Forming Early-Type Galaxies in LambdaCDM Simulations -I. Assembly histories

TL;DR

This study uses high-resolution LambdaCDM simulations to explore the assembly histories of early-type galaxies, revealing two-phase growth, the role of accreted stars, and effects on galaxy structure and dark matter distribution.

Contribution

It provides detailed insights into the two-phase assembly process and the impact of stellar accretion and feedback mechanisms on galaxy evolution in LambdaCDM cosmology.

Findings

Galaxies assemble via in situ star formation followed by accretion of older stars.

Dark matter fractions within stellar radii increase over time from 0.05 to 0.3.

Total density profiles are nearly isothermal at present day.

Abstract

We present a sample of nine high resolution cosmological simulations in the mass range of M_vir=7x10^11-4x10^12 M_sun starting from LambdaCDM initial conditions. Our simulations include primordial radiative cooling, photoionization, star formation, supernova II feedback, but exclude supernova driven winds and AGN feedback. The simulated galaxies assemble in two phases, with the initial growth dominated by compact (r<r_eff) in situ star formation fueled by cold, low entropy gas streams resulting in a very similar mean assembly redshift of z_{f,ins}~2.5 for the in situ stellar component in all galaxies. The late growth is dominated by accretion of old stars formed in subunits outside the main galaxy (r>r_eff) resulting in an assembly redshift of z_{f,acc}~0.5-1.5 with much larger scatter. We find a positive correlation between the fraction of accreted stars and the final mass of our galaxies. We show that gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy color bimodality. The accretion of stellar material is also responsible for the observed size growth of early-type galaxies. In addition, we find that the dark matter fractions within the stellar half-mass radii continuously increase towards lower redshift from about f_DM~0.05 at z~3 to f_DM~0.1-0.3 at z=0. Furthermore, the logarithmic slope of the total density profile is nearly isothermal at the present-day (gamma'~1.9-2.2). Finally, the input of gravitational heating lowers the central dark matter densities in the galaxies, with the effect being smaller compared to simulations without supernova feedback.