[Paper Review] Astro2020 Science White Paper: First Stars and Black Holes at Cosmic Dawn with Redshifted 21-cm Observations

Jordan Mirocha, Daniel Jacobs+40 more|arXiv (Cornell University)|Mar 14, 2019

Radio Astronomy Observations and Technology39 references4 citations

TL;DR

This white paper advocates for low-frequency radio observations of redshifted 21-cm emission to study the formation of the first stars and black holes during cosmic dawn (z ≳ 10). By probing the thermal and ionization state of neutral hydrogen, 21-cm signals offer a unique, indirect window into early astrophysical sources, with the EDGES anomaly at 78 MHz providing a potential early signature of star formation and novel physics at z ≳ 18.

ABSTRACT

The "cosmic dawn" refers to the period of the Universe's history when stars and black holes first formed and began heating and ionizing hydrogen in the intergalactic medium (IGM). Though exceedingly difficult to detect directly, the first stars and black holes can be constrained indirectly through measurements of the cosmic 21-cm background, which traces the ionization state and temperature of intergalactic hydrogen gas. In this white paper, we focus on the science case for such observations, in particular those targeting redshifts z $\gtrsim$ 10 when the IGM is expected to be mostly neutral. 21-cm observations provide a unique window into this epoch and are thus critical to advancing first star and black hole science in the next decade.

Motivation & Objective

• To establish the scientific case for using redshifted 21-cm observations to constrain the formation and properties of the first stars and black holes during cosmic dawn.
• To address the critical lack of direct observational constraints on early star and black hole formation, which remain poorly understood despite their central role in galaxy evolution.
• To highlight the unique sensitivity of 21-cm measurements to the thermal and ionization state of the intergalactic medium (IGM), enabling indirect but powerful constraints on high-redshift sources.
• To emphasize the need for coordinated theoretical and experimental advances to interpret anomalies such as the EDGES 78 MHz feature, which may signal non-standard physics or early star formation.
• To promote the development of both interferometric and global 21-cm experiments to minimize systematics and maximize robustness in detecting the faint cosmic dawn signal.

Proposed method

• Utilize redshifted 21-cm emission from neutral hydrogen to trace the thermal and ionization state of the intergalactic medium (IGM) at z ≳ 10.
• Model the 21-cm brightness temperature contrast (δTb) as a function of redshift, accounting for spin temperature, kinetic temperature, and ionization fraction.
• Apply interferometric techniques to map large-scale 21-cm fluctuations over hundreds of square degrees to minimize cosmic variance and resolve spatial gradients.
• Use global 21-cm signal measurements to detect spectral features such as the anomalous 78 MHz dip reported by EDGES, which may indicate early star formation or dark matter interactions.
• Implement multi-frequency, multi-site observations to reduce instrumental systematics and distinguish foreground contamination from cosmological signals.
• Integrate 21-cm data with galaxy surveys and intensity mapping to cross-correlate sources and disentangle astrophysical and cosmological effects.

Experimental results

Research questions

• RQ1 What are the formation redshifts, masses, and ionizing efficiencies of the first stars and black holes at z ≳ 10?
• RQ2 How do radiative and X-ray feedback from early sources regulate star formation in low-mass halos during cosmic dawn?
• RQ3 Can the anomalous 78 MHz feature in the EDGES global signal be explained by early star formation or non-standard physics such as baryon-dark matter interactions?
• RQ4 What constraints can 21-cm power spectra and brightness temperature fluctuations place on the ionization and heating history of the IGM before reionization?
• RQ5 How can imaging of the 21-cm field at arcminute resolution and large angular scales help distinguish between different models of early black hole and star formation?

Key findings

• The 21-cm background provides a unique, indirect probe of the first stars and black holes by tracing the thermal and ionization state of the neutral intergalactic medium at z ≳ 10.
• The EDGES 78 MHz feature, if confirmed, implies star formation at z ≳ 18 and places constraints on the efficiency of star and black hole formation in small halos at 10 ≲ z ≲ 30.
• 21-cm observations can detect the growth of ionized bubbles and thermal feedback from early sources, even when the sources themselves are too faint to be seen directly.
• Interferometric surveys covering >100 deg² are necessary to minimize cosmic variance and resolve the large-scale structure of the 21-cm signal during cosmic dawn.
• Global 21-cm measurements remain essential despite foreground challenges, especially for detecting spectral features like the EDGES anomaly, which may signal novel physics.
• Future 21-cm experiments must be designed with independent systematics and multiple observing sites to validate anomalous signals and avoid false positives.