We report the discovery of 854 ultra diffuse galaxies ( UDGs ) in the Coma cluster using deep R band images , with partial B , i , and H \alpha band coverage , obtained with the Subaru telescope .
Many of them ( 332 ) are Milky Way-sized with very large effective radii of r _ { e } > 1.5 { kpc } .
This study was motivated by the recent discovery of 47 UDGs by ( 43 ) ; our discovery suggests > 1 , 000 UDGs after accounting for the smaller Subaru field ( 4.1 degree ^ { 2 } ; about 1/2 of Dragonfly ) .
The new Subaru UDGs show a distribution concentrated around the cluster center , strongly suggesting that the great majority are ( likely longtime ) cluster members .
They are a passively evolving population , lying along the red sequence in the color-magnitude diagram with no signature of H \alpha emission .
Star formation was , therefore , quenched in the past .
They have exponential light profiles , effective radii r _ { e } \sim 800 { pc } - 5 { kpc } , effective surface brightnesses \mu _ { e } ( R ) = 25-28 mag arcsec ^ { -2 } , and stellar masses \sim 1 \times 10 ^ { 7 } { M _ { \odot } } - 5 \times 10 ^ { 8 } { M _ { \odot } } .
There is also a population of nucleated UDGs .
Some MW-sized UDGs appear closer to the cluster center than previously reported ; their survival in the strong tidal field , despite their large sizes , possibly indicates a large dark matter fraction protecting the diffuse stellar component .
The indicated baryon fraction \lesssim 1 \% is less than the cosmic average , and thus the gas must have been removed ( from the possibly massive dark halo ) .
The UDG population is elevated in the Coma cluster compared to the field , indicating that the gas removal mechanism is related primarily to the cluster environment .