The Rise of the Galactic Empire: Ultraviolet Luminosity Functions at z ∼ 17 and z ∼ 25 Estimated with the MIDIS+NGDEEP Ultra-deep JWST/NIRCam Data Set

We present a sample of six F200W and three F277W dropout sources identified as 16 < z < 25 galaxy candidates using the deepest JWST/NIRCam data to date (5 σ depths ∼31.5 mag at ≥2 μ m), provided by the MIRI Deep Imaging Survey and the Next Generation Deep Extragalactic Exploratory Public survey. We estimate ultraviolet (UV) luminosity functions and densities at z ∼ 17 and z ∼ 25. The number density of galaxies with absolute magnitudes of −19 < M UV < −18 at z ∼ 17 ( z ∼ 25) is a factor of 4 (25) smaller than at z ∼ 12; the luminosity density presents a similar evolution. Compared to state-of-the-art galaxy simulations, we find the need for an enhanced UV-photon production at z = 17–25 in M DM = 10 8.5−9.5 M ⊙ dark matter halos, provided by an increase in the star formation efficiency at early times and/or by intense compact starbursts with enhanced emissivity linked to strong burstiness, low or primordial gas metallicities, and/or a top-heavy initial mass function. There are a few robust theoretical predictions for the evolution of galaxies above z ∼ 20 in the literature; however, the continuing rapid drop in the halo mass function would predict a more rapid evolution than we observe if photon production efficiencies remained constant. Our z > 16 candidates present mass-weighted ages around 30 Myr, and attenuations A ( V ) < 0.1 mag. Their average stellar mass is M ⋆ ∼ 10 7 M ⊙ , implying a stellar-to-baryon mass fraction around 10% if the emissivity increases with redshift, or significantly higher otherwise. Three candidates present very blue UV spectral slopes ( β ∼ −3) compatible with Population III young (≲10 Myr) stars and/or high escape fractions of ionizing photons; the rest have β ∼ −2.5 similar to the z = 10–12 samples.