My analysis reveals that bulges form in two waves and two modes. The first wave of bulges, which formed at redshift z>3, assembled their mass fast (timescales around 200 Myr). For the second wave of bulges, forming at redshift z<3, a slower mode starts to become relevant. Interestingly, first-wave bulges are more compact than second-wave bulges. Thus, my results suggest that the center of massive disk galaxies actually harbors a fraction of the elusive population of red-nugget relics, which appear to be the oldest spheroids formed in the Universe.
Cartoon summarizing the expected evolutionary paths of first and second-wave bulges.