Mass-to-light ratio gradients in early-type galaxy haloes

Accepted. Received; in original form Since the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterising the halo masses from both observational and theoretical results:∇ℓΥ, the logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from ΛCDM simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies ǫSF. As a pilot study, we assemble the available∇ℓΥ data from kinematics in early-type galaxies—representing the first unbiassed study of halo masses in a wide range of early-type galaxy luminosities—and find a correlation between luminosity and∇ℓΥ, such that the brightest galaxies appear the most darkmatter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the ΛCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency ǫSF> 1. This difficulty is eased if dark haloes are not assumed to have the standard ΛCDM profiles, but lower central concentrations.