UV luminosity density results at z 8 from the first JWST/NIRCam fields: limitations of early data sets and the need for spectroscopy

ABSTRACT We have derived luminosity functions and set constraints on the UV luminosity and SFR density from z ∼ 17 to z ∼ 8, using the three most-studied JWST/NIRCam data sets, the SMACS0723, GLASS Parallel, and CEERS fields. We first used our own selections on two independent reductions of these data sets using the latest calibrations. A total of 18 z ∼ 8, 12 z ∼ 10, 5 z ∼ 13, and 1 z ∼ 17 candidate galaxies are identified over these fields in our primary reductions, with a similar number of candidates in our secondary reductions. We then use these two reductions, applying a quantitative discriminator, to segregate the full set of z ≥ 8 candidates reported over these fields from the literature, into three different samples, ‘robust’, ‘solid’, and ‘possible’. Using all of these samples, we then derive UV LF and luminosity density results at z ≥ 8, finding substantial differences. For example, including the full set of ‘solid’ and ‘possible’ z ≥ 12 candidates from the literature, we find UV luminosity densities, which are ∼7× and ∼20× higher than relying on the ‘robust’ candidates alone. These results indicate the evolution of the UV LF and luminosity densities at z ≥ 8 is still extremely uncertain, emphasizing the need for spectroscopy and deeper NIRCam + optical imaging to obtain reliable results. Nevertheless, even with the very conservative ‘robust’ approach to selections, both from our own and those of other studies, we find the luminosity density from luminous (MUV −19) galaxies to be ∼2 × larger than is easily achievable using constant star formation efficiency models, similar to what other early JWST results have suggested.