A Detailed Model Atmosphere Analysis of Hot White Dwarfs in DESI DR1

Abstract We present a detailed model atmosphere analysis of hot white dwarfs in the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. Our sample includes 19,321 unique targets with G BP − G RP ≤ 0. We use the DESI spectra along with Gaia parallaxes and Sloan Digital Sky Survey (SDSS), Pan-STARRS, and SkyMapper photometry to perform spectroscopic and photometric fits. We find a significant discrepancy between the photometric and spectroscopic masses for DA white dwarfs (a systematic offset of 0.05–0.06 M ⊙ ), indicating problems with the broad hydrogen line profiles in DESI spectroscopy data. Our photometric fits are consistent with a peak at the canonical mass of 0.6 M ⊙ . A remarkable feature of the mass distribution is the prevalence of magnetic white dwarfs among the ultramassive DA population and that of warm DQs in the non-DA distribution. We identify 70 DQs in the DESI hot white dwarf sample, including nine DAQs with carbon and hydrogen atmospheres. We constrain the ratio of non-DA to DA white dwarfs as a function of temperature, and discuss the implications for the spectral evolution of white dwarfs in the temperature range of 10 5 –10 4 K. We also discuss unusual objects in the sample, including metal-rich white dwarfs and extremely low-mass white dwarfs. This analysis provides the first look at the large sample of Gaia-selected white dwarf candidates that will be observed with multiplexed spectroscopic surveys such as DESI, SDSS-V, the 4 m Multi-Object Spectroscopic Telescope, and the William Herschel Telescope Enhanced Area Velocity Explorer over the next several years.