The Baryon Fractions and Mass-to-Light Ratios of Early-Type Galaxies

TL;DR

This study models 22 early-type gravitational lens galaxies to determine their baryon fractions and mass-to-light ratios, revealing low star formation efficiency and the importance of adiabatic compression in modeling galaxy mass distributions.

Contribution

It provides the first joint modeling of multiple lens galaxies with stellar dynamics using standard CDM halo models, constraining average stellar mass fractions and mass-to-light ratios.

Findings

Average stellar mass fraction is 0.026±0.006 without adiabatic compression.

Including adiabatic compression raises the stellar mass fraction to 0.056±0.011.

Adiabatically compressed models are favored when matching local mass-to-light ratios and weak lensing data.

Abstract

We jointly model 22 early-type gravitational lens galaxies with stellar dynamical measurements using standard CDM halo models. The sample is inhomogeneous in both its mass distributions and the evolution of its stellar populations unless the true uncertainties are significantly larger than the reported measurement errors. In general, the individual systems cannot constrain halo models, in the sense that the data poorly constrains the stellar mass fraction of the halo. The ensemble of systems, however, strongly constrains the average stellar mass represented by the visible galaxies to $0.026\pm0.006$ of the halo mass if we neglect adiabatic compression, rising to $0.056\pm0.011$ of the halo mass if we include adiabatic compression. Both estimates are significantly smaller than the global baryon fraction, corresponding to a star formation efficiency for early-type galaxies of $10%-30%$. In the adiabatically compressed models, we find an average local B-band stellar mass-to-light ratio of $(M/L)_0 = (7.2\pm0.5)(M_{\sun}/L_{\sun})$ that evolves by $d\log(M/L)/dz = -0.72\pm0.08$ per unit redshift. Adjusting the isotropy of the stellar orbits has little effect on the results. The adiabatically compressed models are strongly favored if we impose either local estimates of the mass-to-light ratios of early-type galaxies or the weak lensing measurements for the lens galaxies on 100 kpc scales as model constraints.