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

TL;DR

This paper argues that understanding massive stars at extreme metal-poor metallicities is essential for interpreting early galaxy evolution and reionization. It proposes Habitable Worlds Observatory as a unique facility to obtain spatially resolved UV–optical spectroscopy of individual massive stars and their surrounding nebulae in nearby $\lesssim 10\% Z_\odot$ systems, notably I Zw 18, at distances of $\sim$10–20 Mpc. By delivering direct measurements of stellar winds, photospheres, rotation, binarity, and the ionized gas they illuminate, this approach aims to constrain wind mass loss, binary interactions, and the IMF's upper end in the low-metallicity regime, providing empirical anchors for models of early star formation and feedback. The anticipated data will link stellar properties to nebular diagnostics over UV–optical wavelengths, enabling precise mappings of ionizing photon production, gas-phase abundances, and mechanical energy input, with broad implications for interpreting JWST results and understanding the seeds of chemical enrichment and compact-object formation across cosmic time.

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.

Figures (5)

• Figure 1: Schematic representing the chemical evolution of the Universe, and the path towards observing massive stars approaching early Universe metallicities. At present, our only detailed constraints for massive stars across a broad mass range are in the Small and Large Magellanic Clouds, at $\gtrsim 20$% solar metallicity ($Z_\odot$) - corresponding roughly to bulk metallicities characteristic of only the most recent few-billion years madauCosmicStarFormationHistory2014. Pioneering programs with HST have provided a glimpse of lower-metallicity stars in a handful of dwarf irregular galaxies out to the edge of the Local Group, but these only host a small number of massive stars uniformly below 50 $M_\odot$ and extending only to $\sim 10\%$ solar. A facility like HWO is required to capture the first picture of a representative sample of more massive stars at metallicities actually representative of the early (the first Gyr after the Big Bang, at redshift $z>6$) Universe. (Figure adapted from NAOJ by M. Garcia and P. Senchyna)
• Figure 2: A schematic overview of the metal-poor massive star populations available to resolved spectroscopy at present and with HWO. The present state-of-the-art consists only of the Large and Small Magellanic Clouds bestenlehnerXShootingULLYSESMassive2025, and a handful of metal-poor dwarf Irregular galaxies with relatively low star formation rates and consequently lacking in higher mass stars; see Table \ref{['tab:galaxies']} and Section \ref{['sec:physical_parameters']}. This science case is motivated by the lack of resolved star constraints at very low metallicities ($<10\%$ solar) and high stellar masses ($\gtrsim 30$--50 $M_\odot$; pink box), an area of parameter space into which the stars that dominate the integrated light and feedback of the earliest galaxies and those responsible for surprisingly heavy black holes found through LIGO, etc are most likely to fall.
• Figure 3: The far-ultraviolet provides access to a suite of lines formed in the atmospheres of massive stars which are required to understand the winds they drive, especially at very low metallicities where these winds are almost completely hidden at other wavelengths. Here we plot theoretical atmosphere models convolved to $\mathcal{R}\sim 4000$ for a hot main sequence star martinsSpectroscopicEvolutionMassive2021 and a WNL star driving a dense Wolf Rayet wind todtPotsdamWolfRayetModel2015, both at $\lesssim 7\%~Z_\odot$. I Zw 18 is likely to harbor stars spanning the range of wind properties represented here. Many of these same lines (particularly C4 and He2 in addition to others in this range) are known to be excited also in nebular gas in I Zw 18 and other extremely metal-poor galaxies. HWO IFU spectroscopy will simultaneously provide our first glimpse of individual massive stellar atmospheres at such low-metallicity, and constrain their ionizing impact via this extended high-ionization nebular emission.
• Figure 4: A particularly important target is the well-known extremely metal-poor galaxy I Zw 18 -- the nearest bastion of a substantial population of massive ($>30$$M_\odot$) stars significantly below 10% solar metallicity (HST/ACS F606W$+$F814W imaging from GO:10586, PI:Aloisi). The core of the NW component of the galaxy is a cluster complex subtending $4"$ on a side ($\sim 300$ pc at the 18 Mpc distance of I Zw 18). At bottom, we display a simulation of a zoom-in on a 10 pc region of this cluster complex with different UV PSFs, ranging from that achieved by HST/WFC3 ($\sim 80$ mas) down to 5 mas. This 10 pc region subtends a very small area of sky: $0.10"$ at 18 Mpc (the approximate size of just 1 JWST microshutter). We display for reference the approximate pixel size of HST/WFC3/UVIS relative to this scale (left). To produce a simulated view from HWO, we take a cutout of an HST F275W (NUV) image of the core of the most massive star-forming region in the SMC, NGC 346 (GO:17118, PI:Murray). We plot versions of this NGC 346 image convolved to different prospective HWO PSF sizes (and resampled at Nyquist) as labeled. With a UV(-blue optical) IFU (or alternatively, a microshutter array of similar spatial scale and allowing for similarly dense multiplexing) sampling spatial scales $\lesssim 15$ milliarcseconds (mas) over a field of view of $\geq 0.1"$ (ideally, FOV$\sim 3$--$10"$), HWO is uniquely capable of resolving and taking the critical UV spectra of a large population of individual massive $\gtrsim 30$$M_\odot$ stars within the NW star-forming complex of I Zw 18 (with the actual expected yield of resolved stars scaling inversely with the achieved PSF FWHM).
• Figure 5: As one empirical estimate of crowding in I Zw 18, we examine the cumulative distribution of distance to the nearest bright neighbor ($m_V<18$, $M_V<-1$) for O stars evansVLTFLAMESSurveyMassive2006duftonCensusMassiveStars2019 in the cluster NGC 346 in the SMC, cross-matching to the HST photometric catalog of sabbiPresentStarFormation2007. A PSF and spectroscopic aperture $<20$ mas would begin to resolve some O stars in a similar cluster at 18 Mpc; with the fraction resolved scaling strongly with increasingly finer spatial sampling up to approximately half or higher at $<15$ mas.