The ISM in distant star-forming galaxies: Turbulent pressure, fragmentation and cloud scaling relations in a dense gas disk at z=2.3

Abstract: We have used the IRAM Plateau de Bure Interferometer and the Expanded Very Large Array to obtain a high resolution map of the CO(6-5) and CO(1-0) emission in the lensed, star-forming galaxy SMMJ2135-0102 at z=2.32. The kinematics of the gas are well described by a model of a rotationally-supported disk with an inclination-corrected rotation speed, v_rot = 320+/-25km/s, a ratio of rotational- to dispersion- support of v/sigma=3.5+/-0.2 and a dynamical mass of 6.0+/-0.5x10^10Mo within a radius of 2.5kpc. The disk has a Toomre parameter, Q=0.50+/-0.15, suggesting the gas will rapidly fragment into massive clumps on scales of L_J ~ 400pc. We identify star-forming regions on these scales and show that they are 10x denser than those in quiescent environments in local galaxies, and significantly offset from the local molecular cloud scaling relations (Larson's relations). The large offset compared to local molecular cloud linewidth-size scaling relations imply that supersonic turbulence should remain dominant on scales ~100x smaller than in the kinematically quiescent ISM of the Milky Way, while the molecular gas in SMMJ2135 is expected to be ~50x denser than that in the Milky Way on all scales. This is most likely due to the high external hydrostatic pressure we measure for the interstellar medium (ISM), P_tot/kB ~ (2+/-1)x10^7K/cm3. In such highly turbulent ISM, the subsonic regions of gravitational collapse (and star-formation) will be characterised by much higher critical densities, n_crit>=10^8/cm3, a factor ~1000x more than the quiescent ISM of the Milky Way.

The High-Pressure Turbulent ISM in a Star-Forming Galaxy at Redshift 2.3

This paper presents a detailed investigation into the interstellar medium (ISM) of the lensed, star-forming galaxy SMM J2135-0102 at redshift $z = 2.3$. Utilizing high-resolution observations from the IRAM Plateau de Bure Interferometer and the Expanded Very Large Array, the authors provide a comprehensive map of both CO(6–5) and CO(1–0) emissions in the galaxy, offering significant insight into the physical and dynamical conditions of the molecular gas in this system.

Insights into Galactic Dynamics

The observations reveal that the molecular gas in SMM J2135 is organized in a rotationally-supported disk, characterized by an inclination-corrected rotation speed of $v_{\rm rot} = 320\pm 25$ km/s and a ratio of rotational to dispersion support of $v/\sigma = 3.5\pm 0.2$. The dynamical mass is estimated to be $(6.0\pm 0.5)\times 10^{10}$ $M_{\odot}$ within a radius of 2.5 kpc. The Toomre stability analysis results in a $Q$ parameter of $0.50\pm 0.15$, suggesting the disk is unstable and prone to fragmentation into massive clumps.

Cloud Scaling Relations and Dense Gas Conditions

The study identifies star-forming regions on $\sim 400$ pc scales, significantly denser and offset from local molecular cloud scaling relations, specifically, the Larson’s relations. This deviation from local cloud properties indicates that the supersonic turbulence is dominant on scales approximately 100 times smaller than those in the quiescent ISM of the Milky Way. Notably, the molecular gas in SMM J2135 is estimated to be $\sim 50$ times denser than that of the Milky Way across all scales, a condition attributed to the high external hydrostatic pressure measured for the ISM, $P_{\rm tot}/k_{\rm B}\sim (2\pm 1)\times 10^{7}$ K cm$^{-3}$.

Implications for Star Formation Theories

The results suggest that under such high-pressure and highly turbulent environments, regions of gravitational collapse and subsequent star formation will be characterized by densities exceeding $10^8$ cm$^{-3}$—a factor more than 1000 times the quiescent ISM of the Milky Way. This has profound implications for the formation of massive star clusters and the initial mass function (IMF) in early galaxies, supporting theories that predict prominent differences in star formation modes at high redshifts compared to local galaxies.

Future Prospects

This paper underscores the necessity of high-resolution studies of ISM in high-redshift galaxies to validate the scaling laws derived locally and comprehend the drivers behind such extreme star-forming conditions. The anticipated full capabilities of the Atacama Large Millimeter/Submillimeter Array (ALMA) will provide enhanced opportunities to explore these properties further, delivering more detailed insights into the influence of turbulent media on star formation in the early Universe.

By exploiting gravitational lensing and cutting-edge interferometry, the authors deliver a benchmark study that vastly enhances our understanding of star-forming processes over cosmic time, paving the way toward elucidating how prevalent such high-pressure environments are in the star formation history of the Universe.