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 $G_{\rm BP}-G_{\rm RP}\leq0$. We use the DESI spectra along with Gaia parallaxes and 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-$0.06~M_\odot$), 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 $0.6~M_\odot$. 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 9 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 $10^5-10^4$ 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 like DESI, SDSS-V, 4MOST, and WEAVE over the next several years.