The enigmatic origins of supermassive black holes (SMBHs) in the very early Universe have long challenged astronomers and cosmologists alike. How such colossal entities, boasting masses exceeding a billion times that of our Sun, could form in a cosmically brief span has remained one of the paramount puzzles in understanding cosmic evolution. New insights have emerged thanks to observations made possible by the James Webb Space Telescope (JWST), whose unprecedented capability to probe the distant Universe is revealing unprecedented details about the stellar populations in galaxies that harbor these black holes. A recent study, published in Nature Astronomy, leverages JWST’s near-infrared capabilities to investigate two galaxies situated at redshifts greater than 6, when the Universe was less than a billion years old, shedding light on how massive galaxies and their central black holes may have co-evolved during these formative epochs.
These two distant galaxies, each hosting a quasar of moderate luminosity, were scrutinized using JWST’s NIRSpec instrument, which provides highly sensitive rest-frame optical spectra. The analysis was complemented by NIRCam imaging, offering a detailed look at the galaxies’ stellar populations and structural features. Both galaxies exhibit a key spectral signature known as Balmer absorption lines—features typically associated with post-starburst galaxies in the low-redshift Universe. Post-starburst galaxies, often referred to as “E+A” galaxies, are identified by strong Balmer absorption indicating a recent but now quenched burst of star formation. Detecting such spectral fingerprints at these extreme cosmological distances highlights early examples of galaxies undergoing rapid transformations in their star-forming histories.
The stellar mass content of these galaxies is astonishing considering their young cosmic age. Through combined medium-resolution spectroscopic data and multi-band photometry, the research reveals that the bulk of the stellar mass in these systems, each exceeding 10^10.6 solar masses, assembled in intense starburst episodes at redshifts around 9 and 7. To put this into perspective, these redshifts correspond to times just a few hundred million years after the Big Bang, implying that very rapid and efficient star formation processes occurred early in the history of these galaxies. This stellar mass buildup hints at an evolutionary pathway wherein massive galaxies emerge through abrupt, intense star formation events before transitioning to a more passive evolutionary phase.
One of the galaxies stands out by displaying a prominent Balmer break—a spectroscopic feature signaling a sharp drop in stellar continuum emission at wavelengths just beyond the Balmer limit, indicative of an aging stellar population dominated by A-type stars. This galaxy notably lacks spatially resolved H-alpha emission, a prime tracer of ongoing star formation, and its star formation rate places it well below the so-called “star-formation main sequence” for galaxies at redshift 6. Falling under this sequence means the galaxy has largely ceased forming new stars, indicating a quiescent or post-starburst state. Such quiescence at such an early epoch contradicts traditional hierarchical models predicting gradual star formation and suggests abrupt quenching may have played a significant role.
In contrast, the second galaxy invites special attention as it appears to be in transition toward quiescence. While not as fully quenched as its counterpart, it exhibits spectral properties signaling a decline in star formation rates. This intermediate phase reveals that quenching mechanisms might be gradual and that these early galaxies can be caught in the act of evolving from vibrant starbursting systems toward passive, quiescent galaxies. The recognition of such transitional galaxies at extreme redshifts is crucial for testing models of galaxy formation and feedback mechanisms that regulate star formation.
The role of the central supermassive black holes in these galaxies is underscored by the detection of blueshifted wings in their [O III] emission lines, classic indicators of quasar-driven gaseous outflows. These high-velocity outflows are understood as the manifestation of energy and momentum deposited into the galaxy’s interstellar medium by accreting black holes. Through such feedback, quasars can potentially expel or heat the cold gas reservoir of their host galaxies, inhibiting further star formation. This quasar-driven feedback mechanism is widely hypothesized to be key in regulating galaxy growth and facilitating the transition to quiescence, especially at early cosmic times when massive black holes and stars coevolve rapidly.
