheroidal component and reveal some of the clues to the assembly process of agalaxy preserved in the detailed dynamics of old stellar populations.Comment: 15 pages, 14 figures, submitted to Ap