# What is exactly the “progenitor bias”?

I am taking a course in astrophysics and my teacher mentioned different biases that are present when taking a sample of galaxies: the progenitor bias and the Malmquist bias. I understand very well the Malmquist bias but I think that I don't yet really understand the progenitor bias.

What I get is that properties of low redshift galaxies (such as morphological type, ...) are not necessarily conserved when considering high redshift galaxies. What I mean is that, for example, an elliptical galaxy now might have been an elliptical, spiral or irregular galaxy in the past, while if we look at a high redshift elliptical galaxy it should have formed like that.

Am I right? Can you explain what the progenitor bias is in case I am mistaken?

The progenitor bias arises in attempts to study early-type (elliptical) galaxies at higher redshift. The desire is to choose a sample of galaxies at high $z$ that are the analogs of the galaxies that evolved to form the low $z$ sample. The bias arises if one chooses a sample of only early-types at high $z$. Because some late-types eventually evolve into early-types, by selecting only early types, one excludes the high $z$ late-types that will become late-types by low $z$. This introduces a bias in the sample, which tends to underestimate the evolution of the population.