First Light And Reionisation Epoch Simulations (FLARES) VI: The colour evolution of galaxies $z=5-15$

Abstract

With its exquisite sensitivity, wavelength coverage, and spatial and spectral resolution, the James Webb Space Telescope (JWST) is poised to revolutionise our view of the distant, high-redshift (z > 5) Universe. While Webb's spectroscopic observations will be transformative for the field, photometric observations play a key role in identifying distant objects and providing more comprehensive samples than accessible to spectroscopy alone. In addition to identifying objects, photometric observations can also be used to infer physical properties and thus be used to constrain galaxy formation models. However, inferred physical properties from broadband photometric observations, particularly in the absence of spectroscopic redshifts, often have large uncertainties. With the development of new tools for forward modelling simulations it is now routinely possible to predict observational quantities, enabling a direct comparison with observations. With this in mind, in this work, we make predictions for the colour evolution of galaxies at z = 5 − 15 using the First Light And Reionisation Epoch Simulations (FLARES) cosmological hydrodynamical simulation suite. We predict a complex evolution, driven predominantly by strong nebular line emission passing through individual bands. These predictions are in good agreement with existing constraints from Hubble and Spitzer as well as some of the first results from Webb. We also contrast our predictions with other models in the literature: while the general trends are similar we find key differences, particularly in the strength of features associated with strong nebular line emission. This suggests photometric observations alone should provide useful discriminating power between different models.

Seeing sharper and deeper: JWST's first glimpse of the photometric and spectroscopic properties of galaxies in the epoch of reionisation

“…A1 we instead show the predictions from individual snapshots of the FLARES simulations (Lovell et al 2021;Vijayan et al 2021Vijayan et al , 2022Roper et al 2022;Wilkins et al 2022a,b). We do note that interpolating between snapshots can lead to more extreme colour evolution, as emission lines pass in and out of photometric bands (see Wilkins et al 2022b), which is an effect accounted for in the Yung et al (2022) lightcones. We find once again that the F356W−F444W colours (sensitive to the EW of [O III] 𝜆5007 and H𝛽) of our four EoR galaxies (selected from only 5 arcmin 2 of NIRCam imaging), lie within the extreme ends of the FLARES simulation predictions (where the 30 zoom-in simulations have been selected from a parent volume of 3.2 cGpc 3 , thus capturing rare overdense regions and extreme objects in colour space).…”

“…detail in Section 4. For the 𝑧 ∼ 10 galaxies, the red (F356W−F444W) colours are instead likely driven by a prominent Balmer break (see also Wilkins et al 2022b). With the addition of the F115W filter, one should be able to more effectively distinguish between the two redshift regimes, through the presence or absence of a Lyman break in the F115W filter.…”