We propose FOCAS spectroscopy for a newly discovered sample of 5 intensely Lyα-emitting galaxies (Lyα EW0 = 240−500Å) at z=4−5. These large Lyα EW galaxies (LLEGs) are good candidates of high-redshift metal-poor, extremely young galaxies such as a few Myr stellar population with metallicity of only Z ∼ 0.001 that is as low as those of the most metal-poor local galaxies previously identified including I Zw 18 (Z ∼ 0.0006) or possibly even lower. These LLEGs are identified in the latest Hyper Suprime-Cam (HSC) survey data, whose effective volume is more than ten times larger than previous narrowband Lyα emitter searches. We carefully remove transients from our sample by checking multi-epoch observation images. However, without spectroscopy, we cannot certainly rule out the other possibilities of low-z line emitters and AGNs. With the FOCAS spectra, we will spectroscopically confirm our LLEGs at high redshifts and examine the presence of AGN signatures. We will derive the number densities of this very rare population, which will be useful to constrain the duty cycle of intermittent star formation activities. Our spectroscopy program will yield a unique sample of high-redshift galaxies in an early phase of galaxy formation that have been poorly explored so far.
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