Astronomers using data from the extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey find that galaxies that are efficient in forming stars, like discs, are also efficient in retaining angular momentum. In early-type galaxies (ETGs), it is not so: there is a strong dependence as a function of total stellar mass, with the most massive galaxies being least efficient in forming stars and in retaining the angular momentum. This implies that ETGs lose angular momentum during their evolution, and/or retain it in the hot gas component and the satellite galaxies that have not yet merged with the central galaxy.

Galaxies acquire their angular momentum during the very early phases of their lives, from the gravitational torques produced by the large scale tidal fields. These large scale inhomogeneities are generated by structures in the mass distribution in the Universe, e.g. over-densities and voids, dominated by dark matter. The gas, which collapses and forms stars in an isolated dark matter halo, builds up rotation if its angular momentum is approximately conserved. The amount of rotation and its spatial distribution as measured by the velocities of stars and gas in disc galaxies is consistent with these predictions.

For the dark matter haloes in the densest regions of the Universe things get much more complicated. Stars are formed in violent bursts, which together with strong galactic nuclear activity can prevent further accretion of gas. The latter carries most of the angular momentum from the outer regions of the dark matter halos.

Further evolution, including multiple merging with other galaxies, with or without gas, can lead to a further gain or loss of angular momentum. Very simple questions like "does the gain/loss of angular momentum add up?", and "how does it compare with disc galaxies of similar masses?" are indeed very difficult to answer.

The galaxies which are located within the highest mass density regions of the universe are the early type galaxies, or ETGs for short, which include ellipticals and lenticulars. Measuring the total angular momentum of any elliptical galaxy is observationally very challenging. While disc galaxies have extended cold gas discs that allow the measurement of the total mass and angular momentum at large radii because of the simple geometry, ETGs are three-dimensional spheroids, varying from oblate to prolate, and up to triaxial ellipsoids – like rugby balls.

As a further complication, measuring velocities of stars at large radii from the centre of an elliptical galaxy is difficult. Because of the lack of cold gas discs, velocities are measured from absorption lines which can be detected only in the inner bright regions.

In summary, measuring the total angular momentum in ETGs is particularly difficult because of the extended surface brightness profiles, their complex geometry, and the absence of extended gaseous discs. Hence measurements with the goal of constraining the total angular momentum of these complex galaxies was only attempted in a few cases, with limited reproducibility.

The extended Planetary Nebula Spectrograph (ePN.S) survey

Astronomers have long known about planetary nebulae. These are the late phases of Sun-like stars: they are blowing off an envelope of gas which gives off a green glow of an aquamarine hue from its oxygen atoms. The motions of these stars along the line-of-sight are relatively easy to measure from their Doppler shift, because of the bright green oxygen line emitted by their envelope.

Furthermore, as the galaxies become fainter at larger distances from their centres, the planetary nebulae become easier to detect and can be used as beacons to trace the stellar motions out to large distances from the galaxy centres, where the total star light is too faint to analyse.

An international collaboration, including astronomers from Germany and the USA, developed a new strategy to address the conundrum, using a custom-built instrument: the Planetary Nebulae Spectrograph (PN.S) mounted the William Herschel Telescope. The accuracy of the Planetary Nebulae Spectrograph (PN.S) is key to detecting and measuring planetary nebulae in the outer reaches of ETGs.

The observational challenge was indeed overcome by using extragalactic planetary nebulae to measure extended velocity fields out to six effective radii on average for 32 ETGs and these extended velocity fields were key to demonstrating that these galaxies have velocity components along their minor axes, with a substantial component of the angular momentum off axis (see the ING research news released on December 7, 2018).

More recently, Claudia Pulsoni, Ortwin Gerhard, Michael Fall, Magda Arnaboldi and collaborators in a paper published in the Astronomy and Astrophysics journal titled "The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The specific angular momentum of ETGs" explored the relation between mass and angular momentum which are key parameters of any galaxies and specifically ETGs.

While the lower angular momentum content for ETGs had been inferred before (again on the basis of much more spatially-limited velocity fields, for only a handful of galaxies and with strong assumptions on their rotational and three dimensional properties) the analysis carried out by Pulsoni et al. (2023) was achieved using the most extended two velocity fields from the ePN.S survey and a new realistic and sophisticated three dimensional deprojection using ETG analogues from the IllustrisTNG cosmological simulations.

The results of this work show that ETGs have substantially lower specific stellar angular momentum than spiral galaxies with similar total stellar mass (the specific angular momentum is the total angular momentum divided by the total stellar mass). The ETGs have a factor 9 times lower specific angular momentum than disc galaxies with the same stellar mass.

In the accompanying figure, the authors show that galaxies that are efficient in forming stars, like discs, are also efficient in retaining angular momentum and have retention factor fj close to 1. In ETGs, it is not so: there is a strong dependence as a function of total stellar mass, with the most massive galaxies being least efficient in forming stars and in retaining the angular momentum. This implies that ETGs lose specific angular momentum during their evolution, and/or retain it in the hot gas component and the satellite galaxies that have not yet merged with the central galaxy.