Low Surface Brightness Galaxies in z > 1 Galaxy Clusters: HST approaches
  the Progenitors of Local Ultra Diffuse Galaxies
  Ultra Diffuse Galaxies (UDGs), a type of large Low Surface Brightness (LSB)
galaxies with particularly large effective radii (r_eff > 1.5 kpc), are now
routinely studied in the local (z<0.1) universe. While they are found to be
abundant in clusters, groups, and in the field, their formation mechanisms
remain elusive and an active topic of debate. New insights may be found by
studying their counterparts at higher redshifts (z>1.0), even though
cosmological surface brightness dimming makes them particularly diffcult to
detect and study there. This work uses the deepest Hubble Space Telescope (HST)
imaging stacks of z > 1 clusters, namely: SPT-CL J2106-5844 and MOO J1014+0038.
These two clusters, at z=1.13 and z=1.23, were monitored as part of the HST
See-Change program. Compared to the Hubble Extreme Deep Field (XDF) as
reference field, we find statistical over-densities of large LSB galaxies in
both clusters. Based on stellar population modelling and assuming no size
evolution, we find that the faintest sources we can detect are about as bright
as expected for the progenitors of the brightest local UDGs. We find that the
LSBs we detect in SPT-CL J2106-5844 and MOO J1014-5844 already have old stellar
populations that place them on the red sequence. Correcting for incompleteness,
and based on an extrapolation of local scaling relations, we estimate that
distant UDGs are relatively under-abundant compared to local UDGs by a factor ~
3. A plausible explanation for the implied increase with time would be a
significant size growth of these galaxies in the last ~ 8 Gyr, as also
suggested by hydrodynamical simulations.
