Dissipational versus dissipationless galaxy formation and the dark matter content of galaxies

Abstract

We examine two extreme models for the build-up of the stellar component of luminous elliptical galaxies. In one case, we assume the build-up of stars is dissipational, with centrally accreted gas radiating away its orbital and thermal energy; the dark matter (DM) halo will undergo adiabatic contraction and the central DM density profile will steepen. For the second model, we assume the central galaxy is assembled by a series of dissipationless mergers of stellar clumps that have formed far from the nascent galaxy. In order to be accreted, these clumps lose their orbital energy to the DM halo via dynamical friction, thereby heating the central DM and smoothing the DM density cusp. The central DM density profiles differ drastically between these models. For the isolated elliptical galaxy, NGC 4494, the central DM densities follow the power laws r -0.2 and r -1.7 for the dissipational and dissipationless models, respectively. By matching the dissipational and dissipationless models to observations of the stellar component of elliptical galaxies, we examine the relative contributions of dissipational and dissipationless mergers to the formation of elliptical galaxies and look for observational tests that will distinguish between these models. Comparisons to strong lensing brightest cluster galaxies yield median (M */L) B ratios of 2.1 ± 0.8 and 5.2 ± 1.7 at z 0.39 for the dissipational and dissipationless models, respectively. For NGC 4494, the best-fit dissipational and dissipationless models have (M */L) B = 2.97 and 3.96. Comparisons to expected stellar mass-to-light ratios from passive evolution and population syntheses appear to rule out a purely dissipational formation mechanism for the central stellar regions of giant elliptical galaxies. © 2010. The American Astronomical Society.