Figure 3. The formation time, tf, as a function of assembly time, ta, for galaxies at z = 0. Both time-scales are measured as lookback times in Gyr, and the galaxies are classified into three bins of stellar mass by colours. The solid line represents the one to one relation for the two time-scales. Galaxies with stellar mass (M∗ ≤ 1010.5 M ), are distributed along this line, indicating that they assemble most of their stars through in situ star formation. Inmassive galaxies ofM∗ > 1011 M , by contrast, ta lags behind tf, and the galaxies are offset from the ta = tf line, showing the importance of stars formed in other objects and subsequently accreted. The normalized histograms of the tf and ta distributions are shown in marginal panels. The mean and the median of the distributions are indicated by the solid and dotted lines, respectively.
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