The rise of the galactic empire: luminosity functions at $z\sim 17$ and $z\sim 25$ estimated with the MIDIS$+$NGDEEP ultra-deep JWST/NIRCam dataset

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$\ddot{I}ƒ$ depths $\sim 31.5$ mag at $\ge 2$ $\hat{I}¼$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 $z\sim 17$ and $z\sim 25$. The number density of galaxies with absolute magnitudes $-19<M_{\mathrm{UV}}<-18$ at $z\sim 17$ ($z\sim 25$) is a factor of 4 (25) smaller than at $z\sim 12$; 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 ${\mathrm{M}}_{\mathrm{DM}}=10^{8.5-9.5}$ 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 $z\sim 20$ 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 $\mathrm{A}(\mathrm{V})<0.1$ mag. Their average stellar mass is ${\mathrm{M}}_{★}\sim 10^7{\mathrm{M}}_{\odot }$, 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 (${\hat{I}}^2\sim -3$) compatible with Pop III young ($\lesssim 10$ Myr) stars and/or high escape fractions of ionizing photons; the rest have ${\hat{I}}^2\sim -2.5$ similar to $z=10-12$ samples.

Published in ApJ