Warm dark matter constraints from the JWST

ABSTRACT 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 ($z \gtrsim 6$), 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 $M_* \sim 10^7\,\rm M_\odot$) 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 $10^{2}\,\rm M_\odot \rm cMpc^{-3}$ from z ∼ 4 to 17 for $M_* \gtrsim 10^8\,\rm M_\odot$; 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 $10^{-3}\,\rm {cMpc^{-3}}$, 1.5 (3) KeV WDM models do not predict bound objects at $z \gtrsim 12$ (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.