  A class of extremely luminous high-redshift galaxies has recently been
detected in unbiased submillimetre-wave surveys using the Submillimetre
Common-User Bolometer Array (SCUBA) camera at the James Clerk Maxwell
Telescope. Most of the luminosity of these galaxies is emitted from warm
interstellar dust grains, and they could be the high-redshift counterparts of
the low-redshift ultraluminous infrared galaxies (ULIRGs). Only one - SMM
J02399-0136 - has yet been studied in detail. Three other very luminous
high-redshift dusty galaxies with well determined spectral energy distributions
in the mid-infrared waveband are known - IRAS F10214+4724, H1413+117 and APM
08279+5255. These were detected serendipitously rather than in unbiased
surveys, and are all gravitationally lensed by a foreground galaxy. Two - H
1413+117 and APM 08279+5255 - appear to emit a significantly greater fraction
of their luminosity in the mid-infrared waveband as compared with both
low-redshift ULIRGs and high-redshift submillimetre-selected galaxies. This can
be explained by a systematically greater lensing magnification of hotter
regions of the source as compared with cooler regions: differential
magnification. This effect can confuse the interpretation of the properties of
distant ultraluminous galaxies that are lensed by intervening galaxies, but
offers a possible way to investigate the temperature distribution of dust in
their nuclei on scales of tens of parsecs.
