Probing the warm dark matter mass with [C II] intensity mapping

¹ Argelander-Institut für Astronomie, Universität Bonn Auf dem Hügel 71 53121 Bonn, Germany
² SISSA, International School for Advanced Studies Via Bonomea 265 34136 Trieste, Italy
³ INAF, Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste Via Tiepolo 11 34131 Trieste, Italy
⁴ IFPU, Institute for Fundamental Physics of the Universe Via Beirut 2 34151 Trieste, Italy
⁵ INFN, National Institute for Nuclear Physics of the Universe Via Valerio 2 34127 Trieste, Italy
⁶ Laboratoire d’Annecy de Physique Théorique (LAPTh), CNRS/USMB 99 Chemin de Bellevue BP110 Annecy F-74941, France
⁷ ICSC – Centro Nazionale di Ricerca in High Performance Computing, Big Data and Quantum Computing Via Magnanelli 2 Bologna, Italy
⁸ Cardiff Hub for Astrophysics Research and Technology, School of Physics and Astronomy, Cardiff University, Queen’s Buildings Cardiff CF24 3AA, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 December 2025
Accepted: 12 January 2026

Abstract

Context. The nature of dark matter (DM) is still debated. While cold DM (CDM) is the standard paradigm, warm DM (WDM) composed of thermal relics may ease some small-scale tensions in the ΛCDM framework. Line-intensity mapping (LIM) offers a novel probe of DM properties.

Aims. To explore the potential of LIM surveys in constraining the WDM particle mass (m_WDM) by means of the [C II] power spectrum (PS), we provide forecasts for the Deep Spectroscopic Survey (DSS) to be performed with the Fred Young Submillimeter Telescope at z ≃ 3.6 and extend the analysis to larger sky coverage, higher sensitivity, and/or increased spectral resolution.

Methods. We developed a formulation for the [C II] PS based on the halo-model approach, incorporating the uncertainty in the luminosity function (LF) through two alternative parameterisations, one optimistic and the other more conservative. We performed a Bayesian analysis on mock data to derive constraints on m_WDM.

Results. In a CDM universe, the DSS yields lower limits on m_WDM, at a 95% credibility level, of 1.10 keV and 0.58 keV when considering the optimistic and pessimistic LF (α = −1.1), respectively. Ambitious surveys can improve these figures to 5.82 keV and 1.90 keV, and assuming a steeper faint-end slope (α = −1.9) further boosts the constraints even beyond those obtained in the optimistic scenario. A fivefold increase in spectral resolution enhances sensitivity to the damping scale associated with redshift-space distortions, tightening the constraints on m_WDM by a factor of up to ∼1.8. Finally, Bayesian inference on mock data with m_WDM = 3 keV results in a well-constrained and unbiased posterior only in futuristic survey setups.

Conclusions. Upcoming LIM surveys can provide meaningful limits on m_WDM, although the negligible contribution from small haloes reduces the constraining power of the [C II] PS. Future progress will benefit from combining multiple redshifts and emission lines, opening the way to competitive constraints on the nature of DM.