The Spin-Resolved Atomic Velocity Distribution and 21-cm Line Profile of Dark-Age Gas

Abstract

The 21-cm hyperfine line of atomic hydrogen (HI) is a promising probe of the cosmic dark ages. In past treatments of 21-cm radiation it was assumed the hyperfine level populations of HI could be characterized by a velocity-independent ``spin temperature'' T_s determined by a competition between 21-cm radiative transitions, spin-changing collisions, and (at lower redshifts) Lyman-alpha scattering. However we show here that, if the collisional time is comparable to the radiative time, the spin temperature will depend on atomic velocity, T_s=T_s(v), and one must replace the usual hyperfine level rate equations with a Boltzmann equation describing the spin and velocity dependence of the HI distribution function. We construct here the Boltzmann equation relevant to the cosmic dark ages and solve it using a basis-function method. Accounting for the actual spin-resolved atomic velocity distribution results in up to a 2 per cent suppression of the 21-cm emissivity, and a redshift and angular-projection dependent suppression or enhancement of the linear power spectrum of 21-cm fluctuations of up to 5 per cent. The effect on the 21-cm line profile is more dramatic -- its full-width at half maximum (FWHM) can be enhanced by up to 60 per cent relative to the velocity-independent calculation. We discuss the implications for 21-cm tomography of the dark ages.

arXiv comment25 pages, 6 figures, submitted to Mon. Not. Roy. Astron. Soc

References in corpus (28)

• The Cosmic Microwave Background Spectrum from the Full COBE/FIRAS Data Set
• 21-cm Tomography of the Intergalactic Medium at High Redshift
• A New Calculation of the Recombination Epoch
• Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization
• 21 Centimeter Fluctuations from Cosmic Gas at High Redshifts
• Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation
• Multifrequency analysis of 21 cm fluctuations from the Era of Reionization
• The Global 21 Centimeter Background from High Redshifts
• Wouthuysen-Field coupling strength and application to high-redshift 21 cm radiation
• The Spin-Kinetic Temperature Coupling and the Heating Rate due to Lyman Alpha Scattering before Reionization: Predictions for 21cm Emission and Absorption
• A Method for Separating the Physics from the Astrophysics of High-Redshift 21cm Fluctuations
• The CMBR fluctuations from HI perturbations prior to reionization
• Twenty-one centimeter tomography with foregrounds
• On the Direct Detectability of the Cosmic Dark Ages: 21-cm Emission from Minihalos
• HI 21cm absorption beyond the epoch of re-ionization
• The spin temperature and 21cm brightness of the intergalactic medium in the pre-reionization era
• Descending from on high: Lyman series cascades and spin-kinetic temperature coupling in the 21 cm line
• Improving Foreground Subtraction in Statistical Observations of 21 cm Emission from the Epoch of Reionization
• The 21 cm Forest: Radio Absorption Spectra as Probes of Minihalos Before Reionization
• The First Miniquasar
• Growth of Linear Perturbations before the Era of the First Galaxies
• Nonlinear clustering during the cosmic Dark Ages and its effect on the 21-cm background from minihalos
• UV pumping of hyperfine transitions in the light elements, with application to 21-cm hydrogen and 92-cm deuterium lines from the early universe
• The 21 Centimeter Background from the Cosmic Dark Ages: Minihalos and the Intergalactic Medium before Reionization
• Redshifted 21 Centimeter Emission from Minihalos Before Reionization
• Measuring the Primordial Deuterium Abundance During the Cosmic Dark Ages
• Polarization of 21cm Radiation from the Epoch of Reionization
• Polarization Signals of the 21 cm Background from the Era of Reionization

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• 21-cm cosmology
• The Physics of the Intergalactic Medium
• 21 cm fluctuations from inhomogeneous X-ray heating before reionization
• Evolution of the 21 cm signal throughout cosmic history
• HERA Phase I Limits on the Cosmic 21-cm Signal: Constraints on Astrophysics and Cosmology During the Epoch of Reionization
• The 21cm angular-power spectrum from the dark ages
• Fast Large Volume Simulations of the 21 cm Signal from the Reionization and pre-Reionization Epochs
• Eliminating the optical depth nuisance from the CMB with 21 cm cosmology
• 21 cm signal from cosmic dawn: Imprints of spin temperature fluctuations and peculiar velocities
• Primordial non-gaussianity from the bispectrum of 21-cm fluctuations in the dark ages
• Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation
• The subtlety of Ly-a photons: changing the expected range of the 21-cm signal
• A new light on 21 cm intensity fluctuations from the dark ages
• Spin Exchange Rates in Electron-Hydrogen Collisions
• Forecasted 21 cm constraints on compensated isocurvature perturbations
• Spin Exchange Rates in Proton-Hydrogen Collisions
• Interpreting the Global 21-cm Signal from High Redshifts. I. Model Independent Constraints
• Background reionization history from omniscopes
• CMB and 21-cm Signals for Dark Matter with a Long-Lived Excited State
• Particle decay in the early universe: predictions for 21 cm
• New probe of magnetic fields in the prereionization epoch. I. Formalism
• Cosmic 21-cm Fluctuations as a Probe of Fundamental Physics
• Intergalactic heating by Lyman-alpha photons including hyperfine structure corrections
• Primordial Magnetic Fields in Light of Dark Ages Global 21-cm Signal
• The Fundamentals of the 21-cm Line