Little Red Dots as Hidden Neutrino Sources

TL;DR

This work introduces Little Red Dots (LRDs) as high-redshift, hidden neutrino sources produced by jets dissipating inside a dense BH envelope. Using analytic estimates and AMES-based numerics, it shows that photomeson production in the envelope-disk photon field can yield a per-source neutrino luminosity of order $L_{\nu,\rm eff}\sim10^{41}$–$10^{42}$ erg s$^{-1}$ with a peak at $E_ν\sim10^{4.5}$ GeV, and that the cumulative LRD population—modeled with three redshift evolutions—could contribute up to ${\sim}30\%$ of the IceCube diffuse neutrino background in the TeV–PeV band without violating BH mass density or CIB constraints. A key prediction is an energy-dependent flavor composition due to strong muon cooling, transitioning from $(1:1:1)$ to $(1:1.8:1.8)$ at Earth for $E_ν\gtrsim10^{14}$ eV, offering a diagnostic for next-generation detectors. The framework also argues that LRDs are unlikely to produce detectable neutrino multiplets and that their lack of accompanying gamma rays reinforces their role as hidden sources, setting them apart from radio-quiet AGNs and traditional AGN jets.

Abstract

Little Red Dots (LRDs) are enigmatic, compact, red galaxies at high redshift, $z\sim 4$-$7$, discovered by the James Webb Space Telescope. Broad emission lines in the absence of X-ray and radio counterparts suggest that they host accreting supermassive black holes embedded in dense gaseous envelopes. This black-hole-envelope configuration facilitates efficient photohadronic interactions and neutrino production. Remarkably, their observed source number density and luminosity are compatible with the energetics of the diffuse neutrino background. We consider that relativistic jets and outflows are launched from the black hole and propagate through low-density polar funnels within envelopes, where particle acceleration and neutrino emission occur. This leads to LRDs being effectively hidden sources. Our analytic and numerical calculations show that, in an optimistic scenario, LRDs can contribute $\sim 30\%$ of the observed diffuse background at TeV$-$sub-PeV energies, predominantly through photomeson production. At high neutrino energies, $\gtrsim 10^{5.5}~{\rm GeV}$, inverse-Compton cooling of muons modifies the resulting flavor ratio, providing a distinctive diagnostic for IceCube-Gen2 and other upcoming neutrino telescopes.

Figures (7)

• Figure 1: Rescaled population--luminosity diagram in the $(L_{\nu,{\rm eff}},\,\xi_z n_0^{\rm eff})$ plane. Orange-filled stars mark the values of $(L_{\nu,{\rm eff}}, \xi_z n_0^{\rm eff})$. The red solid curve denotes the diffuse requirement (Eq. (\ref{['eq:xiz_n_diff']})). The multiplet--limit curves are shown for IceCube with six-year data as blue-dashed line 2015arXiv151005222T and for IceCube--Gen2 as green-dotted line Gen2_2021. The thin multiplet--limit curves show the multiplet limits based on the SFR evolution, taken from Murase:2016gly, rescaled by the SFR evolution factor $\xi_{z,\rm SFR} \simeq 2.8$. The thick curves represent the multiplet limits after converting the SFR-based result to the LRD case by using the LRD-specific evolution factor $\xi_{z,\rm LRD}$ (Eq. (\ref{['eq:xi_z']})) and luminosity-dependent function $q_{L, \rm LRD}$ (Eq. (\ref{['eq:qL']})). For the LRD point, we use $L_{\nu,{\rm eff}}\simeq5.0\times10^{41}~{\rm erg~s^{-1}}$ (Eq. (\ref{['eq:Lnu_order']})) and the lognormal form of $n(z)$ described in Appendix \ref{['app:numberdensity']}. For comparison, we also show typical locations of FSRQs, BL Lacs, starburst galaxies (SBGs), galaxy clusters or groups (GC/GG--int), radio-loud AGN (RL AGN), radio-quiet AGN (RQ AGN), and low-luminosity AGN (LL AGN) Murase:2016gly.
• Figure 2: Schematic image of the BH-envelope-jet system considered in this work. A central SMBH is embedded in an envelope that extends outward to the photospheric radius $r_{\rm ph}$. In the inner region, gas with nonzero angular momentum circularizes into an accretion disk, whereas along the rotation axis the low-angular-momentum inflow can fall almost in free fall toward the BH, creating a low-density polar funnel. The funnel extends from the base at $r_0=r_{\min}R_g$ near the BH horizon to the covered radius, $r_{\rm cov}$, beyond which the envelope globally confines the jet. The funnel opening angle at its base is $\theta_0$, and a relativistic jet with Lorentz factor $\Gamma_j$ propagates inside the funnel. The orange-shaded region indicates the jet dissipation region. For simplicity, we set the dissipation radius to be close to the covered radius, $r_{\rm dis}\approx r_{\rm cov}\simeq 10^{16}$ cm. In this region, photons from the surrounding accretion flow provide a dense target for cosmic-ray interactions, leading to high-energy neutrino production.
• Figure 3: Acceleration and loss rates at the dissipation region in the comoving frame. Shown are $t_{\rm acc}'^{-1}$ (acceleration; red solid), $t_{p\gamma}'^{-1}$ (photomeson; blue dashed), $t_{\rm BH}'^{-1}$ (Bethe--Heitler; blue dash-dotted), and the total cooling rate $t_{\rm cool}'^{-1} = t_{ p\gamma}'^{-1}+t_{\rm BH}'^{-1}$ (black solid). The horizontal axis is the proton energy $\varepsilon'_p$, and the vertical axis is the inverse timescale $t'^{-1}$. All curves are computed with $r_{\rm dis}=10^{16}$ cm, $L_j=L_{\rm Edd}$ for a $10^{6.5}M_\odot$ BH, $\Gamma_j=2.0$, $\theta_0=1.0$, $\epsilon_p=0.1$, and $\epsilon_B=0.01$ (see Table \ref{['tab:params']}).
• Figure 4: Comoving spectral luminosities $L'$ as functions of comoving energy $\varepsilon'$. The red dashed curve shows the proton injection spectrum, $L'_p(\varepsilon'_p)$, used for the neutrino calculation. The green solid curve shows the resulting all-flavor neutrino spectrum, $L'_\nu(\varepsilon'_\nu)$, computed with the AMES code using the fiducial parameters in Table \ref{['tab:params']}, the same as in Fig. \ref{['fig:timescale']}.
• Figure 5: Redshift-differential diffuse neutrino intensity from LRDs, $d(E_{\nu}^2\Phi_{\nu})/dz$, for two observed energies. Top: $E_{\nu}=10^{4}~\rm GeV$. Bottom: $E_{\nu}=3\times10^{5}~\rm GeV$. Curves: red solid for the Lognormal model, blue dashed for the Spin model, and orange dot-dashed for the Power-law model. We use the same parameters as in Fig. \ref{['fig:timescale']} (see Table \ref{['tab:params']}).