Warm dark matter constraints from the JWST

Warm Dark Matter (WDM) particles with masses ($\sim$ kilo electronvolt) offeran attractive solution to the small-scale issues faced by the Cold Dark Matter(CDM) paradigm. The delay of structure formation in WDM models and theassociated dearth of low-mass systems at high-redshifts makes this an idealtime to revisit WDM constraints in light of the unprecedented data-sets fromthe James Webb Space Telescope (JWST). Developing a phenomenological modelbased on the halo mass functions in CDM and WDM models, we calculatehigh-redshift ($z \gt 6$) the stellar mass functions (SMF) and the associatedstellar mass density (SMD) and the maximum stellar mass allowed in a givenvolume. We find that: (i) WDM as light as 1.5 keV is already disfavoured by thelow-mass end of the SMF (stellar mass $M_* \sim 10^7 \rm{M_\odot}$) althoughcaution must be exerted given the impact of lensing uncertainties; (ii) 1.5 keVWDM models predict SMD values that show a steep decrease from $10^{8.8}$ to$10^{2} ~{\rm M_\odot ~cMpc^{-3}}$ from $z \sim 4$ to 17 for $M_* \gt 10^8\rm{M_\odot}$; (iii) the 1.5 keV WDM model predicts a sharp and earlier cut-offin the maximum stellar masses for a given number density (or volume) ascompared 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 objectsat $z \gt 12$ (18). Forthcoming JWST observations of multiple blank fields cantherefore be used as a strong probe of WDM at an epoch inaccessible by othermeans.