Distant Galaxies Reveal Rotating Discs Challenging Quenching Theories

Analyses of fifteen quiescent galaxies at cosmic noon reveal all ten suitably aligned galaxies exhibit significant rotation exceeding 200 km/s, classifying them as fast rotators. This supports a formation pathway where distant quiescent galaxies evolve into nearby massive ellipticals via mergers, gradually losing rotational support with age. Dynamical masses exceed stellar masses.

Understanding how massive galaxies cease star formation – a process termed ‘quenching’ – remains a central challenge in extragalactic astronomy. New observations utilising the James Webb Space Telescope (JWST) are now providing crucial insights into the dynamics of these galaxies at a period known as ‘cosmic noon’ – roughly 8 billion years ago when star formation peaked. A team led by Martje Slob (Leiden Observatory) and including researchers from institutions including the Max-Planck-Institut für Astronomie, the University of California, Berkeley, and the University of Pittsburgh, present spatially resolved stellar kinematics for 15 quiescent galaxies at a redshift of approximately 1.4. Their findings, detailed in a paper entitled “Fast Rotators at Cosmic Noon: Stellar Kinematics for 15 Quiescent Galaxies from JWST-SUSPENSE”, demonstrate that these early massive galaxies retain significant rotational support, suggesting that quenching did not necessarily disrupt pre-existing disc structures and that mergers play a key role in their subsequent evolution.

JWST Reveals Rotating Progenitors of Today’s Elliptical Galaxies

Recent observations from the James Webb Space Telescope (JWST) are providing detailed insights into the formation and evolution of massive quiescent galaxies – galaxies that have largely ceased star formation – at a period known as cosmic noon, approximately 3 billion years after the Big Bang. This research, utilising data from the JWST-SUSPENSE programme and the Near-Infrared Spectrograph (NIRSpec) Multi-Object Spectroscopy (MSA), focuses on the internal motions of stars within fifteen such galaxies. Analysis reveals that ten of these galaxies exhibit substantial rotational support, with stellar velocities averaging 140 kilometres per second and peaking at 220 km/s, classifying them as ‘fast rotators’ based on their spin parameters.

The JWST-SUSPENSE program delivered unprecedented spectroscopic data, enabling astronomers to dissect the internal kinematics of these distant galaxies with remarkable precision. Researchers employed a sophisticated forward modelling approach, meticulously accounting for observational effects and instrumental limitations to extract accurate measurements of stellar velocities and velocity dispersions. Velocity dispersion describes the range of speeds of stars within a galaxy. This careful analysis revealed a surprising degree of ordered rotation within these massive quiescent galaxies, challenging prevailing theoretical expectations.

Astronomers combined these new observations with data from quiescent galaxies closer to us, uncovering a clear correlation between rotational support and galaxy age. Younger quiescent galaxies consistently exhibit greater rotational support, suggesting a direct evolutionary link between these distant systems and the massive elliptical galaxies observed today. This finding supports a scenario where galaxies initially form with rotating discs, and quenching processes – mechanisms that halt star formation – gradually transform these discs into more spheroidal shapes while preserving a remnant of the initial rotation. The observed trend aligns with predictions from cosmological simulations that incorporate realistic galaxy formation physics, further strengthening the case for an evolutionary connection.

Furthermore, the research indicates a significant discrepancy between the dynamical mass – calculated from the observed stellar motions – and the mass estimated from the stars themselves. This suggests a ‘bottom-heavy’ initial mass function (IMF). The IMF describes the distribution of stellar masses at birth; a bottom-heavy IMF implies a relatively high proportion of low-mass stars. This has important implications for our understanding of stellar populations and galaxy evolution. A bottom-heavy IMF would lead to a higher abundance of massive stars, which have a significant impact on the chemical enrichment of the universe. These massive stars produce heavy elements through nuclear fusion, which are then dispersed into the interstellar medium through supernova explosions.

The observed correlation between rotational support and galaxy age provides strong evidence for an evolutionary connection between distant quiescent galaxies and nearby elliptical galaxies. This connection suggests that elliptical galaxies may form through the gradual transformation of rotating discs, driven by quenching processes and mergers. Understanding this evolutionary pathway is crucial for understanding the formation and evolution of galaxies in the universe.

Extending this research will usher in a new era of galaxy formation and evolution studies. With its unprecedented capabilities, the JWST will continue to provide new insights, revolutionizing our understanding of the universe.

The discovery of significant rotation in these distant quiescent galaxies challenges existing models of galaxy formation and evolution. These models often assume that quenching processes completely destroy any pre-existing disc structures, leading to the formation of spheroidal galaxies with little or no rotation. The observed rotation suggests that quenching processes may be more complex than previously thought, allowing for the preservation of some degree of rotation.

Looking ahead, extending this research will usher in a new era of galaxy formation and evolution studies. The JWST-SUSPENSE program is just the beginning, and with its unprecedented capabilities, the JWST will continue to provide new insights, revolutionizing our understanding of the universe.