Warm dark matter constraints from high-z direct collapse black holes using the JWST

We use a semi-analytic model, Delphi, which jointly tracks the dark matter and baryonic assembly of high-redshift (z ≃ 4-20) galaxies to gain insight on the number density of dicollapse black hole (DCBH) hosts in three different cosmologies: the standard cold dark matter (CDM) model and two warm dark matter (WDM) models with particle masses of 3.5 and 1.5 keV. Using the Lyman-Werner (LW) luminosity of each galaxy from Delphi, we use a clustering bias analysis to identify all, pristine haloes with a virial temperature Tvir ≳ 104 K which are irradiated by an LW background above a critical value as DCBH hosts. In good agreement with previous studies, we find the DCBH number density rises from ∼10-6.1 to ∼10-3.5 cMpc-3 from z ≃ 17.5 to 8 in the CDM model using a critical LW background value of 30J21 (where J21 = 10-21 erg s-1 Hz-1 cm-2 sr-1). We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts (z ≃ 5) as compared to CDM where DCBH hosts only exist down to z ≃ 8. We end by showing how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcoming James Webb Space Telescope can be used to hunt for potential z ≃ 5-9 DCBHs, allowing hints on the WDM particle mass.