[Paper Review] Tomography of the Cosmic Dawn and Reionization Eras with Multiple Tracers

Tzu‐Ching Chang, Angus Beane+42 more|arXiv (Cornell University)|Mar 28, 2019

Solar and Space Plasma Dynamics2 references7 citations

TL;DR

This paper advocates for a multi-tracer tomographic approach to map the Cosmic Dawn and Epoch of Reionization (EoR), combining line intensity mapping (LIM) of multiple emission lines with 21 cm and Lyα tomography to trace the large-scale topology and physical conditions of the intergalactic and interstellar medium. It demonstrates that multi-wavelength LIM surveys can provide direct, statistical constraints on reionization history, metal and dust evolution, and ionizing source properties, significantly improving upon single-tracer 21 cm surveys alone.

ABSTRACT

The Cosmic Dawn and Reionization epochs remain a fundamental but challenging frontier of astrophysics and cosmology. We advocate a large-scale, multi-tracer approach to develop a comprehensive understanding of the physics that led to the formation and evolution of the first stars and galaxies. We highlight the line intensity mapping technique to trace the multi-phase reionization topology on large scales, and measure reionization history in detail. Besides 21cm, we advocate for Lya tomography mapping during the epoch of Wouthuysen-Field coupling as an additional probe of the cosmic dawn era.

Motivation & Objective

• To overcome the limitations of single-tracer 21 cm surveys by introducing multi-tracer line intensity mapping (LIM) to better constrain the large-scale topology and physical conditions during the Cosmic Dawn and Epoch of Reionization (EoR).
• To improve understanding of reionization history, star formation, dust, and metal enrichment by combining LIM of multiple emission lines from different phases of the intergalactic and interstellar medium.
• To advocate for Lyα tomography as a complementary probe during the cosmic dawn, when Wouthuysen-Field coupling may produce a detectable faint glow from early UV photons.
• To enable future cosmological surveys through technological advancements in wideband, high-sensitivity spectroscopy across radio, mm, sub-mm, far-IR, and near-IR bands.
• To integrate theoretical modeling, data analysis, and instrumentation development across CMB, LIM, and EoR communities to accelerate progress in the 2020s and 2030s.

Proposed method

• Utilize line intensity mapping (LIM) to measure collective emission from multiple atomic and molecular transitions (e.g., [C II], [N II], [O III], Hα, Lyα, HeII 1640 Å, H2) on Mpc-scale and larger baselines.
• Combine 21 cm tomography of neutral hydrogen with Lyα tomography of the Wouthuysen-Field coupled gas to trace both neutral and ionized phases during cosmic dawn and EoR.
• Leverage wideband, high-sensitivity spectroscopic instruments across radio (HERA, SKA1-LOW), mm/sub-mm (future on-chip spectrometers), far-IR (Origins Space Telescope), and near-IR (SPHEREx, CDIM) to access multiple redshifted lines.
• Integrate data from LIM surveys with existing constraints from Planck (optical depth τ = 0.054 ± 0.007), HST, ALMA, and global 21 cm measurements (e.g., EDGES) to build a consistent multi-phase picture.
• Apply advanced data analysis techniques and theoretical modeling to disentangle astrophysical foregrounds and extract faint, diffuse line signals from early cosmic structures.
• Use multi-tracer cross-correlations to infer the large-scale distribution of ionizing sources, metallicity, dust content, and gas temperature evolution during reionization.

Experimental results

Research questions

• RQ1 How can multi-tracer line intensity mapping improve constraints on the reionization history compared to 21 cm surveys alone?
• RQ2 What is the contribution of early stars, quasars, and AGN to the ionizing background during the Cosmic Dawn and EoR?
• RQ3 Can Lyα tomography during the Wouthuysen-Field coupling phase reveal the faint, diffuse glow from early UV photons and constrain early structure formation?
• RQ4 How do the physical conditions of the intergalactic medium—such as temperature, ionization state, and metallicity—evolve across cosmic time during reionization?
• RQ5 What instrumental and data analysis advancements are required to detect faint, diffuse line emissions from the EoR and cosmic dawn across multiple bands?

Key findings

• Line intensity mapping of multiple transitions (e.g., [C II], [N II], [O III], Hα, Lyα) enables statistical, large-scale mapping of the intergalactic medium’s physical state during reionization, complementing 21 cm surveys.
• The minimum Lyα photon production rate required for effective Wouthuysen-Field coupling is approximately one Lyα photon per ten hydrogen atoms, setting a detectability threshold for cosmic dawn signals.
• Recombination-produced Lyα photons may dominate the average specific intensity at Lyα frequencies, even though they are localized near sources and not useful for global spin temperature coupling.
• Future instruments such as SPHEREx and the Cosmic Dawn Intensity Mapper (CDIM) can measure Hα from z = 0.5 to 8 and potentially detect Lyα fluctuations during reionization.
• Background-limited far-IR spectrographs on cryogenic space telescopes like the Origins Space Telescope can detect [O I] at z = 5–8 and H2 out to z = 15, enabling deep far-IR LIM.
• Synergies between CMB and LIM communities are expected to accelerate the development of high-sensitivity, wideband, multi-band spectroscopic instruments for EoR and cosmic dawn studies.