Characteristic timescale of transformation

The model assumes that all progenitors of early-type galaxies started forming stars at a time tstart, ended forming stars at a time tstop, and that some time after the end of star formation they are morphologically classified as early-type. Three possible star formation histories are shown here:

The time tstart is the same for all galaxies in the simulation, but the time tstop is different for each galaxy. In the simulation, the values of tstop are drawn from an exponential probability distribution with characteristic scale τ:

P(tstop) ∝ exp((tstart-t)/τ)
for t > tstart, and
P(tstop)=0
for t < tstart.

The value of τ is the characteristic time scale for transformation from star forming galaxy to passively evolving galaxy. The figure below shows distributions of tstop for tstart=0.1 and three different values of τ. Low values of τ imply that most early-type galaxies were formed at high redshift. High values of τ imply that early-type galaxies keep forming at low redshifts.

After entering a value for τ, click "Apply" to calculate the corresponding fraction of today's early-type galaxies that have formed by z=1.