# Ultra-violet photo-ionisation in far-infrared selected sources

## TL;DR

This study investigates the relationship between ultra-violet photo-ionisation and far-infrared emission in Herschel-SPIRE sources, exploring whether AGN activity or massive stars dominate dust heating at high ionising photon rates.

## Contribution

It provides observational evidence on the incidence of FIR emission at high ionising photon rates, challenging the idea of a critical ionisation threshold for FIR detection.

## Key findings

- FIR emission detected at ionising photon rates much higher than HI ionisation threshold.
- No clear critical ionising photon rate for FIR detection observed.
- High ionising rates may be due to AGN or massive stars, not just HI ionisation.

## Abstract

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.