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

Abstract

We present a detailed analysis of the disk component of a simulated galaxy formed in the ΛCDM cosmogony. At redshift z = 0, two distinct dynamical components are easily identified solely on the basis of the orbital parameters of stars in the galaxy: a slowly rotating, centrally concentrated spheroid and a disklike component largely supported by rotation. The disk may be further decomposed into a thin, dynamically cold component with stars on nearly circular orbits and a hotter, thicker component with orbital parameters transitional between the thin disk and the spheroid. Supporting evidence for the presence of distinct thick- and thin-disk components is provided, as in the Milky Way, by the double-exponential vertical structure of the disk and in abrupt changes in the vertical velocity distribution as a function of stellar age. The dynamical origin of these components offers intriguing clues to the assembly of spheroids and disks in the Milky Way and other spiral galaxies. The spheroid is old and has essentially no stars younger than the time elapsed since the last major accretion event, ~8 Gyr ago for the system we consider here. The majority of thin-disk stars, on the other hand, form after the merging activity is over, although a significant fraction (~15%) of thin-disk stars are old enough to predate the last major merger event. This unexpected population of old-disk stars consists mainly of the tidal debris of satellites whose orbital plane was coincident with the disk and whose orbits were circularized by dynamical friction prior to full disruption. More than half of the stars in the thick disk share this origin, part of a trend that becomes more pronounced with age: 9 out of 10 stars presently in the old (age of ≳10 Gyr) disk component were actually brought into the disk by satellites. By contrast, only one in two stars belonging to the old spheroid are tidal debris; the rest may be traced to a major merger event that dispersed the luminous progenitor at z ~ 1.5 and seeded the formation of the spheroid. Our results highlight the role of satellite accretion events in shaping the disk, as well as the spheroidal, component and reveal some of the clues to the assembly process of a galaxy preserved in the detailed dynamics of old stellar populations.