Warm dark matter constraints from the JWST

Warm dark matter (WDM) particles with masses (∼ kilo electronvolt) offer an attractive solution to the small-scale issues faced by the cold dark matter (CDM) paradigm. The delay of structure formation in WDM models and the associated dearth of low-mass systems at high redshifts makes this an ideal time to revisit WDM constraints in light of the unprecedented datasets from the JWST. Developing a phenomenological model based on the halo mass functions in CDM and WDM models, we calculate high redshift (z 6), the stellar mass functions (SMF) and the associated stellar mass density (SMD) and the maximum stellar mass allowed in a given volume. We find that: (i) WDM as light as 1.5 keV is already disfavoured by the low-mass end of the SMF (stellar mass M_* ~ 10⁷\, M_) although caution must be exerted given the impact of lensing uncertainties; (ii) 1.5 keV WDM models predict SMD values that show a steep decrease from 108.8 to 10²\, M_ cMpc⁻³ from z ∼ 4 to 17 for M_* 10⁸\, M_; and (iii) the 1.5 keV WDM model predicts a sharp and earlier cut-off in the maximum stellar masses for a given number density (or volume) as compared to CDM or heavier WDM models. For example, with a number density of 10⁻³\, cMpc⁻³, 1.5 (3) KeV WDM models do not predict bound objects at z 12 (18). Forthcoming JWST observations of multiple blank fields can therefore be used as a strong probe of WDM at an epoch inaccessible by other means.