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Impact of primordial black holes on the formation of the first stars and galaxies (arXiv:2312.04085)
Impact of primordial black holes on the formation of the first stars and galaxies (arXiv:2312.04085)
This is a review of recent progress from the theoretical perspective as a chapter contributed to the book "Primordial Black Holes", ed. Chris Byrnes, Gabriele Franciolini, Tomohiro Harada, Paolo Pani, Misao Sasaki; Springer (2025). In this chapter, we combine semi-analytical analysis and cosmological simulations to explore the possible impact of PBHs on the formation of the first stars and galaxies, taking into account two (competing) effects of PBHs: acceleration of structure formation and gas heating by BH accretion feedback. We find that the impact of stellar-mass PBHs (allowed by existing observational constraints) on primordial star formation is likely minor, although they do alter the properties of the first star-forming halos/clouds and can potentially trigger the formation of massive BHs, while supermassive PBHs serve as seeds of massive structures that can explain the apparent overabundance of massive galaxies in recent observations. Our tentative models and results call for future studies with improved modelling of the interactions between PBHs, particle DM, and baryons to better understand the impact of PBHs on early star/galaxy/structure formation and their imprints in high-redshift observations.
I also contributed to the works by Zhang et al. (2024a, 2024b, 2025a, 2025b) and Casanueva-Villarreal et al. (2024, 2025) on relevant topics. We found that stellar-mass PBHs can make significant/detectable contributions to UV and X-ray radiation backgrounds when they make up >0.1% of dark matter. Besides, the interplay between star formation and BH accretion and feedback in halos seeded by supermassive PBHs shows distinct features compared with the standard CDM case, which can explain JWST observations of overmassive BHs (with respect to the stellar components) and naturally produce SMBH binaries when direct-collapse BHs form in the vicinity of PBHs under their radiative feedback. The discovery of an extremely metal-poor galaxy hosting an unprecedentedly overmassive BH at z ~ 7 reported by Maiolino et al. (2025) and Juodžbalis el al. (2025) offers an excellent candidate for PBH-seeded galaxies.
Recent observations with the James Web Space Telescope (JWST) have identified several bright galaxy candidates at z ≳ 10 (~500 million years after the big bang), some of which appear unusually massive (up to 100 billion solar masses ). Such early formation of massive galaxies is in tension with the predictions of the standard cosmology model, demanding very high star formation efficiency (SFE), possibly even in excess of the cosmic baryon mass budget in collapsed structures (see Labbé et al., 2022; Boylan-Kolchin, 2022). We show that the observed massive galaxy candidates can be explained with lower (and physically valid) SFE if structure formation is accelerated/seeded by massive (above one billion solar masses) primordial black holes (PBHs) that make a up a small fraction (less than 0.1%) of dark matter. More work needs to be done to fully evaluate the viability of such PBH models to explain observations of the early Universe.
The PBH parameters (shaded region) required to explain the observed number density of massive galaxies, assuming that they form in structures seeded by PBHs.
Halo mass threshold for star formation under the influence of PBHs. Predictions from the semi-analytical model are shown with the curves. Data points denote the halo masses at the moment of collapse from the corresponding simulations.
We use cosmological hydrodynamic zoom-in simulations and semi-analytical models to study the effects of PBHs on first star formation. Our models self-consistently combine two competing effects: initial (isocurvature) perturbations induced by PBHs and BH accretion feedback. The former accelerates structure formation, while the latter increases the halo mass threshold for star formation with efficient cooling. Focusing on PBHs with ~30 solar masses, we find that the standard picture of first star formation in molecular cooling minihalos is not changed by PBHs at the scales of star-forming clouds. At halo scales, PBHs tend to shift star formation to more massive halos (see the figure) and reduce the DM density at the centre. As cosmic scales, we estimate that PBHs have minor (with a factor of 2 at z<30) effects on the cosmic star formation history at z>10. We also find that the Lyman-Werner feedback from PBH accretion in atomic-cooling halos may facilitate the formation of direct-collapse BHs.
This work was reported by Nature Astronomy Research Highlight, 6, 881, 2022, TACC and EurekAlert.
Constraining the non-gravitational scattering of baryons and dark matter with early cosmic structure formation (2019, MNRAS, 487, 4711)
Constraining the non-gravitational scattering of baryons and dark matter with early cosmic structure formation (2019, MNRAS, 487, 4711)
The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) measured the 21-cm absorption signal from primordial neutral hydrogen at redshift z∼17, which is (3σ) stronger than the fiducial CDM prediction. The strength of the EDGES signal can be achieved in a class of DM models involving non-gravitational baryon-dark-matter scattering (BDMS).
In this work, we derive new constraints on BDMS models by evaluating the mass thresholds of DM haloes in which primordial gas can cool efficiently to form Pop III stars, based on the timing of the observed 21-cm absorption signal.
Note that BDMS can also suppress small-scale structure formation, such that the region of BDMS parameter space consistent with the EDGES data is very small (Driskell et al., 2022).
Our constraints on BDMS models in the phenomenological parameter space (where the top-left corner enclosed by dashed contours is ruled out).
An example of the intensity maps of free-free emission around the target halo in the CDM model from our simulations.
Free-free and H2 emissions from early structure formation in different dark matter models (2019, MNRAS, 486, 3617)
Free-free and H2 emissions from early structure formation in different dark matter models (2019, MNRAS, 486, 3617)
We use zoom-in simulations to study the free-free and H2 emissions from early structure formation in the standard CDM model and a thermal warm DM (WDM) model with a particle mass of 3 keV. We find that the global free-free signal is not a good diagnostic of the underlying DM model. Massive haloes above 10^12 solar masses will be observable for z<10, with the next generation of far-infrared space telescopes. The biggest difference between the two DM models is that metal enrichment in the WDM cosmology is restricted to dense environments, while low-density gas can also be significantly enriched in the CDM case.
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