Identifying the progenitor set of present-day early-type galaxies: a view from the standard model

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 0.7) spirals have ∼75–95 percent chance of being an early-type progenitor, while the corresponding probability for large blue spirals (MB < −21.5, B −V < 0.7) is ∼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 (MV ≤− 21) of the “red sequence” trace the progenitor set reasonably well, the relationship breaks down at fainter luminosities (MV ≥− 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.