The galaxy bias profile of cosmic voids

Context. Cosmic voids, the largest underdense regions in the Universe, provide unique laboratories for studying galaxy formation in extreme low-density environments and constitute powerful probes of cosmology. Recent work has shown that individual galaxy bias ( b i ), which quantifies how each galaxy traces the underlying dark matter field, exhibits a characteristic radial dependence within spherical voids, defining a void bias profile in which galaxies near void centers display systematically lower bias values. However, the extent to which this bias profile depends on the void identification methodology has so far remained unexplored. Aims. We investigated how the environmental modulation of individual galaxy bias depends on the adopted void-finding algorithm by comparing measurements across five distinct void definitions: spherical voids ( SPARKLING ), watershed-based methods ( ZOBOV and REVOLVER in two modes), and free-form integrated-density voids ( POPCORN ). Methods. We applied these complementary void-finding algorithms to the same galaxy sample drawn from the IllustrisTNG simulation (TNG300-1 at z = 0) and computed individual galaxy bias profiles as a function of distance from void centers. We quantified the correlation between b i and the membership of the void catalogs and explored how this relationship varies with the integrated underdensity threshold for density-based methods. Results. We find that the radial gradient of individual bias within voids, generally increasing from negative values at the void centers to higher values at the boundaries, is robust across most void definitions. However, density-threshold methods ( SPARKLING and POPCORN ) preferentially select galaxies with b i < 0, while watershed methods without density constraints ( ZOBOV ) include substantial contamination from high-bias boundary galaxies. The correlation between negative bias and void membership is systematically strengthened as the integrated underdensity threshold becomes less restrictive, with POPCORN achieving the highest effectiveness in isolating anti-biased populations.