Counting the Unbound Stars of Virgo: Intracluster Planetary Nebulae as a Luminosity Census

Planetary nebulae are the one tracer of intracluster light that can be counted individually and followed spectroscopically, and in the Virgo cluster they provide an estimate of the diffuse population that owes nothing to surface photometry. Adopting a completeness-corrected count of 95 bright intracluster planetary nebulae, within one magnitude of the luminosity-function cutoff, over surveyed fields totalling 2.8 square degrees, and a specific frequency of 2.0e-9 nebulae per solar luminosity for that magnitude interval, the parent stellar luminosity in the surveyed fields is 4.75e+10 solar luminosities, or 1.33e+11 solar masses at a mass-to-light ratio of 2.8. Spread over the fields, that parent population has a mean surface brightness of 28.12 magnitudes per square arcsecond, in agreement with the exponential diffuse profile fitted to Virgo's photometry over the same radial range. The nebula surface density is 33.9 per square degree. Two independent routes to Virgo's unbound stars, counting individual nebulae and integrating faint surface brightness, thus land on the same component with the same radial behaviour, which is the strongest available evidence that the component is a real stellar population and not a photometric artefact.