Ultra Massive Passive Galaxies at z~1.7

At redshift z~1.7 the Universe was at the peak of its star-formation activity. It is thus a puzzle why some galaxies, many of them very massive (M_* >= 10¹¹ M⊙), had already chosen to stop forming stars. These ultra-massive galaxies, guaranteed to be the central galaxies of their host dark matter halos, must have attained very high rates of star formation to assemble their stellar masses in such a short amount of time. Using the largest (to date) K-selected gzK _s survey of passive galaxies (in an effective area of ~ 27.5 deg²) we study the demographics of these dead monsters, hoping to help understand the quenching mechanism that shut them down.