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ING scientific news release|
07 October, 2020
Green Light Unveils the Presence of an Old and Metal-Poor Halo in a Giant Elliptical Galaxy
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.
On-sky distribution of PNe observed with Suprime-Cam on the Subaru Telescope (blue circles) and PN.S on the WHT (red crosses). The dashed blue rectangle shows the outline of the Suprime-Cam field-of-view and the red rectangles the PN.S fields. The background image from the Digitised Sky Survey shows the galaxies NGC 3384 (left) and M105 (centre). The cutouts in the right of the image shows details of the Suprime-Cam images with the detected PNe overplotted. Adapted from Hartke et al. (2020arXiv200801696H). Large format: PNG.
Once the census was completed, Hartke and collaborators found an excess of PNe in the outer halo of M105. This excess of PNe begins at 15 kiloparsecs and can be traced out to 50 kiloparsecs. This is significant because most of the light of this elliptical galaxy is detected within only 3 kiloparsecs. As in a detective story, they engaged in a quest to look for the footprints of the parent stars, and they found previous studies which showed the presence of an old and very metal-poor population of stars, that increased outwards following the same exponential profile as the PNe. The metal-rich stars were, however, concentrated towards the central region of M105. They concluded that the old and very metal-poor population was responsible for generating the excess of PNe in the outer envelope encircling M105.
This outer component is faint as it amounts to only 4% the light from the entire M105 galaxy. It reaches out to 50 kiloparsecs, or 18 effective radii, a region where it becomes possible to test the presence of dark matter and the transition to a group halo.
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