[Paper Review] The ALMA-ALPAKA survey I: high-resolution CO and [CI] kinematics of star-forming galaxies at z = 0.5-3.5

Francesca Rizzo, Fernanda Roman-Oliveira+14 more|arXiv (Cornell University)|Mar 28, 2023

Spectroscopy and Laser Applications8 references4 citations

TL;DR

This study presents the ALPAKA survey, a high-resolution kinematic analysis of CO and [CI] emission in 28 star-forming galaxies at z = 0.5–3.5 using archival ALMA data. It reveals that 19/28 galaxies are rotating disks with high V/σ ratios (median 9⁺⁷₋₂), indicating ordered motion, and provides the largest sample to date of spatially resolved cold gas kinematics in massive, high-redshift galaxies.

ABSTRACT

Spatially-resolved studies of the kinematics of galaxies provide crucial insights into their assembly and evolution, enabling to infer the properties of the dark matter halos, derive the impact of feedback on the ISM, characterize the outflow motions. To date, most of the kinematic studies at z=0.5-3.5 were obtained using emission lines tracing the warm, ionized gas. However, whether these provide an exhaustive or only a partial view of the dynamics of galaxies and of the properties of the ISM is still debated. Complementary insights on the cold gas kinematics are therefore needed. We present ALPAKA, a project aimed at gathering high-resolution observations of CO and [CI] emission lines of star-forming galaxies at z=0.5-3.5 from the ALMA public archive. With 147 hours of total integration time, ALPAKA assembles ~0.25'' observations for 28 star-forming galaxies, the largest sample with spatially-resolved cold gas kinematics as traced by either CO or [CI] at z>0.5. By combining multi-wavelength ancillary data, we derive the stellar masses ($M_{\star}$) and star-formation rates (SFR) for our targets, finding values of $M_{\star}\gtrsim 10^{10}$ M$_{\odot}$ and SFR of 10-3000 M$_{\odot}$/yr. A large fraction of ALPAKA galaxies (19/28) lie in overdense regions (clusters, groups, and protoclusters). We exploit the ALMA data to infer their dynamical state and we find that 19/28 ALPAKA galaxies are rotating disks, 2 are interacting systems, while for the remaining 7 sources the classification is uncertain. The disks have velocity dispersion values that are typically larger in the innermost regions than in the outskirts, with a median value for the entire disk sample of 35$^{+11}_{-9}$ km/s. Despite the bias of our sample towards galaxies hosting very energetic mechanisms, the ALPAKA disks have high ratios of ordered-to-random motion ($V/σ$) with a median value of 9$^{+7}_{-2}$.

Motivation & Objective

• To assemble a large, high-resolution sample of cold gas kinematics in star-forming galaxies at z = 0.5–3.5 using archival ALMA data.
• To investigate the dynamical state of high-redshift galaxies by analyzing CO and [CI] emission lines as tracers of cold gas kinematics.
• To compare cold gas kinematics with existing warm gas tracers (e.g., Hα) to assess the completeness of kinematic diagnostics in galaxy evolution studies.
• To study the role of feedback, accretion, and environment in shaping the kinematics of massive, actively star-forming galaxies at high redshift.

Proposed method

• Collected and reprocessed high-resolution ALMA observations of CO and [CI] emission lines from the public archive, achieving angular resolutions of ~0.25 arcsec.
• Combined ALMA data with multi-wavelength ground- and space-based ancillary data to derive stellar masses (M⋆) and star-formation rates (SFR) for the 28 target galaxies.
• Applied 3D Barolo to model the kinematics, deriving rotation curves and velocity dispersion profiles from the CO and [CI] emission cubes.
• Classified galaxies based on kinematic morphology into rotating disks, interacting systems, or uncertain cases using dynamical diagnostics.
• Computed median values of V/σ and σ for disk galaxies to quantify ordered motion and turbulence in the interstellar medium.
• Publicly released all data cubes and kinematic profiles to enable community-wide follow-up studies.

Experimental results

Research questions

• RQ1 What is the dynamical state of massive, star-forming galaxies at z = 0.5–3.5 as traced by cold gas kinematics?
• RQ2 How do the velocity dispersion profiles and V/σ ratios of high-redshift disks compare to local galaxies and cosmological simulations?
• RQ3 To what extent do warm gas tracers (e.g., Hα) provide a complete picture of galaxy kinematics compared to cold gas tracers like CO and [CI]?
• RQ4 How do environmental overdensities (clusters, groups, protoclusters) influence the kinematic structure of high-redshift star-forming galaxies?
• RQ5 What are the implications of the observed kinematic properties for the role of smooth gas accretion, feedback, and dark matter halo structure?

Key findings

• The ALPAKA survey provides the largest sample to date of spatially resolved cold gas kinematics in star-forming galaxies at z ≥ 0.5, with 28 galaxies observed at ~0.25 arcsec resolution.
• 19 out of 28 galaxies are classified as rotating disks, indicating that ordered rotation dominates the kinematics in massive, high-redshift systems.
• The median V/σ ratio for disk galaxies is 9⁺⁷₋₂, indicating strong ordered motion relative to random motions, even in galaxies with energetic feedback mechanisms.
• Median velocity dispersion (σ) for the disk sample is 35⁺¹¹₋₉ km s⁻¹, with higher values in inner regions, suggesting elevated turbulence in central zones.
• A large fraction (19/28) of the sample resides in overdense environments, suggesting environmental effects may influence kinematic structure, though this requires further study.
• The survey’s data cubes and kinematic profiles are publicly released, establishing a legacy resource for future studies of cold gas dynamics and feedback in high-redshift galaxies.