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We study the origin of high-redshift, compact, quenched spheroids (rednuggets) through the dissipative shrinkage of gaseous discs into compactstar-forming systems (blue nuggets). The discs, fed by cold streams, undergoviolent disc instability (VDI) that drives gas into the centre (along withmergers). The inflow is dissipative when its timescale is shorter than the starformation timescale. This implies a threshold of 0.28 in the cold-to-total massratio within the disc radius. For the typical gas fraction 0.5 at z=2, thisthreshold is traced back to a maximum spin parameter of 0.05, implying thathalf the star-forming galaxies contract to blue nuggets, while the rest formextended stellar discs. Thus, the surface density of blue galaxies is expectedto be bimodal about 10^9 Msun/kpc^2, slightly increasing with mass. Bluenuggets are expected to be rare at low z when the gas fraction is low. The bluenuggets quench to red nuggets by complementary internal and externalmechanisms. Internal quenching by a compact bulge, operating as a fast mode andespecially at high z, may involve starbursts, stellar and AGN feedback, orQ-quenching. Quenching due to hot-medium haloes above 10^{12} Msun providesmaintenance and a slower mode at low redshift. These predictions are confirmedin simulations and are consistent with observations at z=0-3.

Wet Disc Contraction to Galactic Blue Nuggets and Quenching to Red Nuggets