Simulations of Galaxy Formation in a ΛCDM Universe. III. The Dissipative Formation of an Elliptical Galaxy

We examine the dynamical structure of an elliptical galaxy simulated in the ΛCDM scenario. The morphology of the galaxy evolves dramatically over time in response to the mode and timing of mass accretion; smooth deposition of cooled gas leads to the formation of centrifugally supported disks, while major mergers disperse stellar disks into spheroids. In the case we consider here, these two modes of accretion alternate successively until z ~ 0.6, when the galaxy undergoes one last major (1 : 3) merger. The event triggers a starburst that consumes much of the remaining gas into stars. Little gas cools and accretes subsequently, and as a result, most stars at z = 0 are rather old (75% are older than 9 Gyr) and distributed in a spheroidal component that resembles present-day elliptical galaxies. Dynamically, the galaxy is well approximated by an E4 oblate rotator, with rotational support increasing steadily from the center outward. The apparent rotation support, as measured by V rot /σ, correlates strongly with isophotal deviations from perfect ellipses. Boxy isophotes are obtained when the galaxy is seen face-on and V rot /σ ≪ 1. On the other hand, disky isophotes are found for inclinations that maximize V rot /σ. The line-of-sight velocity distribution is nearly Gaussian at all radii, although small but significant deviations are robustly measured. The sign of the Gauss-Hermite skewness parameter h 3 is anticorrelated with the apparent sense of rotation, in agreement with observed trends. Despite its relatively recent assembly, the simulated galaxy has much higher effective surface brightness than normal elliptical galaxies of similar luminosity, in a way reminiscent of the less common type of M32-like "compact elliptical galaxies." This is likely a direct consequence of our star formation and feedback algorithm, which allows for efficient transformation of gas into stars in dense, early collapsing progenitors rather than a definitive prediction for the structure of galaxies assembled in this ΛCDM halo. Despite this limitation, our simulation suggests that dark matter plays a minor role in the luminous regions of compact elliptical galaxies, whose dynamical mass-to-light ratios are thus not dissimilar to those of normal elliptical galaxies. This explains the proximity of compact elliptical galaxies to edge-on projections of the fundamental plane, despite their far greater velocity dispersion at given luminosity. Overall, our simulation shows that repeated episodes of dissipational collapse, followed by merger events, lead to stellar spheroids that are only mildly triaxial and of relatively simple kinematic structure. This is in better agreement with observation than earlier models based on dissipationless mergers of stellar disks and a positive step toward reconciling the observed structure of elliptical galaxies with a hierarchical assembly process where mergers play a substantial role.

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