Low metallicities and old ages for three ultra-diffuse galaxies in the Coma cluster

A large population of ultra-diffuse galaxies (UDGs) was recently discovered in the Coma cluster. Here we present optical spectra of three such UDGs, DF 7, DF 44, and DF 17, which have central surface brightnesses of μ g ≈ 24.4–25.1 mag arcsec⁻². The spectra were acquired as part of an ancillary program within the SDSS-IV MaNGA Survey. We stacked 19 fibers in the central regions from larger integral field units (IFUs) per source. With over 13.5 hr of on-source integration, we achieved a mean signal-to-noise ratio in the optical of 9.5 Å⁻¹, 7.9 Å⁻¹, and 5.0 Å⁻¹, respectively, for DF 7, DF 44, and DF 17. Stellar population models applied to these spectra enable measurements of recession velocities, ages, and metallicities. The recession velocities of DF 7, DF 44, and DF 17 are 6599 ^-25+40 km s⁻¹, 6402 ^{-39 +41} km s⁻¹, and 8315 ^{-43 +43} km s⁻¹, spectroscopically confirming that all of them reside in the Coma cluster. The stellar populations of these three galaxies are old and metal-poor, with ages of 7.9 ^{-2.5 +3.6} Gyr, 8.9 ^{-3.3 +4.3} Gyr, and 9.1 ^{-5.5 +3.9} Gyr, and iron abundances of [Fe/H] -1.0 _^-0.4' ^+0.3 -1.3^-0.4' ^+0.4, and -0.8 ^{-0.5' +0.5}, respectively. Their stellar masses are (3–6) × 10⁸ M _⊙. The UDGs in our sample are as old or older than galaxies at similar stellar mass or velocity dispersion (only DF 44 has an independently measured dispersion). They all follow the well-established stellar mass–stellar metallicity relation, while DF 44 lies below the velocity dispersion-metallicity relation. These results, combined with the fact that UDGs are unusually large for their stellar masses, suggest that stellar mass plays a more important role in setting stellar population properties for these galaxies than either size or surface brightness.