  We present tentative H-alpha emission line detections of four
submillimetre-detected galaxies at z>3.5: the radio galaxies 8C1909+722 and
4C60.07 at signal-to-noise ratios (SNRs) of 3.1 and 2.5, and two
submillimetre-selected galaxies (SMGs) near the first of these at SNRs of 10.0
and 2.4, made with the AKARI Space Telescope as part of the FUHYU mission
program. These are the highest-redshift H-alpha detections in such galaxies,
made possible by AKARI's unique near-infrared spectroscopic capability. The two
radio galaxies had known redshifts and surrounding structure, and we have
detected broad H-alpha components indicating the presence of dust-shrouded
quasars. We conclude that powerful AGNs at z>3.5 occur in peaks of the
star-formation density fields, supporting a close connection between stellar
mass build-up and black hole mass assembly at this redshift. We also show that
4C60.07 is a binary AGN. The H-alpha detections of the two SMGs are the first
redshift determinations for these sources, confirming their physical
association around their companion radio galaxy. The H-alpha-derived star
formation rates (SFRs) for the SMGs are lower than their far-infrared derived
SFRs by a factor of ~10, suggesting a level of dust obscuration similar to that
found in studies at ~1<z<2.7.
