Using the star-forming main sequence to explore quiescent galaxies across cosmic time

In this letter, we explore the quiescent lives of central galaxies using the Semi-Analytical Galaxy Evolution galaxy model and Uchuu dark matter simulation. We ask three questions: (1) How much of a galaxy's life is spent in quiescence? (2) How often do galaxies transit off the main sequence? (3) What is the typical duration of a quiescent phase? We find that the low- and high-mass galaxies spend the highest fraction of their lives in quiescence: 45 +/- 19 per cent for log 10(M-star) < 9.0 (3.68 +/- 1.80 Gyr) and 26 +/- 25 per cent for log 10 (M-star) > 11.5 (3.46 +/- 3.30 Gyr), falling to 7 +/- 13 per cent for galaxies in-between (0.82 +/- 1.57 Gyr). Low-mass galaxies move in and out of quiescence frequently, 2.8 +/- 1.3 times on average, though only for short periods, 1.49 +/- 1.04 Gyr. This can be traced to the influence of supernova feedback on their quite stochastic evolution. Galaxies of higher mass have fewer quiescent periods, similar to 0.7 +/- 0.9, and their length increases with mass, peaking at 1.97 +/- 2.27 Gyr. However, our high-mass population comprises star-forming and quiescent galaxies with diverging evolutionary paths, so the actual time may be even longer. These high-mass trends are driven by radio-mode feedback from supermassive black holes, which, once active, tend to remain active for extended periods.