We calculate the spin temperature and 21 cm brightness of early \HII regions around the first stars. We use outputs from cosmological radiation-hydrodynamics simulations of the formation and evolution of early \HII regions. In the pre-reionization era, \HII regions around massive primordial stars have diameters of a few kpc. The gas within the \HII regions is almost fully ionized, but begins recombining after the central stars die off. The relic \HII regions are then seen as bright {\it emission} sources in hydrogen 21 cm. We make brightness temperature maps of the \HII regions, accounting for radiative coupling with Lyman-$α$ photons in a simplified manner. The spin temperature in the relic \HII region is close to the gas kinetic temperature, generally several hundred to several thousand degrees. We show that the relic \HII region can be as bright as $δT_{\rm b} \sim 100$ mK in differential temperature against the cosmic microwave background for an angular resolution of sub-arcseconds. While individual early \HII patches will not be identified by currently planned radio telescopes, the collective fluctuations from early \HII regions might imprint signatures in the 21 cm background.
Spectral Signatures of Spinning Dust from Grain Ensembles in Diverse Environments: A Combined Theoretical and Observational Study
University of Manchester（曼彻斯特大学）；Jodrell Bank Centre for Astrophysics, University of Manchester（曼彻斯特大学乔德雷尔银行天体物理学中心）；Instituto de Astrofísica de Canarias（加那利群岛天体物理研究所）；Universidad de La Laguna (ULL)（拉帕鲁纳大学）
Smoking-gun signatures of bounce cosmology from echoes of relic gravitational waves
College of Physics, Sichuan University（四川大学物理学院）；Department of Astronomy, School of Physical Sciences, University of Science and Technology of China（中国科学技术大学物理科学学院天文学系）；CAS Key Laboratory for Researches in Galaxies and Cosmology, School of Astronomy and Space Science, University of Science and Technology of China（中国科学院星系与宇宙学重点实验室，中国科学技术大学天文与空间科学学院）
Illuminating Scalar Dark Matter Co-Scattering in EFT with Monophoton Signatures
LAPTh, CNRS, USMB（拉普斯理论物理实验室，法国国家科学研究中心，萨瓦大学）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Department of Physics, Chung-Ang University（中央大学物理系）；Center for Quantum Spacetime, Sogang University（西江大学量子时空中心）；Department of Physics, Sogang University（西江大学物理系）
Gravitational Wave Signatures from Lepton Number Breaking Phase Transitions with Flat Potentials
Instituto de Física Corpuscular, CSIC-Universitat de València（西班牙自然科学研究理事会-巴伦西亚大学粒子物理研究所）；Universitat de València（巴伦西亚大学）；Institute for Basic Science (IBS)（韩国基础科学研究院）；Sogang University（西江大学）
