Researchers face a captivating mystery surrounding the formation of the first galaxies in the universe. Equally intriguing is the enigma surrounding the nature of dark matter, a crucial challenge for fundamental physicists. However, a joint research team from Northeastern University in China and the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has put forth a novel approach to illuminate both the nature of dark matter and the early genesis of galaxies. Their study, published in Nature Astronomy on July 6, introduces a new probe that tackles these intertwined mysteries.
To comprehend dark matter, scientists seek to measure the mass of dark matter particles by studying cosmological observations of small-scale structures. Yet, detecting such structures, devoid of any star formation, proves arduous, particularly during the cosmic dawn. Fortunately, the presence of atomic hydrogen gas in and around these dim, petite structures from the cosmic dawn generates 21-cm absorption lines along the path between Earth and high-redshift radio point sources. This collection of absorption lines is collectively known as the 21-cm forest.
The 21-cm forest probe has been a theoretical concept for over two decades, aiming to explore gas temperatures and potentially unveil dark matter properties during the cosmic dawn. However, scientists have not yet attempted to deploy the probe due to various challenges. These obstacles include extremely faint signals, the difficulty in identifying high-redshift background sources, and the intricate relationship between the mass of dark matter particles and the heating effect. This interplay hampers the probe's ability to constrain either the particle mass or the heating effect emanating from the first galaxies.
Recently, the discovery of numerous high-redshift radio-loud quasars has bolstered the prospects of the 21-cm forest probe. Furthermore, the construction of the Square Kilometer Array (SKA), a colossal international initiative to construct the world's largest radio telescope, commenced in December. These advancements indicate that the utilization of the 21-cm forest probe will soon become viable, holding promise for unraveling the mysteries that surround dark matter and the early formation of galaxies.
Drawing inspiration from the widely utilized power spectrum analyses in cosmological investigations, the researchers at NAOC devised an innovative statistical solution to address the challenges posed by the weak signal and degeneracy problems. They recognized that the distinct scale-dependent characteristics of the signals resulting from the warm dark matter effect and the heating effect could be leveraged to extract key features statistically, enabling differentiation between the two effects.
In their study, the team proposed measuring the one-dimensional (1-D) power spectrum of the 21-cm forest as a novel approach to concurrently tackle the weak signal problem and the degeneracy problem. The amplitude and shape of the 1-D power spectrum unveil the scale-dependent nature of the signal, making the 21-cm forest probe a viable and effective method for probing dark matter properties and the thermal history of the universe simultaneously.
Xu Yidong, the corresponding author of the study, stated, “By measuring the one-dimensional power spectrum of the 21-cm forest, we can not only overcome the challenges of weak signals by enhancing sensitivity but also provide a means to distinguish between warm dark matter models and the early heating process. It's like killing two birds with one stone.”
In scenarios where cosmic heating is relatively mild, the low-frequency array of SKA Phase 1 exhibits sufficient capabilities to constrain both the mass of dark matter particles and gas temperature effectively. However, in cases where cosmic heating is more substantial, the utilization of multiple background radio sources during SKA Phase 2 will enable robust detection capabilities.
The 21-cm forest presents a promising avenue for constraining dark matter in redshift ranges beyond the reach of other observational methods. Additionally, by measuring the level of heating, the 21-cm forest provides insights into the spectral properties of the earliest galaxies and black holes, shedding light on the nature of the universe's first luminous objects. Harnessing the potential of the 21-cm forest probe is vital for advancing our understanding of the early universe and delving into the mysteries surrounding dark matter and the first galaxies.
As the application of the 21-cm forest probe hinges on observations of high-redshift background radio sources, the next steps involve identifying more radio-bright sources during the cosmic dawn (such as radio-loud quasars and gamma-ray burst afterglows) that can be further explored in the era of SKA.
Source: Chinese Academy of Sciences