Extreme submillimetre starburst galaxies
  We use two catalogues, a Herschel catalogue selected at 500 mu (HerMES) and
an IRAS catalogue selected at 60 mu (RIFSCz), to contrast the sky at these two
wavelengths.
  Both surveys demonstrate the existence of extreme starbursts, with
star-formation rates (SFRs) > 5000 Msun/yr. The maximum intrinsic
star-formation rate appears to be ~30,000 Msun/yr. The sources with apparent
SFR estimates higher than this are in all cases either lensed systems, blazars,
or erroneous photometric redshifts.
  At redshifts of 3 to 5, the time-scale for the Herschel galaxies to make
their current mass of stars at their present rate of formation ~ 10^8 yrs, so
these galaxies are making a significant fraction of their stars in the current
star-formation episode. Using dust mass as a proxy for gas mass, the Herschel
galaxies at redshift 3 to 5 have gas masses comparable to their mass in stars.
  Of the 38 extreme starbursts in our Herschel survey for which we have more
complete SED information, over 50% show evidence for QSO-like optical emission,
or exhibit AGN dust tori in the mid-infrared SEDs. In all cases however the
infrared luminosity is dominated by a starburst component. We derive a mean
covering factor for AGN dust as a function of redshift and derive black hole
masses and black hole accretion rates. There is a universal ratio of black-hole
mass to stellar mass, ~ 10^{-3}, driven by the strong period of star-formation
and black-hole growth at z = 1-5.
