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 \mu _ { g } \approx 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 ( S/N ) 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 ^ { +40 } _ { -25 } km s ^ { -1 } , 6402 ^ { +41 } _ { -39 } 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 ^ { +3.6 } _ { -2.5 } Gyr , 8.9 ^ { +4.3 } _ { -3.3 } Gyr , and 9.1 ^ { +3.9 } _ { -5.5 } Gyr , and iron abundances of \mathrm { [ Fe / H ] } -1.0 ^ { +0.3 } _ { -0.4 } , -1.3 ^ { +0.4 } _ { -0.4 } and -0.8 ^ { +0.5 } _ { -0.5 } , respectively .
Their stellar masses are ( 3 - 6 ) \times 10 ^ { 8 } M _ { \odot } .
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 .