A shallow slope for the stellar mass–angular momentum relation of star-forming galaxies at 1.5 &amp;lt; <i>z</i> &amp;lt; 2.5

ABSTRACT We present measurements of the specific angular momentum $j_\star$ of 41 star-forming galaxies at $1.5\lt z\lt 2.5$. These measurements are based on radial profiles inferred from near-infrared Hubble Space Telescope photometry, along with multiresolution emission-line kinematic modelling using integral field spectroscopy (IFS) data from K-band multi-object spectrograph, Spectrograph for Integral Field Observations in the Near Infrared, and OH-Suppressing Infra-Red Imaging Spectrograph. We identified 24 discs (disc fraction of $58.6\pm 7.7~{{\ \rm per\ cent}}$) and used them to parametrize the $j_\star$ versus stellar mass $M_\star$ relation (Fall relation) as $j_\star \propto M_\star ^{\beta }$. We measure a power-law slope $\beta =0.25\pm 0.15$, which deviates by approximately $3\sigma$ from the commonly adopted local value $\beta = 0.67$, indicating a statistically significant difference. We find that two key systematic effects could drive the steep slopes in previous high-redshift studies: first, including irregular (non-disc) systems due to limitations in spatial resolution and second, using the commonly used approximation $\tilde{j}_\star \approx k_n v_s r_\mathrm{eff}$, which depends on global unresolved quantities. In our sample, both effects lead to steeper slopes of $\beta =0.48\pm 0.21$ and $0.61\pm 0.21$, respectively. To understand the shallow slope, we discuss observational effects and systematic uncertainties and analyse the retention of $j_\star$ relative to the angular momentum of the halo $j_h$ (angular momentum retention factor $f_j =j_\star /j_h$). For the $M_\star$ range covered by the sample $9.5 \lt \log _{10} (M_\star /\mathrm{ M}_\odot) \lt 11.5$ (halo mass $11.5 \lt \log _{10} (M_h/\mathrm{ M}_\odot) \lt 14$), we find large $f_j$ values ($\gt 1$ in some cases) in low-mass haloes that decrease with increasing mass, suggesting a significant role of efficient angular momentum transport in these gas-rich systems, aided by the removal of low-$j_\star$ gas via feedback-driven outflows in low-mass galaxies.