2 research outputs found
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Are the ultra-high-redshift galaxies at z > 10 surprising in the context of standard galaxy formation models?
A substantial number of ultra-high redshift (817) galaxy candidates have been detected with JWST, posing the question: Are these observational results surprising in the context of current galaxy formation models? We address this question using the well-established Santa Cruz semi-analytic models, implemented within merger trees from the new suite of cosmological N-body simulations gureft, which were carefully designed for ultra-high redshift studies. Using our fiducial models calibrated at z = 0, we present predictions for stellar mass functions, rest-frame UV luminosity functions, and various scaling relations. We find that our (dust-free) models predict galaxy number densities at z ∼11 (z ∼13) that are an order of magnitude (a factor of ∼30) lower than the observational estimates. We estimate the uncertainty in the observed number densities due to cosmic variance, and find that it leads to a fractional error of ∼20-30 per cent at z = 11 (∼30-80 per cent at z = 14) for a 100 arcmin2 field. We explore which processes in our models are most likely to be rate-limiting for the formation of luminous galaxies at these early epochs, considering the halo formation rate, gas cooling, star formation, and stellar feedback, and conclude that it is mainly efficient stellar-driven winds. We find that a modest boost of a factor of ∼4 to the UV luminosities, which could arise from a top-heavy stellar initial mass function, would bring our current models into agreement with the observations. Adding a stochastic component to the UV luminosity can also reconcile our results with the observations.</p
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Spectral templates optimal for selecting galaxies at z > 8 with the JWST
The selection of high-redshift galaxies often involves spectral energy distribution (SED) fitting to photometric data, an expectation for contamination levels, and measurement of sample completeness—all vetted through comparison to spectroscopic redshift measurements of a sub-sample. The first JWST data are now being taken over several extragalactic fields to different depths and across various areas, which will be ideal for the discovery and classification of galaxies out to distances previously uncharted. As spectroscopic redshift measurements for sources in this epoch will not be initially available to compare with the first photometric measurements of z > 8 galaxies, robust photometric redshifts are of the utmost importance. Galaxies at z > 8 are expected to have bluer rest-frame ultraviolet (UV) colors than typically used model SED templates, which could lead to catastrophic photometric redshift failures. We use a combination of BPASS and Cloudy models to create a supporting set of templates that match the predicted rest-UV colors of z > 8 simulated galaxies. We test these new templates by fitting simulated galaxies in a mock catalog, Yung et al., which mimic expected field depths and areas of the JWST Cosmic Evolution Early Release Science Survey (m 5σ ∼ 28.6 over ∼100 arcmin2). We use EAZY to highlight the improvements in redshift recovery with the inclusion of our new template set and suggest criteria for selecting galaxies at 8 </p