Adding another dimension to the narrative, direct measurements of stellar velocity dispersions within these galaxies offer a striking insight into the black hole–host galaxy relationship at early epochs. While one galaxy conforms to the local universe’s well-established correlation between black hole mass and stellar velocity dispersion—the so-called M–sigma relation—its companion harbors an overmassive black hole that deviates from this scaling relation. Such divergence suggests that black hole growth could precede or outpace stellar mass assembly in at least some early galaxies, challenging the notion of a tight causal link between these two components throughout cosmic history.
Collectively, the findings from this study highlight the existence of massive post-starburst galaxies at redshifts beyond 6, hosting billion-solar-mass black holes in fleeting, intense quasar phases. This discovery reframes our understanding of the rapid and intertwined growth of galaxies and their central black holes during the Universe’s first billion years. The early presence of these post-starburst systems implies that the processes leading to massive galaxy formation and the seeding and build-up of SMBHs were already well underway, operating through mechanisms that abruptly halted star formation on short timescales.
Moreover, the methodology employed—leveraging JWST’s medium-resolution near-infrared spectroscopy—has opened a window into the detailed physical conditions of these high-redshift galaxies. The detection of Balmer absorption and direct stellar velocity dispersions, previously unattainable at such early times, provides robust constraints on the stellar populations and dynamics involved. This represents a significant milestone in observational cosmology, heralding a new era in which the formation histories of the earliest massive galaxies and black holes can be directly probed rather than inferred from indirect metrics.
These observations also provide critical empirical benchmarks that will inform theoretical modeling of galaxy formation. The identification of rapid starburst episodes at redshifts as high as 9 and 7 demands models capable of producing sufficient gas inflow and triggering intense star formation under the conditions of the early Universe. In addition, the evidence of quasar outflows linked to star formation quenching emphasizes the importance of feedback processes in regulating the quick maturity of galaxies.
This research further sheds light on the diversity of evolutionary pathways that massive galaxies can follow in the first billion years. While some galaxies appear to adhere closely to locally observed scaling relations between black holes and stellar velocity dispersions, others highlight deviations that underscore the heterogeneous nature of early galaxy assembly. Understanding the origin and implications of this heterogeneity will require expanding such samples and integrating multiwavelength data to build a comprehensive picture of galaxy–black hole coevolution across cosmic time.
In the broader context, these findings illustrate the symbiosis between cutting-edge observational facilities like JWST and sophisticated theoretical models aiming to trace the assembly of cosmic structure from the primordial Universe to the present day. The ability to directly witness the aftermath of early starburst episodes and quasar-driven feedback episodes provides unprecedented granularity in reconstructing the timelines and mechanisms shaping the Universe’s most massive galaxies and their enigmatic central black holes.
As JWST continues to peer deeper into the cosmos, we can anticipate a growing catalog of such distant quasar-hosting galaxies exhibiting post-starburst signatures. These data will serve as a cornerstone for unraveling the intertwined evolutionary narratives of galaxies and black holes at epochs closer to the Big Bang. Ultimately, this newfound understanding paves the way toward resolving one of cosmology’s outstanding mysteries—the rapid emergence of supermassive black holes and the formation of massive galaxies in the infant Universe.
The unveiling of massive quiescent and transitioning galaxies through unique Balmer absorption fingerprints, direct measurement of stellar velocity dispersions, and tracing quasar-driven feedback heralds a transformative chapter in extragalactic astronomy. With JWST at the forefront, astronomers now possess the tools to map the complex interplay of star formation, black hole accretion, and feedback processes that sculpted the earliest massive galaxies, redefining our comprehension of cosmic dawn.
Subject of Research: Formation and evolution of massive post-starburst galaxies and supermassive black holes at redshifts greater than 6.
Article Title: A post-starburst pathway for the formation of massive galaxies and black holes at z > 6.
Article References:
Onoue, M., Ding, X., Silverman, J.D. et al. A post-starburst pathway for the formation of massive galaxies and black holes at z > 6. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02628-1
