The rise of the galactic empire: luminosity functions at $z\sim17$ and $z\sim25$ estimated with the MIDIS$+$NGDEEP ultra-deep JWST/NIRCam dataset

TL;DR

This study probes the existence and properties of galaxies at $z>16$ by exploiting ultradeep JWST/NIRCam imaging from MIDIS and NGDEEP. It identifies six F200W dropouts and three F277W dropouts, compiling a sample spanning roughly $16<z<21$ and $24<z<28$ over 17.6 arcmin$^2$, enabling robust UV luminosity function measurements at $z\sim17$ and $z\sim25$. The results indicate a strong decline in the number density and UV luminosity density compared to $z\sim12$, suggesting the need for enhanced UV photon production in halos of $M_\mathrm{DM}=10^{8.5-9.5}\,M_\odot$, potentially driven by higher star formation efficiency, intense compact starbursts, low/primordial metallicities, or a top-heavy IMF; and several candidates show very blue UV slopes ($\beta\sim-3$) compatible with Pop III-like populations or high ionizing photon escape fractions. These findings have significant implications for early galaxy formation, reionization, and the interpretation of JWST data, highlighting the role of extreme star-formation modes in the first few hundred million years.

Abstract

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 $\sim31.5$ mag at $\geq2$ $μ$m), provided by the MIRI Deep Imaging Survey (MIDIS) and the Next Generation Deep Extragalactic Exploratory Public survey (NGDEEP). We estimate ultraviolet (UV) luminosity functions and densities at $z\sim17$ and $z\sim25$. The number density of galaxies with absolute magnitudes $-19<M_\mathrm{UV}<-18$ at $z\sim17$ ($z\sim25$) is a factor of 4 (25) smaller than at $z\sim12$; 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 $\mathrm{M}_\mathrm{DM}=10^{8.5-9.5}$ M$_\odot$ 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 few robust theoretical predictions for the evolution of galaxies above $z\sim20$ 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 $\mathrm{A(V)}<0.1$ mag. Their average stellar mass is $\mathrm{M}_\bigstar\sim10^{7}\,\mathrm{M}_\odot$, 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 ($β\sim-3$) compatible with Pop III young ($\lesssim10$ Myr) stars and/or high escape fractions of ionizing photons; the rest have $β\sim-2.5$ similar to $z=10-12$ samples.

Figures (1)

• Figure 1: Compilation of spectroscopically confirmed galaxies (red hexagons, 2023Natur.622..707A2023NatAs...7..622C2024ApJ...972..143C2024Natur.633..318C2025arXiv250511263N) and photometric samples (including 2023ApJ...954L..46L -green pointing-up triangles-, 2024ApJ...969L...2F -green pointing-down triangle-, 2023ApJ...951L...1P -green stars-, 2024arXiv241113640K -green squares-, 2024ApJ...965...98C -green crosses-, 2025arXiv250405893C -orange pentagons; one of their candidates lies outside our plot, at magnitude 25.5, three candidates were spectroscopically confirmed to be at low redshift and are not shown here-, and 2025arXiv250706292W -green plus signs-) of galaxies at $z>11$. The $16<z<25$ galaxy candidates presented in this work are plotted with green and orange diamonds, the color representing the band used in the plot, the one closer to and redward of the Lyman break. We note that for most of the literature points, only the F444W flux is provided and shown in this plot. We also show their magnitude and photometric redshift uncertainties. Lines show the expected magnitudes as a function of redshift of a very young (1 Myr old) starburst with stellar mass $M_\star=10^8\,M_\odot$ according to the 2003MNRAS.344.1000B models (thick dotted line) and a less massive $M_\star=10^7\,M_\odot$ starburst according to the 2017PASA...34...58E BPASS models (thin dotted line). The expected magnitudes for a more extended star formation history (50 Myr constant star formation population) and a dormant galaxy (which experienced an instantaneous burst 30 Myr before the observation) are also shown (thick continuous and dashed-dotted lines, respectively). All models assume no dust. Shaded regions show the 5$\sigma$ depths of the major galaxy surveys used in the identification of $z>11$ galaxy candidates (see second paragraph in Introduction for references). The darkest regions refer to the dataset used in this paper (MIDIS24, which stands for the 2024 depth, including NGDEEP). The color for all data points, lines, and shaded regions indicates the NIRCam band considered.