Globular clusters found on far side of the Galaxy
Until now the stellar clusters have been hidden by dust, and there are likely more to be discovered
T
his month’s research paper takes us to an unexplored corner of our own Galaxy. Our guides are two researchers from Seoul, Jinhyuk Ryu and Myung Gyoon Lee, who have been charting the Milky Way’s furthest reaches in search of new clusters of stars. There’s a particular problem with finding these in the plane of the Milky Way itself; looking at this part of the sky, the view is blocked by nearby dust that hides more distant objects.
This isn’t a new problem. William Herschel’s 18th century attempt to work out the shape of the Galaxy was stymied by the effects of dust, which led him to conclude that the Sun lay at the centre of a flat pancake of stars (so he scores well for correctly deducing we live in a flat disc, but less well for placing us at the centre). Modern astronomers, though, have many more advanced tools at their disposal, and in this case the ability to use telescopes and cameras sensitive to the infrared region of the spectrum proved crucial.
The team worked with data obtained by two venerable surveys: UKIDSS (UKIRT Infrared Deep Sky Survey), which made use of the UK InfraRed Telescope – or UKIRT – on Mauna Kea in Hawaii, and 2MASS (the Two Micron All Sky Survey), which used data from telescopes in Arizona and Chile. Carefully combing through the data in the direction of the galactic centre, the researchers found more than 900 new clusters, of which two were possible globular clusters.
Now known as RLGC1 and 2 (Ryu and Lee Globular Cluster), both are just about visible in infrared images as small groups of resolved stars. A careful counting of stars in both systems reveals the hallmark of a globular cluster, a distribution which leads to a greater density of stars toward the centre; it’s this that distinguishes a globular from a mere open cluster. They hadn’t been spotted before because of their position on the far side of the Milky Way, and because of their distances: RLGC1 is 93,000 lightyears away and RLGC2 52,000.
Globulars tend to include older stars, and that’s true here too; careful measurement suggests both are older than 12 billion years, typical for this type of globular. Both appear to be independent systems, isolated from other clusters and not part of the main body of stars that comprises the Milky Way disc. If I had to guess, these newly-discovered clusters are all that remain of two small galaxies, cannibalised by the Milky Way as it grew rapidly in the distance past. By making sure we have a complete count of these remnants, we can try to understand how that process happened, and why we have the Galaxy that we see today.
The paper ends with an encouraging note for cluster hunters. Of the 214 known Milky Way globulars, 78 lie in or near the disc, but most of those are on the near side. That means there are, the authors reckon, about another 30 to be discovered. Our Milky Way still has plenty of unexplored space with treasures lurking inside.
“If I had to guess, these clusters are all that remain of two small galaxies cannibalised by the Milky Way as it grew rapidly in the distant past”