Signatures of Accreting Black Holes in Line Intensity Mapping

Abstract

Line-intensity mapping (LIM) has attracted growing attention as a powerful technique for probing the large-scale distribution of galaxies and the cosmic history of star formation through unresolved line emission. Existing LIM models for galaxy-associated lines, such as H$α$, often assume that the dominant contribution to observed emission arises from star-forming activity, while the role of accreting black holes (BHs) remains largely unexplored. In this study, we use the IllustrisTNG cosmological hydrodynamical simulation to construct mock intensity maps of H$α$ and He II, including contributions from both star formation and BH accretion. We show that the BH contribution to the mean intensity is significant, reaching $\sim$40--60 per cent for H$α$ and $\sim$60--80 per cent for He II around cosmic noon. Owing to the large luminosity weight of rare, bright sources, BH-powered emission dominates the shot-noise component of the power spectrum and significantly boosts the small-scale clustering amplitude, particularly for He II. We assess the implications for forthcoming LIM surveys and show that SPHEREx can probe the BH-influenced bright end of the H$α$ voxel intensity distribution (VID) at $z\lesssim4$, and a CDIM-like experiment can further access the BH-dominated regime of He II. Our results demonstrate that accreting BHs represent an essential component of LIM signals, which was previously underappreciated. We thus conclude that accurately modeling the BH contribution is crucial for a physically complete interpretation of future LIM observations.