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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?

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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.

So it sounds like your explanation is, more or less, correct. Or at least along the right lines.

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