The Evolution of Rest-Frame K -Band Properties of Early-Type Galaxies from z = 1 to the Present

We measure the evolution of the rest-frame K-band Fundamental Plane from z=1 to the present by using IRAC imaging of a sample of early-type galaxies in the Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We find that $M/L_K$ evolves as $Deltaln{(M/L_K)}=(-1.18pm0.10)z$, which is slower than in the B-band ($Deltaln{(M/L_B)}=(-1.46pm0.09)z$). In the B-band the evolution has been demonstrated to be strongly mass dependent. In the K-band we find a weaker trend: galaxies more massive than $M=2 imes10^{11}M_{odot}$ evolve as $Deltaln{(M/L_K)}=(-1.01pm0.16)z$; less massive galaxies evolve as $Deltaln{(M/L_K)}=(-1.27pm0.11)z$. As expected from stellar population models the evolution in $M/L_K$ is slower than the evolution in $M/L_B$. However, when we make a quantitative comparison, we find that the single burst Bruzual-Charlot models do not fit the results well, unless large dust opacities are allowed at z=1. Models with a flat IMF fit better, Maraston models with a different treatment of AGB stars fit best. These results show that the interpretation of rest-frame near-IR photometry is severely hampered by model uncertainties and therefore that the determination of galaxy masses from rest-frame near-IR photometry may be harder than was thought before.Comment: 5 pages, 3 figures, Accepted for publication in ApJ