Hydrogen Epoch of Reionization Array (HERA)

Abstract: The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization ($z=6-12$), and to explore earlier epochs of our Cosmic Dawn ($z\sim30$). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA's scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.

• The paper outlines HERA's innovative design using 350 14-m dishes to achieve heightened sensitivity for detecting 21 cm cosmological signals.
• It employs redundant calibration and delay-spectrum analysis to distinguish faint 21 cm signals from dominant astrophysical foregrounds.
• The findings are set to refine the reionization timeline, constrain key cosmological parameters, and improve models of early galaxy formation.

The Hydrogen Epoch of Reionization Array (HERA): Addressing Challenges in 21 cm Cosmology

The paper, "Hydrogen Epoch of Reionization Array (HERA)," provides a detailed plan for HERA, an ambitious scientific instrument designed to explore the universe's early epochs, namely Cosmic Dawn and Reionization, through observations of the hydrogen 21 cm line. HERA represents a critical step toward understanding the evolution of the early universe and the formation of the first galaxies and black holes.

HERA is a radio interferometer comprising 350 14-meter parabolic dishes located in the Karoo Desert, South Africa. As a highly sensitive instrument, it aims to detect faint signals from neutral hydrogen atoms present during the universe's formative epochs, specifically focusing on the redshifted 21 cm line. The primary goal of HERA is to measure the 21 cm power spectrum accurately, thus enabling researchers to infer the timing and evolution of reionization, the characteristics of the first galaxies, and the development of large-scale cosmic structures.

Technical Innovations and Approaches

The paper outlines several key technical specifications for HERA that distinguish it from preceding experiments like PAPER, MWA, and LOFAR. The use of large 14-meter dishes is notable, as it significantly increases the collecting area, enhancing the instrument's sensitivity by a substantial margin. The dish design is intended to minimize signal reflections and characterize the effective beam pattern accurately, as verified by simulation and measurement data.

HERA employs a robust calibration approach leveraging redundancy in the antenna layout, allowing for self-calibration without relying heavily on external sky models. This method enhances the precision of the power spectrum measurements by optimizing the short baselines that are least affected by foreground contamination, known as the "wedge."

The strategy involves a combination of delay-spectrum analysis and sophisticated foreground filtering techniques. These methods exploit the spectral smoothness of astrophysical foregrounds to isolate and remove them from the desired 21 cm signal. This technique, validated by precursor experiments, leverages the intrinsic chromatic nature of interferometric measurements.

Scientific Objectives and Implications

The scientific motivations for HERA are compelling. By measuring the 21 cm power spectrum, HERA can provide constraints on several key aspects of early cosmic history:

1. Epoch of Reionization (EOR): Precise measurements of the power spectrum will allow researchers to delineate the reionization timeline and morphology, yielding insights into the interplay between the first stars, galaxies, and the intergalactic medium.
2. Cosmic Dawn and Early Heating: HERA is designed to probe the earlier phase of Cosmic Dawn, seeking evidence of the first generation of luminous sources that heated the neutral hydrogen gas.
3. Precision Cosmology: The data from HERA will mitigate uncertainties in cosmological parameters, such as the optical depth to reionization, which affects CMB interpretations.
4. Theoretical Models: HERA's observations promise to refine our understanding of astrophysical processes and their implications for theoretical models of structure formation and galaxy evolution in the early universe.

Future Prospects and Broader Impacts

The paper suggests that HERA will lay the foundational observational groundwork necessary to fully exploit the capabilities of next-generation instruments like the Square Kilometre Array (SKA). The ability of HERA to integrate theoretical advances and complement observations at other wavelengths demonstrates its potential multidisciplinary impact.

Beyond its primary goals, HERA serves as a platform for conducting a wide array of scientific investigations, including studies of cosmic variance, precursor phenomena, and even potential serendipitous discoveries in transient and exoplanetary radio astronomy.

In conclusion, HERA is a meticulously designed instrument poised to make profound contributions to our understanding of the universe's formative periods. Its deployment represents a crucial advancement in radio astronomy and cosmology, targeting challenges surrounding the extraction of faint 21 cm signals embedded within dominant foregrounds. The successful realization of its scientific potential promises to inform and reshape our comprehension of the cosmos's evolutionary history.