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 30m 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 NGC6946. 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.

💡 Research Summary

The paper argues that a 30‑ to 40‑meter class telescope in the northern hemisphere will be essential for advancing massive‑star science in the 2040s. It identifies three overarching research themes that will dominate the field: (1) the influence of metallicity on massive‑star evolution and on the production of gravitational‑wave progenitors, (2) the shape and possible variations of the initial mass function (IMF) from the most massive stars down to sub‑stellar objects, and (3) the role of the local environment in shaping different modes of star formation, from compact clusters to loosely bound associations.

Because no 30‑m telescope is currently under construction in the north, the authors assume a timeline in which such a facility would become operational only in the late 2030s or early 2040s, with instrumentation comparable to that planned for the European Extremely Large Telescope (ELT). They then map the scientific opportunities that a northern 30‑m telescope would unlock across three distance regimes: (i) the Milky Way, focusing on the Cygnus sightline, (ii) the two other Local Group spirals, M31 and M33, and (iii) a set of bright, face‑on grand‑design spirals within ~25 Mpc (M101, M51, NGC 6946).

For the Milky Way, the Cygnus tangent offers a unique combination of very recent massive‑star formation (Cyg OB2), the most massive O‑type binary within 1 kpc (the Bajamar star), and heavily extincted but intrinsically bright objects such as Cyg X‑3. The authors argue that optical multi‑object spectroscopy (MOS) with a 30‑m telescope would obtain high‑S/N blue‑violet spectra of OB stars even through A_V > 10 mag, while near‑infrared MOS would simultaneously probe the low‑mass pre‑main‑sequence population. This dual approach would enable a fully sampled IMF from brown dwarfs to >120 M_⊙, allow a direct census of binarity and dynamical ejections, and test whether the canonical Kroupa IMF holds in a high‑metallicity, high‑density environment.

In M31, the nearest large spiral, the high inclination (77°) and strong internal extinction have so far limited metallicity gradient measurements to a handful of blue supergiants. A 30‑m telescope equipped with high‑resolution near‑infrared spectrographs and adaptive‑optics assisted integral‑field units (IFU) would resolve individual massive stars and compact clusters, delivering precise abundances, wind parameters, and rotational velocities across the disk. This would finally produce a robust metallicity map and enable IMF studies as a function of galactocentric radius.

M33, at a similar distance but nearly face‑on, already hosts extensive H II region spectroscopy and deep HST imaging (PHATTER). Its well‑known radial abundance gradient (≈ –0.06 dex kpc⁻¹) and the presence of the prototypical Scaled OB Association NGC 604 make it an ideal laboratory for testing metallicity‑dependent wind theory, massive‑star feedback, and IMF variations. The authors stress that a 30‑m telescope could resolve the core of NGC 604 and other compact clusters down to 0.01″, allowing individual stellar spectroscopy even in the crowded outskirts where metallicities drop to LMC/SMC levels.

Beyond the Local Group, the authors consider M101, M51, and NGC 6946. Although individual stars cannot be fully resolved at 7–8 Mpc, the high collecting area and spatially resolved IFU spectroscopy would permit “spectral synthesis” of small (∼100 pc) regions, yielding average stellar metallicities, IMF upper‑end slopes, and feedback signatures. M51’s recent interaction with NGC 5195 provides a natural testbed for environment‑driven star‑formation changes, while NGC 6946’s high star‑formation rate and azimuthal metallicity variations offer a complementary perspective.

The paper concludes with a set of technical requirements: a suite of optical MOS, high‑resolution near‑infrared spectrographs, adaptive‑optics assisted IFU, and the capability for deep, high‑S/N observations in the blue‑violet despite heavy extinction. The authors argue that only a northern 30‑m class facility can provide the necessary spatial resolution (sub‑0.01″) and sensitivity to address the three key science drivers, thereby delivering a comprehensive picture of how metallicity, IMF, and environment shape massive‑star evolution and, ultimately, the gravitational‑wave landscape of the universe.