The discrepancy between dynamical and stellar masses in massive compact galaxies traces non-homology

Abstract

For many massive compact galaxies, their dynamical masses (Mdyn ασ2re) are lower than their stellar masses (M*). We analyse the unphysical mass discrepancy M*/Mdyn > 1 on a stellarmass-selected sample of early-type galaxies (M* ≥ 1011M⊙) at redshifts z̃0.2 to z̃1.1.We build stacked spectra for bins of redshift, size and stellar mass, obtain velocity dispersions, and infer dynamical masses using the virial relationMdyn = K σ2e re/G with K = 5.0; this assumes homology between our galaxies and nearby massive ellipticals. Our sample is completed using literature data, including individual objects up to z̃2.5 and a large local reference sample from the Sloan Digital Sky Survey (SDSS). We find that, at all redshifts, the discrepancy between M* and Mdyn grows as galaxies depart from the present-day relation between stellar mass and size: the more compact a galaxy, the larger its M*/Mdyn. Current uncertainties in stellar masses cannot account for values of M*/Mdyn above 1. Our results suggest that the homology hypothesis contained in theMdyn formula above breaks down for compact galaxies.We provide an approximation to the virial coefficient K ̃ 6.0[re/(3.185 kpc)]-0.81[M*/(1011M⊙)]0.45, which solves the mass discrepancy problem. A rough approximation to the dynamical mass is given by Mdyn ̃ [σe/(200 km s-1)]3.6[re/(3 kpc)]0.352.1 × 1011M⊙. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.