Direct Collapse Black Hole Candidates from Decaying Dark Matter

TL;DR

This work investigates whether decaying axion-like dark matter can supply photons in the 1–13.6 eV range to suppress molecular hydrogen in early, pristine halos, enabling atomic cooling and the formation of heavy, direct-collapse black hole seeds. Using a semi-analytic chemo-thermal one-zone model for a benchmark halo, the authors track H2 chemistry, heating, and cooling while incorporating intergalactic photon flux from axion decay with a line-resolved Lyman–Werner treatment and redshift smearing. They identify a viable axion parameter window around $m_a/2\in[24.5,26.5]$ eV with couplings as low as $g_{a\gamma\gamma}\sim 3\times 10^{-12}$–$4\times 10^{-12}\ \mathrm{GeV}^{-1}$, within which halos can reach the atomic cooling limit and form heavy seeds, potentially explaining the high-redshift SMBH population observed by JWST and other surveys. The results highlight the importance of the Lyman–Werner line structure for narrow spectra and indicate a substantial, testable link between dark matter properties and early black-hole demographics, while noting substantial uncertainties and the need for dedicated simulations to connect to the full high-redshift population.

Abstract

Injecting 1-13.6 eV photons into the early universe can suppress the molecular hydrogen abundance and alter the star formation history dramatically enough to produce direct collapse black holes. These, in turn, could explain the recently observed population of puzzling high-redshift supermassive black holes that appear to require super-Eddington accretion. We show that axion dark matter decay in the intergalactic medium can account for this energy injection. We use a single zone model of the gas core and semi-analytically evolve its chemo-thermal properties to track the conditions for which the system becomes an atomic cooling halo-a necessary precursor for the production of heavy black hole seeds to explain the high-redshift black hole population. Windows of axions masses between 24.5-26.5 eV with photon couplings as low as $4\times 10^{-12}$/GeV may realize this atomic cooling halo condition. We highlight the significance of the band structure of molecular hydrogen on the effectiveness of this process and discuss estimates of the heavy seed population and prospects for testing this model.

Figures (10)

• Figure 1: Halo evolution with redshift. left: total halo mass $M(z)$ compared to the filtering mass $M_\text{F}$. middle: gas (H nuclei) number density in the core, $n_\text{p}(z)$. right: gas temperatures, $T(z)$.
• Figure 2: left: Model halo history in the absence of additional photons from axion decay. .95Pop III star formation begins when the .93H$_2$ fraction, $x_{\text{{\scalefont{.93}{H}}$_2$}}{}$, crosses the critical fraction. The halo properties are constructed so that in the absence of .93H$_2$, atomic cooling would begin at the left edge: $z=10$ ($T=10^4\,\text{K}$). The $x_\text{e}$, critical .93H$_2$, baryon density curves do not change when photons from axion decay are introduced. right: The heating (red), .93H$_2$ cooling (solid blue), .93H$\alpha$ cooling (dashed purple), free fall rate (dashdot black), and Hubble rate (pink) in this work. The conservative condition for forming a heavy seed candidate is that the .93H$_2$ cooling rate stays below the Hubble rate until the atomic cooling threshold ($T=10^4\text{K}$, left edge) when the .93H$\alpha$ cooling activates. The condition that .93H$_2$ cooling is slower than the free fall rate is a weaker, but more realistic condition. We choose the conservative condition because our halo model breaks down when the .93H$_2$ cooling rate surpasses the dynamical heating rate (red); at this point the halo temperature no longer tracks the virial temperature and a more detailed analysis is necessary.
• Figure 3: Left: the spectrum of photons from axion decay in the intergalactic medium (IGM) smeared out by reshifting and may excite some of the $\mathcal{O}(70)$ Lyman--Werner states of .93H$_2$ that could then decay into 2.93H. Right: Photons above $13.6\,\text{eV}$ or with one of the $n \geq 3$ Lyman energies do not reach the halo because the .93IGM is opaque to these photons due to a large density of .93H.
• Figure 4: Left: Photon intensity contribution and spectral profile at the halo from axion decay in the intergalactic medium. The axion coupling is set to $g_{a \gamma\gamma} = 10^{-11} \, \text{GeV}^{-1}$ and the observation redshift is $z=10$. For each color, the right boundary is $m_\text{a} / 2$, and the left boundary is the position of a $n\geq 3$ Lyman line for atomic hydrogen. The dashed grey lines show first 6 of these Lyman lines. Right:.93H$_2$ self-shielding factor as a function of the .93H$_2$ column density in the conditions of interest, $n_\text{p} = 1 \, \text{cm}^{-3}$. The column density is the product of the core density and the core radius, which is approximately a tenth of the virial radius, $R_\text{core} \sim \mathcal{O}(10^3\,\text{pc})$ at $z=10$.
• Figure 5: Halo gas evolution with decaying axions: .93H$_2$ fraction (left) and heating/cooling rates (right). Viable .93DCBH candidates remain below red line (left: critical molecular hydrogen fraction, right: Hubble rate) at $z=10$, at which point the halo reaches the atomic cooling limit. This line represents the condition \ref{['eq:.xH2.crit']}. The benchmark halo follows the history in Fig. \ref{['fig:standard:halo:history:and:thermal.rates']}.