Why the northern hemisphere needs a 30-40 m telescope and the science at stake: Massive stars in spiral galaxies

This document discusses the three main lines expected to dominate massive-star research in the 2040s, namely: (1) The role of metallicity in stellar evolution, especially in determining the end products such as gravitational-wave progenitors. (2) The initial mass function from the most massive stars to substellar objects. (3) The role of the environment in the different modes of star formation from compact star clusters to born-this-way associations and from massive clusters to small stellar groups. More specifically, we present the contributions to such science that would be enabled by a 30~m type telescope in the northern hemisphere studying spiral galaxies. Those can be grouped in three: our own Galaxy, the Milky Way; the other two spiral galaxies in the Local Group, M31 and M33; and other galaxies within 25 Mpc, such as M101, M51, and NGC~6946. This work is based on the fact that, as of today, no construction of a 30~m telescope has yet started in the northern hemisphere, so even in the best case scenario of such a hypothetical telescope, its full operation would not start until the late 2030s or early 2040s. It makes no assumptions about its location but supposes an instrumentation development similar to that of ELT.