Fuzzy dark matter soliton core hosting a supermassive black hole as a dense low-mass perturber in strong gravitational lensing

TL;DR

The paper tackles the puzzle of dense, low-mass perturbers inferred from strong gravitational lensing by proposing that a fuzzy dark matter soliton core, augmented by a central supermassive black hole, can act as the perturber. It combines soliton- plus SMBH-constructed mass profiles with Schrödinger-Poisson modeling to fit the observed perturber in JVAS B1938+666, deriving $mc^2 \approx 3.6\times10^{-21}$ eV and a halo mass $M_h \approx 7.1\times10^6 M_\odot$, with an SMBH of $M_{SMBH} \approx 4\times10^5 M_\odot$ at the center. The authors explore three origin scenarios: (i) FDM parameter choices that reproduce the profile, (ii) tidal evolution that modifies the soliton mass while keeping the SMBH, and (iii) the possibility of heavy SMBH seeds through direct collapse or primordial black holes. While the framework offers a compelling lensing-based probe of FDM substructure, it also faces constraints on FDM mass ranges and uncertainties in the soliton–halo scaling, inviting further work on compound DM models and baryonic effects. The study highlights how dense low-mass perturbers could serve as clean laboratories for testing the physics of FDM and SMBH seeding in low-mass halos.

Abstract

Recent high-resolution imaging observations of strong lens systems reveal dense low-mass perturbers. We propose a soliton core, whose central density is boosted by a supermassive black hole (SMBH), in the fuzzy dark matter (FDM) model as an efficient perturber in strong gravitational lensing. The higher central density makes it less efficient in the tidal mass loss, and leads to the higher impact in gravitational lensing. We show that the mass profile of a $\sim 10^6M_\odot$ perturber in JVAS B1938+666, which does not resemble any known astronomical object, can be wel explained by a soliton core in the FDM model with the mass of $4\times 10^{-21}$eV hosting an SMBH with the mass of $4\times 10^5M_\odot$. The high mass of the SMBH may be explained by several scenarios that predcit heavy SMBH seeds such as the direct collapse black hole formation and primordial black holes.

Figures (4)

• Figure 1: Mass profiles of the soliton core with ( red) and without ( blue) an SMBH with the ratio of the mass of the SMBH to that of the soliton core of $M_{\mathrm{SMBH}}/M_{\mathrm{sol}} =0.24$. Dotted lines indicate the fitting from given by Eq. \ref{['eq:rho_sol']} with the transformation of $\rho_{\mathrm{sol}}(r)\rightarrow f^2\rho_{\mathrm{sol}}(f^{2/3}r)$ with $f=1$ and $2.79$ for the case without and with an SMBH, respectively.
• Figure 2: The cylindrical mass $M_{\mathrm{cyl}}(<R)$ as a function of the projected radius $R$ for the UD+PM model, which corresponds to the best-fitting model in Ref. 2026arXiv260102466V, as well as $M_{\mathrm{cyl}}(<R)$ for an SMBH plus a soliton core in the FDM model. We choose $M_{\mathrm{SMBH}}=M_{\mathrm{PM}}$, $M_{\mathrm{sol}}=M_{\mathrm{UD}}$, and $r_{\mathrm{c}}=90$ pc.
• Figure 3: The FDM mass $mc^2$ and the halo mass $M_{\mathrm{h}}$ that explain the $\sim 10^6M_\odot$ perturber in JVAS B1938+666. The dark shaded region is a forbidden region defined by $M_{\mathrm{h}}<M_{\mathrm{sol}}$. The filled point for $\lambda=1$ indicates the parameters that explain the observation in the absence of any tidal mass loss of the soliton core. The dotted line shows a track for different values of $\lambda$ that is defined by the ratio of the soliton core masses after and before the tidal mass loss. The light shaded region indicates a rough range of the halo mass that is consistent with the SMBH mass of $M_{\mathrm{SMBH}}=4.25\times 10^5M_\odot$ based on the extrapolation of the SMBH mass--halo mass from higher masses.
• Figure 4: The density ratio $\mu=\rho_{\mathrm{sol}}(0)/\bar{\rho}$ between the central density of the soliton core with ( solid) and without ( dashed) the SMBH and the density of the main halo as a function of the distance $r/r_{\mathrm{vir}}$ of the soliton core from the center of the main halo. The horizontal dotted line indicates the rough condition for the tidal disruption, $\mu \lesssim 70$2018PhRvD..97f3507D. The gray dotted line shows the enclosed mass fraction $M(<r)/M_{\mathrm{vir}}$ of the main halo as a function of $r/r_{\mathrm{vir}}$.