Transforming gas-rich low-mass disky galaxies into ultra-diffuse galaxies by ram pressure

Up to 40% of ultra-diffuse galaxies could have formed via stripping of material by ram pressure. A study on 11 low-mass post-starburst galaxies in the Coma and Abell 2147 clusters shows ubiquitous marks of ram-pressure stripping and recent star formation, and most of them will evolve into ultra-diffuse galaxies in the next 10 Gyr. Faint extended elliptically shaped ultra-diffuse galaxies and slightly brighter and more compact dwarf elliptical and lenticular stellar systems are common in galaxy clusters. Their poorly constrained evolutionary paths can be studied by identifying young ultra-diffuse galaxy and dwarf elliptical analogues populated with bright, massive stars. Using data mining we identified 11 such low-mass (2 x 10(8) M-circle dot < M-star < 2 x 10(9) M-circle dot) galaxies with large half-light radii (2.0 kpc < r(e) < 5 kpc) and recently quenched star formation in the Coma and Abell 2147 galaxy clusters. All galaxies happen to have ram-pressure-stripped tails with signs of current or recent star formation. Deep spectroscopic observations revealed rotating stellar disks containing 70-95% dark matter by mass. A large fraction of the disk stars (10-60%) formed in intense star bursts 180-970 Myr ago, probably triggered by ram pressure. Observed global gradients of stellar age corroborate this scenario. Passive evolution in the next 10 Gyr will transform 9 of the 11 galaxies into ultra-diffuse galaxies. If we assume a constant rate of galaxy infall, 44 +/- 16% of the most luminous present-day ultra-diffuse galaxies in Coma must have formed via ram-pressure stripping of disky progenitors.