(Proposal ID) S16B-023 (PI) Kajisawa, Masaru (Proposal Title) What drives the quenching of star formation in high-z massive galaxies ? (Abstract) We have recently discovered six "MAssive Extremely STrong Lymanalpha Objects (hereafter MAESTLOs)" at z~3 in the COSMOS field using our Suprime-Cam imaging survey with 12 intermediate band filters. The MAESTLOs have the following observational properties; 1) large stellar mass of M_{star} >10^{10.5} M_sun, 2) extremely strong Lyalpha emission of EW_0(Lyalpha) > 100 Angstrom, 3) their Lyalpha emission tends to be spatially extended with r_{hl} ~ 4-7 kpc, although the rest-UV continuum images show a very compact morphology, 4) old stellar population indicated by strong Balmer/4000 Angstrom break, and 5) no evidence for AGN. These suggest that MAESTLOs are in the final stage of massive galaxy formation where their SFRs decrease as gas is ejected from the galaxy by the superwind-driven outflows. Here we propose spectroscopic follow-up observations of these 6 MAESTLOs with Subaru/FOCAS and Keck/MOSFIRE to test this scenario. Our main purposes are 1) to measure their spectroscopic redshifts and Lyalpha luminosity accurately, and 2) to examine their detailed Lyalpha line profiles comparing with the systemic velocity estimated from [OIII]lambda5007. The proposed observations provide us important clues to understand the quenching of the star formation in massive galaxies at the cosmic noon.