New morpho-kinematic classification: The two-dimensional spatial distribution of stellar specific angular momentum in late-type galaxies

¹ Aix-Marseille Univ., CNRS, CNES, LAM, 38 Rue Frédéric Joliot Curie, 13338 Marseille, France
² Canada-France-Hawaii Telescope, 65-1238 Mamalahoa Highway, Kamuela, HI 96743, USA
³ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Universidad de Concepción, Departamento de Astronomía, Barrio Universitario s/n, Concepción, Chile

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

Received: 16 September 2025
Accepted: 16 January 2026

Abstract

Aims. The two-dimensional spatial distribution of stellar specific angular momentum (sAM) within galaxies has never been analysed. We seek to identify the morpho-kinematics of this property and its correlation with the total stellar sAM (j_★) and the total stellar mass (M_★) of late-type galaxies in the local Universe.

Methods. We estimated the two-dimensional distribution of the stellar sAM of 30 spiral and irregular galaxies part of the Gassendi Hα survey of SPirals (GHASP). The galaxies are mostly located in nearby low-density environments. To perform the estimations, we produced, for the first time, high-resolution stellar sAM surface density (sAMSD) maps by using a formalism that combines the 3.4 μm WISE photometry with Hα velocity fields and H I rotation curves. We quantified the features of these new maps using standard non-parametric morphological indicators such as concentration, asymmetry, and smoothness, augmented by two additional coefficients that respectively measure the degree of similarity to an axisymmetric Freeman disc and the prominence of bi-symmetric substructures within the stellar sAMSD space.

Results. Each galaxy in our sample was classified into one of the five new morpho-kinematic categories proposed in this study: j_★- ring, j_★- spiral, j_★- bar, j_★- clump, and j_★- irregular. These classes are named after the predominant galactic substructure of their stellar sAMSD map and form a new classification scheme that combines both morphological characteristics and dynamic properties. For 14 galaxies, the morphological classification does not coincide with the corresponding morpho-kinematic category. As expected, there is a high correlation between j_★ and M_★ for our sample. Similarly, our galaxies exhibit a correlation between j_★ and their star formation rate, and between j_★ and their total H I mass. The mean j_★ and M_★ are distributed along different regions along the Fall relation for all j_★ types, although there is a considerable scatter within each type.

Conclusions. There is a link between the two-dimensional distribution of stellar sAMSD and j_★. The location of the different j_★ types in the j_★-M_★ diagram could indicate a morpho-kinematic evolution of late-type galaxies within the hierarchical paradigm of galaxy formation. The physical mechanisms capable of modifying the shape of galaxies in the stellar sAMSD space seem to be related with the stability of the galactic disc. The spatial redistribution of angular momentum for low-mass late-type galaxies may be driven by a combination of feedback, dynamical friction, shock waves, and resonances, while the quasi-stationary rotating density wave may be responsible for the aforementioned process in the massive spirals.