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 SURVEY LAYOUT:
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.