Echoes of the First Stars: Massive Star Evolution in Extremely Metal-Poor Environments with the Habitable Worlds Observatory

TL;DR

This paper discusses how the Habitable Worlds Observatory will enable detailed study of massive stars in extremely metal-poor galaxies, shedding light on early stellar populations and cosmic evolution.

Contribution

It proposes using HWO's UV-optical spectroscopy to resolve individual massive stars in distant, metal-poor dwarf galaxies, a breakthrough in understanding early universe star formation.

Findings

HWO will observe massive stars in galaxies 10-20 Mpc away.

This will provide direct data on stars with >30 solar masses and <10% solar metallicity.

It will significantly advance knowledge of early stellar populations.

Abstract

A remarkable span of frontier astrophysics, from gravitational-wave archaeology to the origin of the elements to interpreting snapshots of the earliest galaxies, depends sensitively on our understanding of massive star formation and evolution in near-pristine, relatively enriched gas. From the surprisingly massive black holes detected by LIGO/Virgo to highly ionized nebulae with peculiar enrichment patterns observed in galaxies at Cosmic Dawn, evidence is mounting that our understanding of massive-star populations at very low metallicity remains critically incomplete. The fundamental limitation is the hand nature has dealt us: only a few star-forming galaxies within $\lesssim$1 Mpc can currently be resolved into individual stars, and none reach the extreme metallicities and star-formation intensities that characterized the early Universe. With an ultraviolet integral-field spectrograph aboard the Habitable Worlds Observatory (HWO), this barrier will finally be broken. HWO will bring rare, actively star-forming, extremely metal-poor dwarf galaxies at $\sim$10-20 Mpc such as I Zw 18 within reach of resolved UV-optical spectroscopy, providing our first direct, statistical view of individual massive stars and the feedback they drive at $>$30 $M_\odot$ and $<$10% $Z_\odot$. This science is deeply synergistic with many next-generation facilities, yet requires the unique combination of spatial resolution and UV/optical sensitivity that only HWO can provide. The massive star science enabled by HWO within the Local Volume represents a transformational advance in our ability to probe the earliest stellar populations - those that seeded the Milky Way and other galaxies with the first heavy elements, and paved the way for life in the transparent, reionized Universe we inhabit today.