Elliptical galaxies/stellar halos connection

TL;DR

This paper investigates the connection between the stellar halos and the inner regions of early-type galaxies, using deep photometry and spectroscopy to understand their formation histories and compare observations with cosmological simulations.

Contribution

It introduces a method to measure the transition radius between galaxy inner regions and halos, linking structural and kinematic properties to galaxy formation models.

Findings

Identification of potential discontinuities between galaxy cores and halos.

Correlation of transition radius with galaxy size and stellar mass.

Comparison of observed transition features with simulation predictions.

Abstract

Cosmological simulations predict that early-type galaxies (ETGs) are the results of extended mass accretion histories. The latter are characterized by different numbers of mergers, mergers mass ratios and gas fractions, and timing. Depending on the sequence and nature of these mergers that follow the first phase of the in-situ star formation, these accretion histories may lead to ETGs that have low or high mass halos, and that rotate fast or slow. Since the stellar halos maintain the fossil records of the events that led to their formation, a discontinuity may be in place between the inner regions of ETGs and their outer halos, because the time required for the halos stars to exchange their energies and momenta is very long compared with the age of these systems. Exquisite deep photometry and extended spectroscopy for significant samples of ETGs are then used to quantify the occurrence and significance of such a transition in the galaxies structural and kinematical parameters. Once this transition radius is measured, its dependency with the effective radius of the galaxies light distribution and total stellar masses can be investigated. Such correlations can then be compared with the predictions of accreted, i.e. ex-situ vs. in-situ components from cosmological simulations to validate such models.