Exploring the Mass–Metallicity Relationships with the DESI BGS and ELG Samples

The mass–metallicity relation (MZR) serves as a cornerstone for understanding the chemical evolution of galaxies by correlating the stellar mass of galaxies with their gas-phase metallicity. Using data from the Bright Galaxy Survey (BGS) and emission-line galaxy (ELG) samples from the early data release of the Dark Energy Spectroscopic Instrument, we examine the MZR at z ∼ 0.18 and z ∼ 0.86, respectively. The analysis includes 30,943 star-forming galaxies from BGS and 22,784 from ELG. Both samples exhibit typical MZR trends, but with a significantly steeper slope in the ELG sample. The BGS sample shows clear redshift evolution, with metallicity increasing over time at fixed stellar mass, whereas such evolution is not evident in the ELG sample, likely due to large stellar mass uncertainties. We further investigate the fundamental metallicity relation (FMR) for both populations. The inclusion of star formation rate (SFR) in the FMR framework reduces the scatter compared to the MZR in both samples, and the BGS FMR supports a nonevolving M ⋆ –SFR–metallicity relation. The comparison of the FMR slopes between BGS and ELG reveals that, even after accounting for the effect of SFR, the two populations still exhibit residual differences. This result indicates that the FMR only partially reconciles their metallicity scaling relations and that ELGs deviate from the nonevolving relation derived from the BGS.