Resolved molecular gas and star formation in massive unquenched spirals

I. UGC 8179

¹ LUX, Observatoire de Paris, PSL Research University, Sorbonne Université, CNRS, 75014 Paris, France
² Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
⁴ Observatoire astronomique, Université de Strasbourg, CNRS UMR 7550, Strasbourg, France
⁵ Departamento de Física Téorica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁶ Instituto Carlos I de Física Téorica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
⁷ Collège de France, 11 place Marcelin Berthelot, 75231 Paris, France
⁸ IRAM, 300 rue de la Piscine, 38400 Saint-Martin-d’Hères, France

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

Received: 9 December 2025
Accepted: 25 February 2026

Abstract

Recent studies have uncovered a rare population of supermassive (M_★ ≳ 10¹¹ M_⊙), yet actively star-forming spiral galaxies – super spiral galaxies (SSGs) – whose existence challenges classical mass- and environment-quenching scenarios. We investigate the resolved star-forming and molecular-gas properties of the nearby SSG UGC 8179 (z = 0.052, log(M_★/M_⊙) = 11.62) in order to assess whether its local star formation (SF) follows the same physical processes as those observed in typical star-forming main-sequence (SFMS) spirals. We combined the first spatially resolved CO(1–0) interferometric observations of an SSG with pixel-by-pixel SED fitting, based on archival UV–to–mid-IR imaging. Our 3″ × 3″ (∼25 kpc²) pixel maps provide spatially resolved measurements of stellar mass, star formation rate (SFR), and molecular gas surface densities across its extended disk. UGC 8179 hosts a massive rotating molecular gas reservoir of M_H2 = 1.02 × 10¹⁰ M_⊙, yielding a standard molecular gas fraction and a typical depletion time of ∼1 Gyr in the region observed by NOEMA, despite its extreme mass. We derive lower limits of log₁₀f_mol ≥ −1.61 ± 0.06 and log₁₀τ_dep ≥ 8.82 ± 0.13 at the scale of the galaxy. The large spatial extent of UGC 8179 enables us to probe low surface-density regimes hardly accessible in nearby disks (Σ_★ < 10⁷ M_⊙ kpc⁻²; Σ_SFR < 10^−3.5 M_⊙ yr⁻¹ kpc⁻²). All three resolved scaling relations (resolved SFMS – rSFMS, resolved Kennicutt Schmidt – rKS, and resolved molecular gas main sequence – rMGMS) are well defined. The rKS slope (0.87 ± 0.09) is broadly consistent with unity, indicating standard local SF processes. The rSFMS shows a shallower global slope (0.80 ± 0.02) due to a central suppression in specific SFR (∼ − 0.5 dex), but a two-component fit restores agreement with literature relations at Σ_★ ≲ 10^7.2 M_⊙ kpc⁻². This break suggests the influence of a bulge – and possibly a bar – driving a transition to a more dynamically regulated SF regime in the inner disk. A similar flattening in the rMGMS supports this interpretation. UGC 8179 provides evidence that SSGs can sustain standard local SF processes while exhibiting central dynamical regulation at high stellar surface densities. Extending this analysis to our full sample of 19 SSGs and nearby massive unquenched spirals will enable us to test whether such regulation is a common feature among massive unquenched spirals.