Detection of H-alpha emission from z>3.5 submillimetre luminous galaxies with AKARI-FUHYU spectroscopy

TL;DR

This study reports the first detection of H-alpha emission from galaxies at redshifts greater than 3.5 using AKARI, revealing insights into early galaxy evolution, black hole growth, and star formation obscuration.

Contribution

It presents the highest-redshift H-alpha detections in submillimetre luminous galaxies, demonstrating AKARI's unique spectroscopic capabilities and revealing the presence of dust-shrouded quasars and binary AGNs.

Findings

Detection of broad H-alpha components indicating dust-shrouded quasars.

Confirmation of physical association of SMGs with radio galaxy.

Star formation rates from H-alpha are lower than infrared estimates by a factor of ~10.

Abstract

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.