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(< r) profiles of intermediate redshift (0.4 < z < 1.4) star-forming galaxies (SFGs) in the relatively unexplored regime of low masses (down to M_⋆ ∼ 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 GAlPAK^3D 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 j̃_⋆(<_r) 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 < log M_⋆/M_⊙ < 9 mass range. We find that the (SFR-weighted) sAM relation follows j̃_⋆ ∝ M_⋆^α with an index α 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 j̃_⋆ ∝(1+z)^a, with a = −0.27_−0.56^+0.42 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 logj|_M⋆ ∝ 0.65_−0.08^+0.06 × log(V_max/σ), where σ is the velocity dispersion at 2R_e. In TNG50, SFGs also form a j̃_⋆−M_⋆−(V/σ) 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.