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

Abstract

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 $M_{\star }\sim 10^8$M${}_{\odot }$), and small sizes (down to $R_{\mathrm{e}}\sim 1.5$ kpc) and characterize the sAM scaling 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{} 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 $j(<r)$ profiles down to low effective signal-to-noise ratio (SNR) of $⪆3$. We applied our methodology blindly to a sample of 494 \OII{}-selected SFGs in the MUSE Ultra Deep Field (UDF) 9~arcmin${}^2$ mosaic data, covering the unexplored $8<\log M_{\ast }/$M${}_{\odot }<9$ mass range. We find that the (SFR-weighted) sAM relation follows $j\propto M_{\star }^{\hat{I}}\pm$ with an index $\hat{I}\pm$ varying from $\hat{I}\pm =0.3$ to $\hat{I}\pm =0.5$, from $\log M_{\star }/$M${}_{\odot }=8$ to $\log M_{\ast }/$M${}_{\odot }=10.5$. The UDF sample supports a redshift evolution 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 $\log j{|}_{M_{\ast }}\propto 0.65\times \log (V_{\max }/\ddot{I}ƒ)$ where $\ddot{I}ƒ$ is the velocity dispersion at $2R_{\mathrm{e}}$. In TNG50, SFGs also form a $j-M_{\star }-(V/\ddot{I}ƒ)$ plane but it correlates more with galaxy size than with morphological parameters. Our results suggest that SFGs might experience a dynamical transformation before their morphological transformation to becoming passive via either merging or secular evolution.

20 pages, 12 figures, accepted version; The TNG50 data is publicly available at https://www.tng-project.org