We present a comprehensive theoretical study , using a semi-analytical model within the standard LCDM framework , of the photometric properties of the progenitors of present-day early-type galaxies in the redshift range 0 < z < 1 .
We explore progenitors of all morphologies and study their characteristics as a function of the luminosity and local environment of the early-type remnant at z = 0 .
In agreement with previous studies , we find that , while larger early-types are generally assembled later , their luminosity-weighted stellar ages are typically older . In dense cluster-like environments , \sim 70 percent of early-type systems are ‘ in place ’ by z = 1 and evolve without interactions thereafter , while in the field the corresponding value is \sim 30 percent .
Averaging across all environments at z \sim 1 , less than 50 percent of the stellar mass which ends up in early-types today is actually in early-type progenitors at this redshift , in agreement with recent observational work . The corresponding value is \sim 65 percent in clusters , due to faster morphological evolution in such dense environments . We develop probabilistic prescriptions which provide a means of including spiral ( i.e .
non early-type ) progenitors at intermediate and high redshifts , based on their luminosity and optical colours .
For example , we find that , at intermediate redshifts ( z \sim 0.5 ) , large ( M _ { V } < -21.5 ) , red ( B - V > 0.7 ) spirals have \sim 75 - 95 percent chance of being an early-type progenitor , while the corresponding probability for large blue spirals ( M _ { B } < -21.5 , B - V < 0.7 ) is \sim 50 - 75 percent . The prescriptions developed here can be used to address , from the perspective of the standard model , the issue of ‘ progenitor bias ’ , whereby the exclusion of late-type progenitors in observational studies can lead to inaccurate conclusions regarding the evolution of the early-type population over cosmic time . Finally , we explore the correspondence between the true ‘ progenitor set ’ of the present-day early-type population - defined as the set of all galaxies that are progenitors of present-day early-types regardless of their morphologies - and the frequently used ‘ red-sequence ’ , defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects . We find that , while more massive members ( M _ { V } \leq - 21 ) of the ‘ red sequence ’ trace the progenitor set reasonably well , the relationship breaks down at fainter luminosities ( M _ { V } \geq - 21 ) .
Thus , while the results of recent observational studies which exploit the red sequence are valid ( since they are largely restricted to massive galaxies ) , more care should be taken when deeper observations ( which will probe fainter luminosities ) become available in the future .