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

Abstract

The two-dimensional spatial distribution of stellar specific angular momentum (sAM) within galaxies has never been previously analysed. We investigate its morpho-kinematics and its relation to total stellar sAM (jstar) and stellar mass (Mstar) for 30 spiral and irregular galaxies from the GHASP survey. We constructed high-resolution stellar sAM surface density (sAMSD) maps by combining 3.4 micron WISE photometry with Halpha velocity fields and HI rotation curves. Their structure was quantified using non-parametric morphological indicators (concentration, asymmetry, smoothness) plus two additional coefficients measuring similarity to an axisymmetric Freeman disc and the strength of bisymmetric substructures in sAMSD space. Each galaxy was assigned to one of five new morpho-kinematic classes based on its dominant sAMSD feature: jstar-ring, jstar-spiral, jstar-bar, jstar-clump, and jstar-irregular. This defines a classification scheme that combines directly morphology and dynamics. For 14 galaxies, the classical morphological type differs from the sAMSD-based category. As expected, jstar correlates strongly with Mstar. We also find correlations between jstar and star formation rate, and between jstar and total HI mass. The mean jstar and Mstar for the different jstar types occupy distinct regions along the Fall relation, with significant internal scatter. The link between the two-dimensional sAMSD distribution and global jstar, together with the location of each type in the jstar-Mstar plane, suggests a possible morpho-kinematic evolutionary sequence for late-type galaxies. The mechanisms reshaping galaxies in sAMSD space appear to be related to disc stability: in low-mass systems, angular momentum redistribution may arise from feedback, dynamical friction, shocks, and resonances, whereas in massive spirals it is likely driven by quasi-stationary rotating density waves.

Figures (12)

• Figure 1: Maps of $B_\star$, $v_{\rm los}$, and stellar sAMSD for UGC9969. Left panel: SB in the 3.4 $\mu$m WISE $W_1$ band. The transparent pixels correspond to the regions where the SB profile was used to fill the map after the foreground star removal process. Middle panel: H$\alpha$ velocity map (solid colour $v_{\rm los}$) on top of the H$\alpha$ + H i RC projected on the sky (transparent $v_{\rm los}$). Right panel: Stellar sAMSD per pixel computed using Eq. (\ref{['eq: iota_pix']}).
• Figure 2: Maps of $B_\star$ and stellar sAMSD for UGC6537, UGC10470, UGC8490, and UGC10359 (from left to right). The first row is the SB in the 3.4 $\mu$m WISE $W_1$ band for each galaxy. The second row shows the stellar sAMSD per pixel for each galaxy, computed using Eq. (\ref{['eq: iota_pix']}).
• Figure 3: Fall relation for our sample. The x-axis is the stellar mass, in logarithmic scale, calculated following the method described in Sect. \ref{['sec: Mass']}. The y-axis is the total stellar sAM, in logarithmic scale, computed using Eq. (\ref{['eq: j']}). The error bars represent the variation in $j_\star$ when uncertainties in inclination and PA are considered. Each of the five $j_{\star}$ types described in Sect. \ref{['sec: sAM maps results']} is represented by a different colour and marker, as can be seen in the legend box. The dotted grey line and the shaded region around it are the best fit of the Fall relation for our sample, with its respective $1\sigma$ uncertainty. The alpha coefficient, the zero point, and the intrinsic scatter for the relation are displayed in the upper-left corner.
• Figure 4: Morpho-kinematic evolution diagram along the Fall relation. The average $M_\star$ and $j_\star$ for each morpho-kinematic class, and their 1$\sigma$ standard deviations, are represented following the colour and marker scheme introduced in Fig. \ref{['fig: Fall_relation']}. The stellar sAMSD maps for the representatives of the five $j_{\star}$ types are arranged on the $j_{\star}$-$M_{\star}$ plane in the location corresponding to their stellar disc mass and total stellar sAM. Each galaxy is labelled according to its morpho-kinematics. Starting from the lower-left corner and moving up along the best-fit line for our sample, we have UGC8490, UGC10359, UGC10470, UGC6537, and UGC9969. The axes and the parameters in the upper-left corner are the same as in Fig. \ref{['fig: Fall_relation']}.
• Figure 5: Average $f_{\rm gas}$, $c$, and $\rho_{\rm o}$ for each $j_{\star}$ type. The values for each property and their 1$\sigma$ standard deviation are represented following the colour and marker scheme introduced in Fig. \ref{['fig: Fall_relation']}.