A first look at the SMACS0723 JWST ERO: spectroscopic redshifts, stellar masses and star-formation histories

Abstract

We present a first-look analysis of the JWST ERO data in the SMACS J0723.3-7327 cluster field. We begin by reporting 10 new spectroscopic redshifts from 𝜆 obs = 1.8 − 5.2𝜇m NIRSpec medium-resolution (𝑅 = 𝜆/Δ𝜆 = 1000) data. These are all determined via multiple high-SNR emission line detections, with 5 objects at 1 < 𝑧 < 3 displaying multiple rest-frame near-infrared Hydrogen Paschen lines, and 5 objects at 5 < 𝑧 < 9 displaying rest-frame optical Oxygen and Hydrogen Balmer lines. For the 5 higher-redshift galaxies we extract fluxes in 6 NIRCam bands spanning 𝜆 obs = 0.8 − 5𝜇m and perform spectral energy distribution fitting using these data in combination with existing HST photometry. The 7 < 𝑧 < 9 objects exhibit a U-shaped pattern across the F277W, F356W and F444W bands, indicating the presence of a Balmer break seen in emission (Balmer jump) and high-equivalent-width [O ] emission. This is indicative of an extremely young stellar population, with the bulk of the current mass having formed within the past 10 Myr. We report robust stellar masses and mean stellar ages from our spectral fitting, with the four 𝑧 > 6 galaxies exhibiting low stellar masses from log 10 (𝑀 * /M ) = 7.4 − 8.6 and correspondingly young mean stellar ages of only a few Myr. This work highlights the critical importance of combining large upcoming NIRCam surveys with NIRSpec follow-up to measure the spectroscopic redshifts necessary to robustly constrain physical parameters.

Discovery and properties of ultra-high redshift galaxies ($9

“…The redshift range allowed is 0 < 𝑧 < 15, and we apply dust attenuation following Calzetti et al (2000). The treatment for the IGM attenuation derived in Madau We compare our photometric redshifts with spectroscopic redshifts for a NIRSpec sample measured by Carnall et al (2022). As can be seen, we find a generally good agreement between these spectroscopic redshifts and our photometric ones, with some scatter.…”

“…All three F090W break objects are correctly identified as being at high redshifts, 𝑧 > 7, but our redshift measures are found to be systematically higher in redshift. The cause of this is likely the very high H𝛽 and [OIII] equivalent widths determined in the spectrophotometric analysis conducted in Carnall et al (2022). This would inflate the F444W measurement and shift a photometric redshift higher if the lines strengths in the templates are not as high as in reality.…”

“…To validate the photometric redshifts, we compare to some of the initial NIRSpec spectroscopic analysis of Carnall et al (2022). We use the 10 spectroscopic redshifts provided by this study, 5 for lower redshift sources and 5 for higher redshift sources.…”

“…For the purposes of this work, we limit our structural analysis to the light profiles of our sources through single-component Sérsic profiles in the F444W band as measured by Morfometryka (Ferrari et al 2015). We include in this analysis the sources verified as being at 𝑧 > 7 from Carnall et al (2022). As input into this part we use 64x64 pixels stamps as well as a PSF generated with .…”

The evolution of the galaxy UV luminosity function at redshifts z ~ 8-15 from deep JWST and ground-based near-infrared imaging

“…We make a conservative estimate for the area in SMACS0723, including the removal of the highly-lensed region centred on the cluster. Only 1 of the 16 galaxies found in the SMACS0723 field lay within the excluded area, SMACS-34086, and therefore this is not included in the calculation of the luminosity function (note that this highlylensed galaxy has been spectroscopically confirmed with NIRSpec at 𝑧 = 8.948; Carnall et al 2022).…”