Purely dry mergers do not reproduce the observed evolution of early-type galaxies

Alessandro Sonnenfeld1, Carlo Nipoti2, Tommaso Treu1,3

1University of California Santa Barbara, 2Bologna University, 3University of California Los Angeles

Observations: the slope of the total density profile from strong lensing and stellar kinematicsWhat is the density profile of ETGs and how does it evolve? Strong gravitational lensing is an excellent tool for studying the mass content of ETGs, as it allows for measurements of the total mass of objects at cosmological distance with a few percent accuracy. Stellar kinematics information, i.e. velocity dispersion measurements, provide an independent constraint on the gravitational potential that can be used in combination with strong lensing to measure the slope of the total density profile. In practice, this is done by fitting a power-law density profile to the lensing and velocity dispersion measurements:

We measured the density slope for a sample of 83 lenses in the redshift range 0 < z < 0.8 (Sonnenfeld et al. 2013) and were able to constrain the evolution of this quantity. We found that ETGs evolve at approximately constant density slope.

Schematics of a density slope measurement in a strong gravitational lens system. Strong lensing gives the total projected mass within a cylinder of radius Reinst, while stellar kinematics is mostly sensitive to the mass enclosed within the sphere of the scale of the half-light radius. By fitting a power-law density profile to these two independent probes of the gravitational potential we can measure the slope of the total density profile.

IntroductionEarly-type galaxies (ETGs) are believed to grow as a result of mergers, but the details of this process are still largely unknown. Do the mergers involve significant amounts of gas (wet) or are they dissipationless (dry)?Theoretical studies aimed at matching the observed size evolution of quiescent galaxies have focused on dry mergers, as the low star formation rates measured in these galaxies leaves little room for a significant occurrence of wet mergers. The predicted and observed merger rates seem to be able to account for the late (z < 1.5) size evolution of quiescent galaxies. Can we then explain all observations with dry mergers? What about the evolution of the density profile?

Sonnenfeld et al. (2013). Denstiy slope vs. redshift (left) and vs. Stellar mass density (right). The density slope anti-correlates with the former and correlates with the latter. Since ETGs become less concentrated with time (decreasing stellar mass density), the net result is that ETGs evolve while keeping their density slope approximately constant.

Does the dry merger evolution scenario reproduce the observation of a density slope that stays roughly constant with time? We used numerical simulations to answer this question. We first looked at the effect of isolated dry mergers on the density slope with high-resolution N-body simulations, then folded the results into a model for the evolution of the population of massive ETGs between z=1 and z=0, where galaxies grow as a result of dry mergers with rates suggested by the Millennium simulation.This purely dry merger model predicts a strong decrease of the density slope with time, ruled out by observations with a 3-sigma significance. A purely dry merger scenario then, while reproducing the observed size evolution of ETGs, fails in matching density slope observations.

Toy model: adding dissipationA more successful model might be one where we allow for some dissipation. We test this scenario with a toy model, where we assume that a small fraction of the accreted baryons is in the form of gas. This gas falls to the center of the galaxy and forms stars, while the density profile contracts adiabatically. Allowing for dissipation results in a better agreement with observations.

Sonnenfeld et al. (2014). Top panel: change in density slope for unit increase in stellar mass as a function of merger mass ratio for a series of binary dry mergers with different initial conditions. All mergers tend to decrease the density slope. Minor mergers (left side of the plot) have a stronger impact on the slope than major mergers, for the same accreted mass.Bottom panel: evolution of the average density slope for a mock population of massive ETGs (solid line) and as observed with strong lensing and stellar kinematics (shaded region). The discrepancy between model and observation is larger than 3-sigma.

Comparison with theory: predicted dry merger evolution of the density slope

Sonnenfeld et al. (2014). Evolution of the average density slope for a mock population of massive ETGs where 10% of the accreted baryonic mass is in the form of gas (solid line) and as observed with strong lensing and stellar kinematics (shaded region). Predictions for a few cosmological simulations are also shown.References:

Nipoti C., Treu, T., Leauthaud, A., et al. 2012, MNRAS, 422, 1714Sonnenfeld, A., Treu, T., Gavazzi, R., et al. 2013, ApJ, 777, 98Sonnenfeld, A., Nipoti, C., Treu, T. 2014, ApJ, 786, 89