Life beyond 30: probing the -20<M_UV<-17 luminosity function at 8<z<13 with the NIRCam parallel field of the MIRI Deep Survey

Abstract

We present the ultraviolet luminosity function and an estimate of the cosmic star formation rate density at $8<z<13$ derived from deep NIRCam observations taken in parallel with the MIRI Deep Survey (MDS) of the Hubble Ultra Deep Field (HUDF), NIRCam covering the parallel field 2 (HUDF-P2). Our deep (40 hours) NIRCam observations reach a F277W magnitude of 30.8 ($5\sigma$), more than 2 magnitudes deeper than JWST public datasets already analyzed to find high redshift galaxies. We select a sample of 44 $z>8$ galaxy candidates based on their dropout nature in the F115W and/or F150W filters, a high probability for their photometric redshifts, estimated with three different codes, being at $z>8$, good fits based on $\chi^2$ calculations, and predominant solutions compared to $z<8$ alternatives. We find mild evolution in the luminosity function from $z\sim13$ to $z\sim8$, i.e., only a small increase in the average number density of $\sim$0.2 dex, while the faint-end slope and absolute magnitude of the knee remain approximately constant, with values $\alpha=-2.2\pm0.1$ and $M^*=-20.8\pm0.2$ mag. Comparing our results with the predictions of state-of-the-art galaxy evolution models, we find two main results: (1) a slower increase with time in the cosmic star formation rate density compared to a steeper rise predicted by models; (2) nearly a factor of 10 higher star formation activity concentrated in scales around 2 kpc in galaxies with stellar masses $\sim10^8$ M$_\odot$ during the first 350 Myr of the Universe, $z\sim12$, with models matching better the luminosity density observational estimations $\sim$150 Myr later, by $z\sim9$.