Boosting the cosmic 21-cm signal with exotic Lyman-$α$ from dark matter

TL;DR

This work introduces a novel DM-induced Lyman-$\alpha$ channel that seeds early Wouthuysen-Field coupling and strengthens the 21-cm signal during cosmic dawn, reducing dependence on uncertain first-star UV flux. By considering decays $\chi\to\gamma\gamma$ with $m_\chi \in [20.4,50]$ eV, and in particular $m_\chi \in [20.4,27.2]$ eV where the photons populate the Lyman-$\alpha$ band, they derive constraints that are largely decoupled from astrophysical heating uncertainties. They implement energy deposition via DM21cm/DarkHistory, correct threshold/binning artifacts, and forecast sensitivities for HERA and SKA1-Low, obtaining competitive bounds on light decaying DM and axion-like particles (e.g., SKA1-Low limits imply $g_{a\gamma\gamma} < 4.1\times10^{-13}$ GeV$^{-1}$ for $m_a=20.4$ eV). The results remain robust across different astrophysical scenarios and offer a path to probing otherwise unconstrained DM parameter space even with non-detections, with future lunar/space-based 21-cm experiments further enhancing reach.

Abstract

The 21-cm signal from the epoch of cosmic dawn ($z \sim 10-30$) offers a powerful probe of new physics. One standard mechanism for constraining decaying dark matter from 21-cm observations relies on heating of the intergalactic medium by the decay products, an effect whose observability is entangled with the uncertain Lyman-$α$ fluxes and X-ray heating from the first stars. In this Letter, we explore a novel mechanism, where the Lyman-$α$ photons produced from dark matter decay initiate early Wouthuysen-Field coupling of the spin temperature to the gas temperature, thereby boosting the 21-cm signal. This mechanism provides constraints on dark matter that are less dependent on uncertainties associated with star formation than constraints on exotic heating. We study this effect for decaying dark matter with masses $m_χ\sim20.4-27.2$ eV, where diphoton decay efficiently produces Lyman-series photons. We present forecasts for the Hydrogen Epoch of Reionization Array and the Square Kilometre Array, showing their potential to probe an unconstrained parameter space for light decaying DM, including axion-like particles.

Figures (8)

• Figure 1: The deposition of energy by channel. We show the fraction $f$ of energy deposited into the relevant channels $c$ for different masses $m_\chi$, at $z = 35$, computed using DarkHistoryLiu:2019bbm, with lifetime $\tau = 10^{28}$ s. The bin boundaries from DarkHistory are shown as a step function, and bin centers marked with a dot. The shaded gray regions indicate where the DarkHistory computation fails, due to binning issues, and we show with the colored bands the corrected behavior of each channel.
• Figure 2: The effect of decaying dark matter on the global signal and power spectrum. We show the effect of decaying dark matter with various lifetimes $\tau$ and with masses $m_{\chi} = 20.4~{\rm eV}$ and $m_{\chi} = 50~{\rm eV}$, on global signal (top) and power spectrum (bottom) at $k \simeq 0.2$ Mpc$^{-1}$. The $m_{\chi} = 20.4~{\rm eV}$ scenario, labeled by "Ly$\alpha$", enhances the signal in both figures, whereas the $m_{\chi} = 50~{\rm eV}$ scenario, labeled by "Heat", suppresses the signal in both figures. We include the 1$\sigma$ experimental thermal error computed for SKA1-Low in the bottom panel. We use the same color bar for both scenarios, noting that they are separated by the dotted line denoting the decay-free scenario.
• Figure 3: Projected 95th percentile limits on monochromatic decays, with HERA and SKA1-Low. We also show bounds computed using the 21-cm signal for molecularly cooled galaxies (21-cm MCG HERA) and atomically cooled galaxies (21-cm ACG HERA) Facchinetti:2023slb, CMB limits computed below and above the ionization thresholds (CMB) xu_cmb_2024, HST limits from galaxy UV searches (HST UV) Todarello:2024qci, limits from anomalous heating of the Leo T dwarf galaxy (Leo T) Wadekar:2021qae, and the region of interest identified for Lyman-Werner direct collapse black holes (DC-SMBH), with shielding parameters $\varepsilon_{sh} = 0.1, \, 0.5, \, 1.0$Lu:2024zwa.
• Figure 4: Comparison of the impact of exotic Lyman-$\boldsymbol{\alpha}$ on the 21-cm observables for different astrophysics scenarios. We show the Lyman-$\alpha$--induced shift to the power spectrum for a decaying dark matter particle with mass $m_\chi = 20.4$ eV, at wavenumber $k \simeq 0.2$ Mpc$^{-1}$, as in Fig. \ref{['fig:decaying_dm_global_PS']}. The different astrophysical scenarios are defined in Table \ref{['tab:fiducial_scenarios']}.
• Figure S1: Comparison of the 21-cm power spectrum at different scales. We show the Lyman-$\alpha$--induced shift to the power spectrum for a decaying dark matter particle with mass $m_\chi = 20.4$ eV, as in Fig \ref{['fig:decaying_dm_global_PS']}, but for different wavenumbers, $k$. All panels include the 1$\sigma$ experimental thermal error computed for SKA1-Low, and the fiducial scenario without decays.