The MUSE Hubble Ultra Deep Field Survey. XVI. The angular momentum of low-mass star-forming galaxies: A cautionary tale and insights from TNG50

We investigate the specific angular momentum (sAM) j ( ) profiles of intermediate redshift (0.4  ⋆  ∼ 108  M ⊙ ) and small sizes (down to R e  ∼ 1.5 kpc), and we characterize the sAM scaling relation (i.e., Fall relation) and its redshift evolution. We have developed a 3D methodology to constrain sAM profiles of the star-forming gas using a forward modeling approach with GAlPAK3D that incorporates the effects of beam smearing, yielding the intrinsic morpho-kinematic properties even with limited spatial resolution data. Using mock observations from the TNG50 simulation, we find that our 3D methodology robustly recovers the star formation rate (SFR)-weighted profiles down to a low effective signal-to-noise ratio of ⪆3. We applied our methodology blindly to a sample of 494 [O II]-selected SFGs in the MUSE Ultra Deep Field (UDF) 9 arcmin2 mosaic data, covering the unexplored 8 ⋆ /M ⊙ varying from α  = 0.3 to α  = 0.5, from log M ⋆ /M ⊙ = 8 to log M ⋆ /M ⊙ = 10.5. The UDF sample supports a redshift evolution , with which is consistent with the (1 + z )−0.5 expectation from a universe in expansion. The scatter of the sAM sequence is a strong function of the dynamical state with , where σ is the velocity dispersion at 2R e . In TNG50, SFGs also form a plane, but it correlates more with galaxy size than with morphological parameters. Our results suggest that SFGs might experience a dynamical transformation, and lose their sAM, before their morphological transformation to becoming passive via either merging or secular evolution.