The Fundamental Plane of Cluster Ellipticals at z = 1.25

Using deep HST ACS imaging and VLT FORS2 spectra, we determined the velocity dispersions, effective radii and surface brightnesses for four early-type galaxies in the $z=1.237$ cluster \cl. All four galaxies are massive, $> 10^{11} M_{\sun}$\mathrm{.}Thesefourgalaxies,combinedwiththreefrom\text{lynx}at$z=1.276$, establish the Fundamental Plane of massive early-type cluster galaxies at $\bar{z}=1.25$. The offset of the Fundamental Plane shows that the luminosity evolution in rest-frame $B$ is $\Delta \ln M/L_B = (-0.98 \pm 0.06) z$forgalaxieswith$M > 10^{11.5} M_{\sun}$. To reproduce the observed mass-to-light ratio ($M\mathrm{/}L$) evolution, we determine the characteristic age of the stars in these $M>{10}^{11.5}{M}_{\text{sun}}$ galaxies to be $3.0_{-0.3}^{+0.3}$ Gyrs, {\em i.e.} $z_{\ast }=3.4_{-0.4}^{+0.5}$. Including selection effects caused by morphological bias (the ``progenitor bias’‘), we estimate an age of $2.1_{-0.2}^{+0.2}$ Gyrs, or $z_{\ast }=2.3_{-0.2}^{+0.2}$ for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived $M\mathrm{/}L$ values, which is confirmed by the spread in the galaxies’ colors. Two lower mass galaxies in our $\bar{z}=1.25$ sample have much lower $M\mathrm{/}L$ values, implying significant star-formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, $M \simeq 10^{11} M_{\sun}$,galaxiesat$z\simeq 1.5$.