Resolving the molecular gas emission of the z ∼ 2.5–2.8 starburst galaxies SPT 0125-47 and SPT 2134-50

¹ Department of Space, Earth & Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
² Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, Versoix 1290, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 July 2024
Accepted: 13 November 2025

Abstract

Context. The comoving cosmic star formation rate density peaks at z ∼ 2–3, with dusty star-forming galaxies being significant contributors to this peak. These galaxies are characterized by their high star formation rates and substantial infrared (IR) luminosities. The formation mechanisms remain an open question for these galaxies, particularly with respect to how such intense levels of star formation are triggered and maintained.

Aims. We aim to resolve CO(3–2) emission toward two strongly lensed galaxies, SPT 0125-47 and SPT 2134-50, at z ∼ 2.5–2.8 to determine their morphology and physical properties.

Methods. We used high-resolution ALMA band 3 observations of CO(3–2) emission toward both sources to investigate their properties. We performed parametric and nonparametric lens modeling using the publicly available lens modeling software PYAUTOLENS. We divided the CO(3–2) emission line into two bins corresponding to the red and blue portions of the emission line and nonparametrically modeled the source plane emission for both bins.

Results. We found that both sources are well described by a single Sérsic profile in both the parametric and nonparametric models of the source plane emission, in contrast to what was previously found for SPT 0125-47. Parametric lens modeling studies of the red and blue bins have reported distinctive differential magnification across the line spectrum. We performed a basic analysis of the morphology and kinematics in the source plane using nonparametric lens modeling of the red and blue bins. We found tentative evidence of a velocity gradient across both sources and no evidence of any clumpy structure, companions, or ongoing mergers.

Conclusions. The previously calculated high star formation rates and low depletion times of both SPT 0125-47 and SPT 2134-50 suggest that these galaxies are undergoing a dramatic phase in their evolution. Given the lack of evidence of ongoing interactions or mergers in our source plane models, we suggest that the intense star formation was triggered by a recent interaction and/or merger. We also consider the possibility that these galaxies might be in the process of settling into disks.