FRESCO: The First Reionization Epoch Spectroscopically Complete Observations

Revealing the dramatic build-up of galaxies during the first 1 billion years to the peak of star formation at z~2-3 is one of Hubble's greatest achievements. Yet huge gaps in our understanding remain since our galaxy samples are incomplete due to the uncertainties of photometric selection. The highly-incomplete spectroscopic information at z>6 means that we lack physical understanding of the processes driving early galaxy assembly. To date, less than 1% of known galaxies in the epoch of reionization at z>6 have confirmed redshifts, and basic quantities such as mass-to-light ratios can be uncertain by factors of 5-10 — due to the unknown contributions of strong emission lines in the photometry used to derive stellar masses. As a result, we still only have a broad, phenomenological picture of early galaxy formation and growth. FRESCO exploits JWST's remarkable new spectroscopic capabilities to remedy this situation in a maximally-efficient way. By obtaining 2 hr deep NIRCam/grism observations with just the F444W filter, FRESCO will yield redshifts over a wide redshift range for a complete sample of ~330 z~7-9 galaxies, as well as ~1200 z~5-6.5 galaxies, in the Deep CANDELS areas of the GOODS-S and GOODS-N fields. FRESCO will yield an unprecedented Legacy archive, for the first time, of spectroscopic redshifts and emission line measurements from [OIII]+H-beta, H-alpha, and even Pa-alpha at low redshifts. FRESCO's grism observations provide the total line fluxes for estimating galaxy stellar mass and critically-needed slit-loss calibrations of NIRSpec/MSA spectra. We are not requesting proprietary time to ensure that FRESCO will be a key Legacy dataset for the community.



FRESCO is designed to optimally cover the two CANDELS/Deep regions in the GOODS-S and -N fields, which are among the most valuable extragalactic legacy fields in the full sky. Since more than 40% of all known z ∼ 7 − 8 candidate galaxies lie in these fields, they are the obvious choice for complete spectroscopic follow-up. Additionally, the two fields show different large scale structures, with GOODS-N featuring an overdensity of z ∼ 7 − 8 candidates. The FRESCO mosaics are designed to cover a total area of 7 x 8.6 sq-arcmin both with grism spectroscopy and medium band imaging, which will significantly add to the legacy of these fields.

More detailed information on the FRESCO survey can be found in the overview paper Oesch et al. (2023).