Ultra-violet photo-ionisation in far-infrared selected sources
  It has been reported that there is a deficit of stellar heated dust, as
evident from the lack of far-infrared (FIR) emission, in sources within the
Herschel-SPIRE sample with X-ray luminosities exceeding a ``critical value'' of
L~10^37 W. Such a scenario would be consistent with the suppression of star
formation by the AGN, required by current theoretical models. Since absorption
of the 21-cm transition of neutral hydrogen (HI), which traces the star-forming
reservoir, also exhibits a critical value in the ultra-violet band (above
ionising photon rates of Q ~ 3 x 10^56 s^-1), we test the SPIRE sample for the
incidence of the detection of 250 micron emission with Q. The highest value at
which FIR emission is detected above the SPIRE confusion limit is Q = 8.9 x
10^57 s^-1, which is ~30 times that for the HI, with no critical value
apparent. Since complete ionisation of the neutral atomic gas is expected at Q
> 3 x 10^56 s-1., this may suggest that much of the FIR must arise from heating
of the dust by the AGN. However, integrating the ionising photon rate of each
star over the initial mass function, we cannot rule out that the high observed
ionising photon rates are due to a population of hot, massive stars.
