A dynamically cold disk galaxy in the early Universe (2009.01251v1)

Abstract: The extreme astrophysical processes and conditions that characterize the early Universe are expected to result in young galaxies that are dynamically different from those observed today. This is because the strong effects associated with galaxy mergers and supernova explosions would lead to most young star-forming galaxies being dynamically hot, chaotic and strongly unstable. Here we report the presence of a dynamically cold, but highly star-forming, rotating disk in a galaxy at redshift ($z$) 4.2, when the Universe was just 1.4 billion years old. Galaxy SPT-S J041839-4751.9 is strongly gravitationally lensed by a foreground galaxy at $z = 0.263$, and it is a typical dusty starburst, with global star-forming and dust properties that are in agreement with current numerical simulations and observations of its galaxy population. Interferometric imaging at a spatial resolution of about 60 pc reveals a ratio of rotational-to-random motions of $V/\sigma = 9.7\pm 0.4$, which is at least four times larger than expected from any galaxy evolution model at this epoch, but similar to the ratios of spiral galaxies in the local Universe. We derive a rotation curve with the typical shape of nearby massive spiral galaxies, which demonstrates that at least some young galaxies are dynamically akin to those observed in the local Universe, and only weakly affected by extreme physical processes.

• The paper demonstrates that SPT0418-47 exhibits a high V/σ ratio (9.7 ± 0.4), confirming a dynamically cold disk in the early Universe.
• It employs high-resolution ALMA imaging and 3D lens-kinematic modelling to accurately derive the galaxy’s gas kinematics and dynamic properties.
• The findings challenge current models by revealing stable, rapidly rotating disks at a time when turbulent, dispersion-dominated systems were expected.

A Dynamically Cold Disk Galaxy Observed in the Early Universe

The research presented in "A dynamically cold disk galaxy in the early Universe" documents a pivotal observation of a young galaxy, SPT-S J041839-4751.9 (SPT0418-47), located at a redshift of 4.2. This paper reveals a dynamically cold and rapidly rotating disk in a star-forming galaxy from a period when the Universe was merely 1.4 billion years old. Key to this discovery was the exploitation of gravitational lensing by a foreground galaxy at redshift 0.263, along with high-resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA).

Methodology

The authors employed sophisticated interferometric imaging to capture the dynamics and properties of the galaxy's disk. The data revealed the ratio of rotational-to-random motions as $V/\sigma = 9.7 \pm 0.4$, notably higher than predictions from contemporary galaxy evolution models, which anticipated a lower value due to intense astrophysical processes expected during this epoch. This observation was supported by a rigorous 3D lens-kinematic modelling approach, which allowed for an accurate determination of the galaxy's gas kinematics and subsequent dynamic analysis.

The rotation curve derived for SPT0418-47 is reminiscent of those seen in nearby massive spiral galaxies. These kinematic features suggest that some early galaxies maintained stable, dynamically cold configurations akin to local Universe counterparts, defying traditional evolutionary models which predict highly turbulent, dispersion-dominated systems due to processes like galaxy mergers and supernova feedback.

Results

The paper quantified the global parameters of SPT0418-47, reporting a stellar mass of $1.2^{+0.2}_{-0.1} \times 10^{10}$ $M_{\odot}$, a dark matter mass of $1.7^{+0.3}_{-0.3} \times 10^{12}$ $M_{\odot}$, and a gas fraction of $0.53^{+0.06}_{-0.08}$. Additionally, the galaxy exhibited an intrinsic infrared luminosity of $(2.4 \pm 0.4) \times 10^{12}$ $L_{\odot}$, corresponding to a star formation rate (SFR) of $352 \pm 65$ $M_{\odot} \text{yr}^{-1}$. The gas depletion timescale was calculated to be $38 \pm 9$ Myr.

Astrophysical models predicted that dynamically hot systems with high turbulent motions should dominate such epochs, due to significant feedback mechanisms. However, this galaxy's V/$\sigma$ ratio contradicts these expectations, as the derived Toomre parameter indicated a marginally stable rotating disk prone to clumpy star formation and without prevalent violent disk instabilities. This empirical evidence challenges existing models of galaxy formation and evolution, particularly concerning the early Universe.

Implications and Future Directions

The findings impose significant constraints on theories of early galactic dynamics and evolution. They suggest that models require revision to account for the existence of dynamically cold galaxies at such high redshifts. It becomes crucial to consider the mechanisms allowing some galaxies to maintain such stability despite intense environmental factors.

Future studies could harness advanced telescopes and simulations to explore the conditions under which dynamically cold disks can form and persist in the early Universe. This may include deepening the understanding of the interplay between internal processes and environmental interactions. Furthermore, investigating a broader sample of galaxies at similar epochs could determine whether SPT0418-47 is an anomaly or indicative of a broader cosmic phenomenon.

In conclusion, the documented findings of SPT0418-47 present critical insights into the nature of young galaxies and compel the scientific community to revisit existing paradigms of early cosmic structure development.