Intermediate-mass black holes and contribution to extragalactic background light from Population III stars in Milky Way-like galaxies

TL;DR

This work addresses whether intermediate-mass black holes (IMBHs) formed from Population III stars contribute significantly to the local IMBH population and the extragalactic background light, under constraints from gravitational-wave BBH mergers. It combines the A-SLOTH semi-analytical simulation of Pop III star formation, remnant mapping, and Tanikawa et al.'s BBH merger framework to estimate the number of unmerged IMBHs in a Milky Way–like galaxy, finding about $N_U\approx1.3\times10^2$ under SWT-suggested SFRs. The study also quantifies the Pop III progenitors' contribution to the near-infrared EBL, finding it to be well below current measurements (often $<0.1\%$, potentially up to $\sim1\%$ under aggressive SFR scaling). These results imply that Pop III IMBHs, while present, are not readily detectable via EBL or current gamma-ray searches, and that gravitational-wave constraints strongly limit their abundance; future observations (gamma-ray, GW detectors) could further refine these limits and test DM-spike signatures around IMBHs.

Abstract

The mass range of observed black holes extends from stellar-mass to supermassive scales, yet the existence of objects in the intermediate-mass range of $10^{2} - 10^{5} \text{M}_{\odot}$ remains unconfirmed. Black holes are suspected to compress the surrounding dark matter distribution, forming a ``spike''. If dark matter is self-annihilating, the spike could produce gamma-ray emission sufficiently luminous to be detected. This work aims to estimate the number of expected unmerged intermediate-mass black holes in a Milky Way-like galaxy that could form such spikes. These intermediate-mass black holes are assumed to have formed from the collapse of high-mass Population III stars, such that the resulting merger rate is constrained by observations of gravitational wave emission. It is furthermore estimated to what extent the progenitor Population III stars contribute to the extragalactic background light. The Population III stars are simulated and tracked using the A-SLOTH semi-analytical simulation code and the resulting number of intermediate-mass black holes is constrained by applying the Population III binary black hole merger rate to an effective volume determined from the Population III star formation rate. In this framework, $\sim 130$ unmerged IMBHs from Population III stars are expected to reside in a Milky Way-like galaxy. The contribution of their progenitors to the extragalactic background light in the near-infrared is less than $10^{-3} \text{nW} \text{m}^{-2} \text{sr}^{-1}$, well below previous estimates.

Figures (8)

• Figure 1: SFRs from 100 A-SLOTH simulations (blue and green lines). The averaged SFR is shown in white with the overlayed parametrization in red.
• Figure 2: Comparison of Pop III A-SLOTH, Jaacks and SWT SFRs with MF17. A-SLOTH Pop III star formation is terminated at $z = 3.357$.
• Figure 3: Remnant mass function of metal-free stars following Fryer_2012. The mass range above $260M_{\odot}$ is not shown, though it simply follows the dotted line (BH retains full progenitor mass). For a comparison of the stellar model with inefficient convective overshoot (this plot) with a model with efficient convective overshoot see Fig. 2 in Tanikawa_2021.
• Figure 4: Averaged mass distribution of Pop III stars from 100 A-SLOTH simulations. The sampling reflects the log-flat IMF. Coloring indicates different stellar evolutionary paths depending on the ZAMS mass.
• Figure 5: Total dead mass density of all IMBH remnants (green) and dead mass densities for the individual IMBH capable bins (blue lines).