Fast Spectrophotometric Modeling of 40 Million DESI Spectra

The Dark Energy Spectroscopic Instrument (DESI) is a redshift-measuring machine; in just 3.5 years of science operations, the DESI survey has obtained precise redshifts for more than 40 million extragalactic targets and produced the largest and most detailed three-dimensional map of the universe from z=0-4. The observed-frame optical spectra (360-980 nm) of the DESI targets contain a wealth of information beyond just their redshift, however, creating an acute need for new, computationally efficient tools for extracting the physical and spectrophotometric properties of each target from the rich DESI dataset. In this contribution, we present FastSpecFit (https://fastspecfit.readthedocs.io), a new open-source Python package written from scratch to quickly and accurately model each observed DESI spectrum and its ultraviolet through infrared broadband photometry using physically motivated stellar and quasar continuum and emission-line templates. We describe the code's physical and computational underpinnings, and showcase just a handful of the extensive scientific inquiries being carried out using DESI and FastSpecFit. Finally, we highlight the FastSpecFit value-added catalogs which are being publicly released as part of the DESI Data Release 1 (DESI/DR1).