The H$\alpha$ specific angular momentum of dwarf galaxies

Abstract

The relationship between a galaxy's specific angular momentum $j$ and its mass $M$, parameterised by $j\propto M^{\alpha }$ and known as the Fall relation, has emerged as a fundamental scaling relation reflecting key physical and morphological properties of galaxies. This relation has been well studied for galaxies with masses above $10^9$ $M_{\odot }$. However, whether or not it holds for the low-mass dwarf galaxies, especially given their varied morphologies, remains uncertain. Here we use H$\alpha$ observations of 49 star-forming dwarf galaxies from the SH$\alpha$DE survey, as well as 20 high-mass `control' galaxies, to investigate the stellar $j_{\ast }$-$M_{\ast }$ relation down to masses below $10^6$ M${}_{\odot }$. We find that the star-forming dwarf galaxies follow the same $j_{\ast }$-$M_{\ast }$ relation as high-mass disk-like galaxies, with $\alpha =0.53\pm 0.4$, demonstrating that the relation holds across 5 orders of magnitude in mass. We then select a matching sample from the IllustrisTNG cosmological simulation and create mock observations resembling the SH$\alpha$DE survey. We find that the simulated dwarf galaxy population follows the extrapolated $j_{\ast }$-$M_{\ast }$ relation with a flattening and significant scatter towards lower $j_{\ast }$ values. Following the evolution of these galaxies, we find that dwarf galaxies experience a gradual loss in $j_{\ast }$ with time, while high-mass galaxies experience a sudden jump in $j_{\ast }$ before settling into a stable state. This dynamical evolution leads to a redshift dependence in $\alpha$, with $\alpha =0.45$ at $z=2$ and 0.55 at $z=0$. Despite the apparent simplicity in the present-day $j_{\ast }$-$M_{\ast }$ relation over a wide mass range, the evolution of $j$ leading to this relation is complex and dynamic.

21 pages, 17 figures, accepted for publication in PASA