Astronomers using Suprime-Cam on the Subaru Telescope and the Planetary Nebula Spectrograph (PN.S) on the William Herschel Telescope (WHT) have been able to unveil the presence of an old and metal-poor halo in the outskirts of a giant elliptical galaxy in a loose group. This is the first study to clearly establish the link between a metal poor population of stars and the excess of planetary nebulae (PNe) in the outer regions of an elliptical galaxy.

Galaxies are seldom found in isolation. Instead, most of them are found in larger structures that are classified as groups or clusters, depending on their size and number of galaxies. These hierarchical structures are bound together by gravity, and the outer regions of galaxies in these structures can "feel" each other because of their gravitational interactions.

In 1951, the astronomer Fritz Zwicky formulated the hypothesis that stars unbound as a consequence of galaxy interaction during the formation of the Coma cluster were responsible for the missing mass and thus providing the gravitational pull to bind the cluster. We now know that stars are not enough to gravitationally bind clusters, and dark matter could be responsible for the gravity force in the outer region of galaxies. Still, stars are found in the empty regions between galaxies that are in groups or clusters, which proves that galaxies are somehow modified by being "immersed" in the group/cluster structures. It then becomes important to understand when these "free floating stars" began to appear and populate the empty spaces among galaxies in groups.

The Leo I galaxy group is located at a distance of about 10 Mergaparsecs and it is the closest group that contains all types of galaxies (elliptical, spiral, and dwarf). A team of astronomers using data from the Subaru and William Herschel telescopes studied the elliptical galaxy M105 (NGC 3379) which is located at its centre. The goal was to identify single stars whose motions are relatively easy to measure, and use them to resolve the transition from the bound halo of M105 to a population of stars orbiting in the group.

They used PNe as mass tracers. PNe are the late stages of stars like our own Sun. During the PN phase, the expelled outer layers of the central star shine with an aquamarine colour due to the oxygen emission at 5007 Angstroms, similar to the colour of the northern lights. Using this bright emission, astronomers can unveil the structure of the outermost regions of galaxies.

They used the Suprime-Cam camera on the 8-meter Subaru Telescope and the Planetary Nebula Spectrograph (PN.S) mounted on the WHT to carry out a complete census of PNe in the outer regions of M105 and measure their motions.