Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Bingjie Wang, Bingjie Wang, , ,
| Summary:
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 of LRDs. 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$beta$ ($sim10^-2$, $<20times$ local AGN median) further
points to an unusually soft ionizing spectrum. We extend our analysis to dense
gas environments (the “black-hole star” hypothesis), 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; a very high it 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 immediate 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.
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