Constraints on dynamically-formed massive black holes in Little Red Dots from X-ray non-detections

Abstract

The existence of massive, compact galaxies (Little Red Dots, LRDs) at $z \sim 2$ challenges early structure formation models, suggesting rapid stellar and black hole (BH) assembly. While LRDs are efficient environments for BH growth, many show no X-ray evidence of strong AGN emission. We utilize a subsample of X-ray non-detected LRDs to test the compatibility of collision-based BH formation scenarios and constrain physical parameters like metallicity and column density. Our results indicate LRDs are ideal birthplaces for massive BHs, particularly given a mass-radius relation $R_{gal} \propto M_{gal}^{0.6}$. Collision-based models suggest seed masses larger than those in the local Universe, consistent with high-redshift BH mass-radius relations. We modeled BH seed formation and X-ray emission (0.3-7 keV) against observed upper limits. We find that mass-radius exponents $> 0.55$ favor the collision-based scenario; however, consistency with stacked X-ray analysis requires specific accretion and obscuration parameters. Constant or increasing SFR scenarios with high Eddington ratios are feasible but necessitate larger column densities or higher metal enrichment. Alternatively, moderate sub-Eddington accretion reconciles massive seeds with observed masses and X-ray weakness. We conclude that even if LRDs began as starbursts, they should eventually evolve into AGNs.

Figures (19)

• Figure 1: Galaxy stellar masses obtained for the subsample of SACCHI2025 using the previous observed scaling correlation between ${\rm M}_{\rm UV}$ and $M_{\rm gal}$ (Eq. \ref{['eq:stellarMassCorrelation']}) compared to the stellar masses of the galaxies in the sample of AKINS2024, both shown as a function of redshift.
• Figure 2: Probability density distribution of effective radii ($R_{\text{gal}}$) for the galaxy sample from AKINS2024. The shaded red histogram shows the observed data binned in logarithmic intervals. The solid curves represent the best-fit Gaussian mixture models, assuming a single component (purple) and two components (cyan). We show the reduced chi-squared ($\chi^2_\nu$) for each model.
• Figure 3: Mass-radius diagram for stellar systems to illustrate relevant timescales and the position of different sources in different parts of the parameter space. Red dots are LRDs from AKINS2024, gray dots are NSCs in the local universe GEORGIEV2016NEUMAYER2020, blue dots are well resolved BHs, while orange dots are spatially unresolved BHs GULTEKIN2009ESCALA2021. The solid black line represents the collisional timescale $t_{\rm coll} = 13.7$ Gyr, while the dashed black line is the relaxation time for $t_{\rm relax}=13.7$ Gyr. We include (in green) a fiducial galaxy with mass $10^{10}$ M$_\odot$ and radius $20$ pc to show its trajectory in the mass-radius plane for $\beta=1$ and $0.5$ defined in Eq. \ref{['eq:radius']}. The time evolution of the green line is from the left to the right.
• Figure 4: Fraction of galaxies where BH formation is driven by the condition $t=t_{\rm coll}$, as a function of the power-law exponents $\alpha$ and $\beta$ . The x-axis represents the exponent in the mass-radius relation $R_{\rm gal}\propto M_{\rm gal}^{\beta}$ and the y-axis represents the exponent in the mass-time relation $M_{\rm gal}\propto t^{\alpha}$. The colorbar shows $f_{\rm coll}$ indicates the percentage of galaxies in which BH formation specifically satisfies $t=t_{\rm coll}$.
• Figure 5: SFR distribution of our galaxy sample as function of $\alpha$. We show the distribution for $\alpha=0.3$ (blue), $1.0$ (green), and $2.9$ (orange). The cyan area shows the SFR range for the J0647_1045 galaxy at $z\sim4.53$KILLI2024, the violet area shows the range of three LRDs at $z\sim7-8$WANG2024, and in gray an extreme case of a LRD at $z\sim 4.47$ with a high SFR LABBE2024.