GA-NIFS: A smouldering disk galaxy undergoing ordered rotation at z=4.26

Gareth C. Jones, Roberto Maiolino, Francesco D'Eugenio, Santiago Arribas, Andrew J. Bunker, Stephane Charlot, Michele Perna, Bruno Rodriguez del Pino, Hannah Übler, Torsten Böker, Giovanni Cresci, Isabella Lamperti, Eleonora Parlanti, Robert Pascalau, Jan Scholtz, Sandra Zamora

First listed 2025-12-04 | Last updated 2025-12-04

Abstract

Rotating galaxies with relaxed gaseous disks have been discovered across cosmic time, from the local Universe to high redshift (z>4). But few such sources have been confirmed at z>4, making them a precious sample to examine what conditions result in such ordered kinematics in an early, more chaotic Universe. One of the best examples of this sample is the galaxy DLA0817g1 (z=4.2603), which shows remarkably clear rotation in ALMA [CII] data. We present recent JWST/NIRSpec IFU data (R~2700) of DLA0817g1, which we combine with archival ALMA [CII] observations to place constraints on its ISM conditions and morpho-kinematics. From a combination of line ratios, we find a high gas-phase metallicity (~0.7 solar), high fraction of obscured star formation, low ionisation (compared to other high-redshift galaxies observed with JWST), and no significant evidence for AGN (based on the WHAN diagnostic). Dynamical modelling with 3DBarolo reveal nearly identical rotation in Halpha and [CII], but with a higher velocity dispersion in the former. Using our metallicity estimate and previous CO and [CII] detections, we derive a new estimate of the molecular gas mass, relieving a previous strain in the mass budget. Altogether, we suggest that this is a 'smouldering' galaxy, where past star formation resulted in significant chemical enrichment (i.e., Zgas and Mdust), but the current activity is low (i.e., lower ionisation parameter and electron temperature). These new observations have opened a window into questions regarding the interplay of gas, metallicity, star formation, and kinematics in a prototypical early disk galaxy.

Short digest

GA-NIFS combines NIRSpec IFU (R≈2700) with archival ALMA [C II] to dissect DLA0817g1 at z=4.2603, a prototypical rotating disk. Line ratios point to near-solar metallicity (~0.7 Z⊙), a high obscured star-formation fraction, low ionisation, and no significant AGN by WHAN. 3DBarolo shows near-identical rotation in Hα and [C II], with Hα carrying the higher velocity dispersion, and a metallicity-informed revision of the molecular gas mass eases the previous mass-budget tension. The authors frame the system as a “smouldering” disk—chemically enriched from past bursts, but presently low-excitation.

Key figures to inspect

• Figure 1: Compare Hα, [N II], and [C II] intensity maps to see how the ionised and cool gas disks overlap and whether [C II] extends beyond the Hα-emitting region; check the overlaid Hα vs [C II] peak contours for spatial alignment.
• Figure 2: Rotation-velocity and dispersion maps for Hα versus [C II]; verify ordered rotation in both tracers and the systematically higher σ in Hα that underpins the multiphase kinematics claim.
• Figure 3: Integrated spectrum fits with two-Gaussian components; inspect centroid offsets and widths used for WHAN placement and to rule out strong AGN-like broad wings.
• Figure 4: Constraints from [N II] and [S III] ratios; examine how the curves support high metallicity and significant dust/obscuration consistent with the low-ionisation interpretation.