Unveiling the First Stars: How Population III Stars Impact the 21cm Signal

Population III (Pop. III) stars, the first generation of stars in the universe, played a crucial role in shaping early cosmic structures. However, detecting them remains an ongoing challenge. In their study, Ventura et al. explore how these ancient stars influence the 21cm signal, which is a powerful tool for probing the early universe. The authors use a semi-analytical model, meraxes, to estimate the impact of Pop. III stars on the heating and ionization of the intergalactic medium (IGM), focusing on how these effects manifest in the 21cm power spectrum.

Simulating Pop. III Star Formation

The authors improve upon previous models by incorporating Pop. III star formation in mini-halos—small early-universe structures where these first stars likely formed. Because directly simulating Pop. III formation on large cosmological scales is computationally prohibitive, they calibrate their semi-analytical model using high-resolution simulations and scaling relations that account for star formation efficiency, initial mass function (IMF), and X-ray luminosity per unit star formation rate (LX/SFR). By applying these relationships to a large (310 cMpc) simulation, they can estimate the Pop. III star formation rate and its effect on the cosmic radiation background.

Impact on the 21cm Global Signal

The 21cm signal, which comes from neutral hydrogen, is sensitive to the heating and ionization history of the universe. The authors find that Pop. III stars do not significantly alter the overall reionization timeline but do impact the thermal state of the IGM. If Pop. III stars produce strong X-ray emissions, they heat the surrounding gas at redshifts z ≥ 15, affecting the 21cm signal earlier than previously thought. In scenarios where Pop. III stars have higher X-ray emission, the signal transitions from absorption to emission at higher redshifts, making it easier to detect.

Effects on the 21cm Power Spectrum

The power spectrum of the 21cm signal, which measures fluctuations in hydrogen brightness temperature, is another key observational tool. The authors find that models with stronger X-ray emission from Pop. III stars lead to increased power in the 21cm spectrum at redshifts z ≤ 10. This means that future radio telescopes, such as the Square Kilometer Array (SKA), might be able to distinguish between different Pop. III scenarios by measuring these fluctuations.

Observability with SKA

The authors assess the feasibility of detecting Pop. III effects using SKA1-low, a next-generation radio telescope. They simulate the expected 21cm power spectrum noise and find that, with 1000 hours of observation time, SKA should be able to differentiate between models where Pop. III stars produce significant X-ray heating and those where they do not. This suggests that SKA could provide crucial constraints on the properties of the first stars.

Conclusions

This study highlights the importance of Pop. III stars in early cosmic heating and their potential impact on the 21cm signal. By developing a scalable framework for including Pop. III star formation in large-scale simulations, the authors provide a roadmap for future observations. If the SKA detects the predicted power spectrum features, it could offer the first indirect evidence of the universe’s very first stars, bringing us closer to answering the question: When and where did the first stars shine?

Source: Ventura