Revealing Galaxy Candidates out to $z \sim 16$ with JWST Observations of the Lensing Cluster SMACS0723

Abstract

One of the main goals of the JWST is to study the first galaxies in the Universe. We present a systematic photometric analysis of very distant galaxies in the first JWST deep field towards the massive lensing cluster SMACS0723. As a result, we report the discovery of two galaxy candidates at 𝑧 ∼ 16, only 250 million years after the Big Bang. We also identify two candidates at 𝑧 ∼ 12 and 11 candidates at 𝑧 ∼ 10 − 11. Our search extended out to 𝑧 21 by combining color information across seven NIRCam and NIRISS filters. By modelling the Spectral Energy Distributions (SEDs) with EAZY and BEAGLE, we test the robustness of the photometric redshift estimates. While their intrinsic (un-lensed) luminosity is typical of the characteristic luminosity L * at 𝑧 > 10, our high-redshift galaxies typically show small sizes and their morphologies are consistent with disks in some cases. The highest-redshift candidates have extremely blue UV-continuum slopes −3 < 𝛽 < −2.5, young ages ∼ 10 − 100 Myr, and stellar masses log(𝑀 ★ /M ) = 8.4 − 8.8 inferred from their SED modeling which indicate a rapid build-up of their stellar mass. Our search clearly demonstrates the capabilities of JWST to uncover robust photometric candidates up to very high redshifts, and peer into the formation epoch of the first galaxies.

The brightest galaxies at Cosmic Dawn

“…Thus a reduction in dust attenuation may play an important role in explaining excesses of bright 𝑧 ∼ > 8 galaxies (e.g., Bowler et al 2020). Indeed, we note that of the brightest 𝑧 ∼ > 11 candidates found with JWST, all show very blue UV slopes 𝛽 ∼ < − 2.1, indicating negligible dust attenuation (Naidu et al 2022;Atek et al 2022;Donnan et al 2022). Though c.f.…”

“…The James Webb Space Telescope (JWST) is pushing our observational horizon even further, expanding our view to 5𝜇m (with the ★ E-mail: charlotte.mason@nbi.ku.dk NIRCam instrument, Rieke et al 2005), making it possible to observe rest-frame UV emission from 𝑧 ∼ < 30 galaxies, when the universe is just 100 Myr old. Interestingly, recent works have shown JWST to preliminary confirm HST galaxy counts at 𝑧 ∼ 7 − 9 (Leethochawalit et al 2022b) but claimed discovery of a relatively high number of galaxy candidates at 𝑧 ∼ 10 − 17 from early release science observations, suggesting the excess of sources relative to UV LF predictions extends to higher redshifts (Castellano et al 2022;Naidu et al 2022;Morishita & Stiavelli 2022;Donnan et al 2022;Atek et al 2022;Finkelstein et al 2022). It is too early to draw definitive conclusions, as these sources are not spectroscopically confirmed and the photometric candidates are based on reductions using preliminary calibrations (indeed, some of the same sources are reported with significantly different fluxes by different groups), and cover small fields subject to cosmic variance (e.g., Trenti & Stiavelli 2008).…”

Star formation at the smallest scales; A JWST study of the clump populations in SMACS0723

“…We focus on the portion of data that cover the centre of the cluster, that is entirely contained in the NIRCam module B. We use the reduced data as presented in Atek et al (2022) and summarise here the main reduction steps. The raw uncalibrated data have been reduced using the JWST calwebb pipeline v1.5.2.…”

“…The raw uncalibrated data have been reduced using the JWST calwebb pipeline v1.5.2. To reduce the significant levels of background residuals in the calibrated images, Atek et al (2022) use the grizli (1.5.1.dev6) pipeline (Brammer (2019)) to fit and subtract the sky background and register the images before creating the final mosaic with A . SW images have been reduced with a pixel scale of 0.02", while a pixel scale of 0.04" for the LW filters has been applied.…”

UV Slopes at All Redshifts Are Consistent with H = 1 Stochastic Star Formation Histories

“…With H = 1 SFHs providing a prior on the underlying growth trajectories of early galaxies, values of −2.8 ≤ β ≤ −2.5 should be common (e.g., Atek et al 2022) irrespective of the presence of, e.g., Population III stars. Towards lower redshifts, the wider range and higher median is expected.…”