Letter to the Editor

A tight angular-momentum plane for disc galaxies^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: pavel@astro.rug.nl
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ Observatoire Astronomique de Strasbourg, Université de Strasbourg, 11 rue de l’Université, 67000 Strasbourg, France
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 16 June 2021
Accepted: 5 July 2021

Abstract

The relations between the specific angular momenta (j) and masses (M) of galaxies are often used as a benchmark in analytic models and hydrodynamical simulations as they are considered to be amongst the most fundamental scaling relations. Using accurate measurements of the stellar (j_*), gas (j_gas), and baryonic (j_bar) specific angular momenta for a large sample of disc galaxies, we report the discovery of tight correlations between j, M, and the cold gas fraction of the interstellar medium (f_gas). At fixed f_gas, galaxies follow parallel power laws in 2D (j, M) spaces, with gas-rich galaxies having a larger j_* and j_bar (but a lower j_gas) than gas-poor ones. The slopes of the relations have a value around 0.7. These new relations are amongst the tightest known scaling laws for galaxies. In particular, the baryonic relation (j_bar − M_bar − f_gas), arguably the most fundamental of the three, is followed not only by typical discs but also by galaxies with extreme properties, such as size and gas content, and by galaxies previously claimed to be outliers of the standard 2D j − M relations. The stellar relation (j_* − M_* − f_gas) may be connected to the known j_* − M_*-bulge fraction relation; however, we argue that the j_bar − M_bar − f_gas relation can originate from the radial variation in the star formation efficiency in galaxies, although it is not explained by current disc instability models.