On the importance of medium-resolution continuum spectroscopy for JWST's extragalactic legacy

Abstract

We argue for a significant investment of JWST time into an extremely deep medium-resolution NIRSpec MSA survey targeting a large, representative sample of high-redshift $(z>1.5)$ galaxies, specifically intended to obtain high-SNR continuum detections. This is only possible with JWST, and no planned future instrumentation has similar capabilities, meaning that obtaining such observations with JWST must be urgently prioritised. The vast majority of existing archival NIRSpec observations are not deep enough for continuum science, being focused on the detection of emission lines. Deeper continuum data opens up the ability to constrain a much wider range of key galaxy physical parameters, including stellar population ages and metallicities, stellar kinematics and neutral gas physical properties, inflows and outflows. Such measurements will provide crucial insight into quenching physics, galaxy dynamical masses, merger histories, pristine early star-formation, reionization and the physics of Little Red Dots, as well as enabling all of the emission-line science that can be achieved with shallower observations. To facilitate future survey planning, we present an empirically derived relationship between rest-optical continuum SNR in JWST NIRSpec medium-resolution data, rest-optical apparent magnitude and exposure time, which we show to be independent of redshift (and hence the grating used). We then provide a 1000-hour sketch survey plan based on achieving the continuum SNR $=20$/pixel necessary to constrain key physical properties and scaling relations for a large galaxy sample at $z>1.5$. We finally discuss NIRSpec sky subtraction strategies, which are of critical importance for continuum SNR.