Which of the following statements is not an assumption in models of galaxy formation?

In the context of models of galaxy formation, the statement that stars formed before gas contracted into disks is not an assumption typically made in these models. Instead, the prevailing understanding is that gas plays a crucial role in the formation of structures within galaxies. In fact, gas is believed to collapse under its own gravity to form disks, and then stars form from that gas within those disks. This process emphasizes the importance of the gas in the early stages of galaxy evolution, making the notion that stars predate this gas contraction inconsistent with standard galaxy formation theories.

On the other hand, the other statements reflect common assumptions within these models. For instance, the idea that all gas clouds have similar angular momentum stems from the need for certain conditions to lead to organized structures. The notion that gas forms disks, which then lead to star formation, aligns with the understanding of how galaxies evolve. Additionally, the evolution of galaxies over time due to interactions, such as mergers and gravitational influences, is widely accepted as a fundamental aspect of how galaxies develop and change over cosmic timescales. Thus, these concepts are integral to the theoretical frameworks surrounding galaxy formation, contrasting with the incorrect assertion regarding the sequence of star and gas formation.