Origin of the ISRF dropoff with distance

Determine whether the detectable decrease in LightCube interstellar radiation field intensity with distance from the Sun, particularly beyond approximately 800 pc, reflects a genuine decrease in the volume density of photons emitted by ultraviolet-bright stars or a selection effect caused by the limited sensitivity of the input stellar dataset and associated Malmquist bias.

Background

The LightCube model shows a detectable decline in interstellar radiation field intensity as a function of distance from the Sun, including changes in both the highest and lowest intensity percentiles. The authors identify two possible explanations: a real decrease in the spatial density of ultraviolet-bright stars, or incompleteness in the stellar catalog caused by its sensitivity limit and Malmquist bias.

Although the authors argue that the behavior of the most luminous stars is probably physical and conclude that LightCube is largely reliable out to at least 800 pc, they acknowledge that the strong clustering of young stars makes it difficult to exclude the possibility that the observed decline is partly or wholly a selection effect. Resolving the physical origin of the dropoff is therefore left uncertain.

References

We find that within 800 pc of the Sun there is a detectable dropoff in ISRF intensity as a function of distance from the Sun, for both the highest and the lowest percentiles of the ISRF (see Figure \ref{fig:percrad}). This effect could be either be a decrease in the actual volume density of photons emitted from UV-bright stars, or a selection effect due to our input stellar data set's limited sensitivity (Malmquist bias). Because young stars are so heavily clustered, it is not obvious that we can rule out the first possibility.

— LightCube: A Parsec-Resolution 3D Model of the Local Far-Ultraviolet Interstellar Radiation Field  (2608.27389 - Bish et al., 27 Aug 2026) in Section 3.2, “Completeness” (subsection labeled \ref{subsec:completeness})