On the minimal angular size in open, Lambda CDM, and scalar field cosmologies

We propose a simple method for determining the redshift z(m) at which the a ngular size of an extragalactic source with fixed proper diameter takes its minimal value. A closed analytical expression, which is often convenient f or numerical evaluation is derived. The method is exemplified with the foll owing FRW type expanding universes: the open matter dominated models (Omega (Lambda) = 0), a critical density model with cosmological constant (Omega(L ambda) not equal 0), and the class of scalar field cosmologies proposed by Ratra and Peebles. The influence of systematic evolutionary effects is brie fly discussed.