The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars

TL;DR

This study uses recombination lines to analyze Little Red Dots' ionizing spectra, revealing they are softer than typical AGN spectra and suggesting alternative models like cold accretion disks or combined AGN-star systems.

Contribution

It introduces HeII emission as a robust diagnostic for the central engine of Little Red Dots, challenging standard AGN models and proposing new explanations for their ionizing properties.

Findings

Hα equivalent widths indicate excess H-ionizing photons beyond typical AGN spectra.

HeII emission is weak, consistent with very soft ionizing spectra.

Standard AGN accretion disk models are inconsistent with observed ionization signatures.

Abstract

The nature of Little Red Dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine. Our analysis first reveals a tension between the ionizing properties implied from H$\alpha$ and HeII$\,\lambda$4686. The high H$\alpha$ EWs require copious H-ionizing photons, more than the bluest AGN ionizing spectra can provide. In contrast, HeII emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low HeII/H$\beta$ ($\sim10^{-2}$, $<20\times$ local AGN median) further points to an unusually soft ionizing spectrum. We extend our analysis to dense gas envelopes (``quasi-star''/``black-hole star''), and find that hydrogen recombination lines become optically thick and lose diagnostic power, but HeII remains optically thin and a robust tracer. Photoionization modeling with Cloudy rules out standard AGN accretion disk spectra. Alternative explanations include: exotic AGN with red rest-optical emission; high average optical depth ($>10$) from gas/dust; and/or soft ionizing spectra with abundant H-ionizing photons, consistent with e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the HeII-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs.