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The Clustering of Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

M. Grant Roberts, Aarna Garg, Tesla Jeltema, Stefano Profumo

TL;DR

The paper addresses whether Little Red Dots (LRDs) at high redshift can originate from seeds produced by ultra-strongly self-interacting dark matter (uSIDM). It develops a formation-based framework mapping the LRD mass function to host-halo properties and computes the effective clustering bias $b_{eff}$ using both a power-law fit and full Monte Carlo sampling of accretion and merger histories. It finds $b_{eff} ≈ 4.5$ and a characteristic halo mass $m_{200}^{char} ≈ 8 × 10^{10}$ solar masses, robust to variations in the uSIDM cross-section and fraction, implying LRDs inhabit lower-mass halos than high-redshift quasars. This provides a falsifiable test for the uSIDM heavy-seed scenario with upcoming JWST data and helps distinguish heavy-seed formation channels from other LRD origins.

Abstract

We predict the effective clustering bias parameter, $b_{\rm{eff}}$, at $z\sim5$ for Little Red Dots (LRDs) seeded by Ultra-Strongly Self-Interacting Dark Matter (uSIDM). From our model, we find that $b_{\rm{eff}}\sim4.5$, thus we infer that LRDs seeded by uSIDM would populate halos of typical masses $\sim 8\times10^{10}~M_{\odot}$; this bias factor is consistent with LRDs being a distinct population from high redshift quasars. To the extent that we are aware, this is the first formation-based theoretical prediction of LRD clustering from a model consistent with the LRD mass function. We find that this bias and clustering is insensitive to a wide range of the underlying uSIDM microphysics parameters, including the uSIDM cross-section $σ/m$ and uSIDM fraction $f$. This is therefore a robust prediction from the uSIDM model, and will allow for direct probes of the uSIDM paradigm as the origin of LRDs in the next few years. Upcoming \texttt{JWST} observations will constrain the population of LRDs, including directly measuring their clustering.

The Clustering of Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

TL;DR

The paper addresses whether Little Red Dots (LRDs) at high redshift can originate from seeds produced by ultra-strongly self-interacting dark matter (uSIDM). It develops a formation-based framework mapping the LRD mass function to host-halo properties and computes the effective clustering bias using both a power-law fit and full Monte Carlo sampling of accretion and merger histories. It finds and a characteristic halo mass solar masses, robust to variations in the uSIDM cross-section and fraction, implying LRDs inhabit lower-mass halos than high-redshift quasars. This provides a falsifiable test for the uSIDM heavy-seed scenario with upcoming JWST data and helps distinguish heavy-seed formation channels from other LRD origins.

Abstract

We predict the effective clustering bias parameter, , at for Little Red Dots (LRDs) seeded by Ultra-Strongly Self-Interacting Dark Matter (uSIDM). From our model, we find that , thus we infer that LRDs seeded by uSIDM would populate halos of typical masses ; this bias factor is consistent with LRDs being a distinct population from high redshift quasars. To the extent that we are aware, this is the first formation-based theoretical prediction of LRD clustering from a model consistent with the LRD mass function. We find that this bias and clustering is insensitive to a wide range of the underlying uSIDM microphysics parameters, including the uSIDM cross-section and uSIDM fraction . This is therefore a robust prediction from the uSIDM model, and will allow for direct probes of the uSIDM paradigm as the origin of LRDs in the next few years. Upcoming \texttt{JWST} observations will constrain the population of LRDs, including directly measuring their clustering.

Paper Structure

This paper contains 4 sections, 5 equations, 3 figures.

Table of Contents

  1. Introduction
  2. uSIDM Phenomenology and Black Hole Formation
  3. LRD Clustering and the Effective Bias
  4. Summary and Conclusions

Figures (3)

  • Figure 1: Here we reproduce Fig. 2 (right) from Roberts_LRD. The uSIDM LRD mass function (colored dots) for different uSIDM parameters is shown. The uSIDM results are plotted against the SIDM simulation results from fangzhou_LRD (shaded red), and the LRD mass function data points from Kokorev_2024 (red diamonds). The red diamonds are the derived BH masses, assuming $\lambda_{\rm{Edd}} = 1$; for lower accretion rates these would shift to larger BH masses as depicted by the horizontal lines which extend down to $\lambda_{\rm{Edd}} = 0.1$. The simulations for the red shaded region assume $\varepsilon_{\rm{DC}} = 1$ whereas we assume $\varepsilon_{\rm{DC}} = 0.01$.
  • Figure 2: Here we show the range of $b_{\rm{eff}}$ from the three power-law fits to the uSIDM LRD mass function, $3.81 \lesssim b_{\rm{eff}} \lesssim 5.67$ (gray shaded region), overlaid onto the Tinker2010 halo bias function (red line). We find a rough scale for $m_{200}^{\rm{char}}$ to be $\left({0.4-1.8}\right) \times 10^{11}~M_{\odot}$. We also show the inferred mass range from Schindler+25 in shaded orange, with the median inferred value as the dashed orange vertical line. In blue we plot the various cases explored by Pizzati+25.
  • Figure 3: Here we show $b_{\rm{eff}}$ from the three representative full uSIDM LRD mass function curves. We find that the full mass function yields $b_{\rm{eff}}$ values that are very nearly insensitive to the underlying particle physics. They all yield $b_{\rm{eff}}\sim4.5$ and thus $m_{200}^{\rm{char}}\sim8\times10^{10}~M_{\odot}$. To show the small deviations in $b_{\rm{eff}}$, we zoom into bias values between $4$ and $5.5$. As in Fig. \ref{['fig:power-law-bias']}, we plot the inferred mass regions from Schindler+25 in orange and Pizzati+25 in blue. We find that we are consistent with the low mass and low duty cycle case in Pizzati+25.