An ALMA Survey of Submillimetre Galaxies in the Extended Chandra Deep
  Field South: The Far-Infrared Properties of SMGs
  We exploit ALMA 870um (345GHz) observations of submillimetre sources in the
Extended Chandra Deep Field South to investigate the far-infrared properties of
high-redshift submillimetre galaxies (SMGs). Using the precisely located 870um
ALMA positions of 99 SMGs, together with 24um and radio imaging of this field,
we deblend the Herschel/SPIRE imaging of this region to extract their
far-infrared fluxes and colours. The median photometric redshifts for ALMA LESS
(ALESS) SMGs which are detected in at least two SPIRE bands increases with
wavelength of the peak in their SEDs, with z=2.3+/-0.2, 2.5+/-0.3 and 3.5+/-0.5
for the 250, 350 and 500-um peakers respectively. We find that 34 ALESS SMGs do
not have a >3-sigma counterpart at 250, 350 or 500-um. These galaxies have a
median photometric redshift of z=3.3+/-0.5, which is higher than the full ALESS
SMG sample; z=2.5+/-0.2. Using the photometric redshifts together with the
250-870um photometry, we estimate the far-infrared luminosities and
characteristic dust temperature of each SMG. The median infrared luminosity of
the S_870um>2mJy SMGs is L_IR=(3.0+/-0.3)x10^{12}Lo(SFR=300+/-30Mo/yr). At a
fixed luminosity, the characteristic dust temperature of these high-redshift
SMGs is 2-3K lower than comparably luminous galaxies at z=0, reflecting the
more extended star formation occurring in these systems. By extrapolating the
870um number counts to S_ 870um=1mJy, we show that the contribution of
S_870um>1mJy SMGs to the cosmic star formation budget is 20% of the total over
the redshift range z~1-4. We derive a median dust mass for these SMGs of
M_d=(3.6+/-0.3)x10^8Mo and by adopting an appropriate gas-to-dust ratio, we
estimate an average molecular mass of M_H2=(4.2+/-0.4)x10^{10}Mo. Finally, we
use our estimates of the H2 masses to show that SMGs with S_870um>1mJy contain
~10% of the z~2 volume-averaged H2 mass density at this epoch.
