The progenitor set of present-day early-type galaxies

TL;DR

This study uses a semi-analytical model to analyze the progenitors of present-day early-type galaxies, revealing that many progenitors are non-early-type galaxies and that the red sequence is an unreliable proxy at lower luminosities.

Contribution

It provides probabilistic prescriptions for identifying spiral progenitors based on luminosity and color, and assesses the red sequence's effectiveness as a progenitor proxy.

Findings

Less than 50% of stellar mass in early-types at z~1 is in early-type progenitors.

In clusters, about 65% of the mass is in early-type progenitors due to faster transformations.

Red sequence is a good proxy for bright progenitors but not for fainter ones.

Abstract

We present a comprehensive theoretical study, within a fully realistic semi-analytical framework, of the photometric properties of early-type progenitors in the redshift range 0<z<1, as a function of the luminosity and local environment of the early-type remnant at present-day. We find that, averaging across all environments at z~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. The corresponding value is ~65 percent in clusters due to faster morphological transformations in the 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 (BVK) colours. For example, at intermediate redshifts (z~0.5), large (M_B<-21.5), red (B-V>0.7) spirals have ~75-95 percent chance of being a progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is ~50-75 percent. Finally, we explore the correspondence between the true progenitor set of present-day early-types and the commonly used `red-sequence', defined as the set of galaxies within the part of the colour-magnitude space which is dominated by early-type objects. While large members (M_V<-22) of the `red sequence' trace the progenitor set accurately in terms of numbers and mass, the relationship breaks down severely at fainter luminosities (M_V>-21). Hence the red sequence is generally not a good proxy for the progenitor set of early-type galaxies.