2019A&A...627A..93C

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_X∼10³⁷W. Such a scenario would be consistent with the suppression of star formation by the AGN, required by current theoretical models. Since absorption of the 21cm transition of neutral hydrogen (HI), which traces the star-forming reservoir, also exhibits a critical value in the ultraviolet band (above ionising photon rates of Q~=3x10⁵⁶s^–1), we test the SPIRE sample for the incidence of the detection of 250µm emission with Q. The highest value at which FIR emission is detected above the SPIRE confusion limit is Q=8.9x10⁵⁷s^–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≥3x10⁵⁶s^–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.