S19A0017

We propose FOCAS spectroscopy for a newly discovered sample of 5 remarkably blue galaxies with UV slope of β ∼ −3 at z ∼ 4. These very blue galaxies (VBGs) are good candidates of high-z extremely young, metal-poor galaxies such as a few Myr stellar population with as low metallicity as those of the most metal-poor local galaxies including I Zw 18 (Z ∼ 0.0006) or possibly even lower. Interestingly, their very blue colors can only be explained by cases with a non-legligible amount of ionizing photon leakage, which suggests that they may significantly contribute to the ionizing background at that epoch. These very rare VBGs are identified in the HSC survey data, whose effective volume is more than 1 Gpc3. We carefully remove transients from our sample by checking multi-epoch observation images. However, without spectroscopy, we cannot rule out the possibilities of low-z line emitters and AGNs. With the FOCAS spectra, we will spectroscopically confirm our VBGs at high redshifts and examine the presence of AGN signatures. Our spectroscopy program will yield a unique sample of high-z galaxies in an early phase of galaxy formation that have been poorly explored so far.


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