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 dynamical properties of a simulated disk galaxy assembled hierarchically in the ΛCDM cosmogony. At 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 disk-like component largely supported by rotation. These components are also clearly recognized in the surface brightness profile of the galaxy, which can be very well approximated by the superposition of an R 1/4 spheroid and an exponential disk. However, neither does the dynamicallyidentified spheroid follow de Vaucouleurs' law nor is the disk purely exponential, a result which calls for caution when estimating the importance of the disk from traditional photometric decomposition techniques. 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 found, as in the Milky Way, in the double-exponential vertical structure of the disk and in abrupt changes in the vertical velocity distribution as a function of age. The dynamical origin of these components offers intriguing clues to the assembly of spheroids and disks in the Milky Way and other spirals. 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: nine out of ten stars presently in the old (τ ∼ > 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.

The role of mergers and halo spin in shaping galaxy morphology

“…In fact, throughout this paper κrot will represent a whole class of kinematic morphological parameters, most of which are based on the distribution of the 'orbital circularity' parameter . This parameter is usually defined as = jz/j(E), where j(E) is the maximum specific angular momentum possible for a star with specific binding energy E (Abadi et al 2003), or as V = jz/rvc(r), where vc(r) = GM (< r)/r is the circular velocity at the distance r (Scannapieco et al 2009(Scannapieco et al , 2012. Some examples of such circularity-based morphological parameters include the discto-total ratio, defined as the fraction of stars with sufficiently circular orbits, typically D/T = f ( > 0.7) ≈ f ( V > 0.8) (Aumer et al 2013;Marinacci, Pakmor & Springel 2014), or the bulge-tototal fraction, usually defined as B/T = 2 × f ( < 0) (but see Martig et al 2012;Zavala et al 2016, for an improved B/T measurement).…”

Constraining the Milky Way assembly history with Galactic Archaeology

“…They could also have been created through accretion of galaxy satellites (Abadi et al 2003;Meza et al 2005), where thick disc stars then have an extragalactic origin.…”

The relation between galaxy morphology and colour in the EAGLE simulation

“…Snyder et al 2015) or quantifying the rotational support based on stellar motions (e.g. Abadi et al 2003;Sales et al 2012;Dubois et al 2016;Rodriguez-Gomez et al 2017). Although the results of these methods correlate, the scatter is large and photometric decompositions tend to result in lower bulge-to-disc ratios (e.g.…”