Vol. 619
In section 4. Extragalactic astronomy

Extreme submillimetre starburst galaxies

How extreme can the starburst galaxies be? The authors have mined starburst objects in an Herschel catalogue at 500 micron and an IRAS catalogue at 60 micron. They found starbursts with star formation rate (SFR) in the range 5 000 to 30,000 Mo/yr. Higher than this are special cases, either lensed systems, blazars, or erroneous photometric redshifts. At redshifts of between three and five, the timescale for the Herschel galaxies to make their stellar mass (M*) at their present SFR is ~10^8 yr, so these galaxies are making a significant fraction of their stars in the current starburst. Their gas fraction is 50%, determined from dust emission. Of the 54 extreme with complete spectral energy distribution (SED) information, just over 50% show evidence for AGN emission, but the infrared luminosity is dominated by the starburst. Using assumptions about the Eddington ratio and accretion efficiency, it is possible to derive black hole (BH) masses. There is a universal ratio Mbh ∼ 0.001M*, at these redshifts, driven by the strong period of star formation and black-hole growth at z = 2-5. Then the SFR falls by a factor of 1000, and the BH accretion rate falls by only a factor of 30, suggesting a decoupling of star formation and the feeding of the nuclear black hole.