New Hubble Space Telescope observations of globular star clusters in a tiny galaxy called the Fornax dwarf spheroidal galaxy (Fornax dSph) show these objects are very similar to those found in our own Milky Way Galaxy, and so must have formed in a similar way. One of the leading theories on how globular clusters form predicts that they should only be found nestled among large quantities of old stars. But these old stars, though rife in our Galaxy, are not present in the Fornax dSph, so the mystery only deepens.
This image shows four globular clusters in the Fornax dwarf spheroidal galaxy, from left to right, top to bottom: Fornax 1, Fornax 2, Fornax 3 and Fornax 5. Image credit: NASA / ESA / S. Larsen, Radboud University, the Netherlands.
Globular clusters are large balls of stars that orbit the centers of galaxies, but can lie very far from them. These objects remain one of the biggest cosmic mysteries.
They were once thought to consist of a single population of stars that all formed together.
However, a number of studies have since shown that many of the Milky Way’s globular clusters are made up of at least two distinct populations of stars and had far more complex formation histories.
Of these populations, around half the stars are a single generation of normal stars that were thought to form first, and the other half form a second generation of stars, which are polluted with different chemical elements.
The proportion of polluted stars found in the Milky Way’s globular clusters is much higher than astronomers expected, suggesting that a large chunk of the first generation star population is missing.
A leading explanation for this is that the clusters once contained many more stars, but a large fraction of the first generation stars were ejected from the cluster at some time in its past.
This explanation makes sense for globular clusters in our Galaxy, where the ejected stars could easily hide among the many similar, old stars in the vast halo, but the new observations call this theory into question.
An international team of astronomers used the Wide Field Camera 3 aboard NASA’s Hubble Space Telescope to observe four globular clusters in the Fornax dSph – an elliptical dwarf galaxy located in the constellation Fornax, about 460,000 light-years away. This galaxy is a satellite of our Milky Way Galaxy and contains six globular clusters.
The Hubble observations show that the Fornax dSph’s globular clusters also contain a second polluted population of stars and indicate that not only did they form in a similar way to one another, their formation process is also similar to clusters in our Galaxy.
Specifically, the team measured the amount of nitrogen in the cluster stars, and found that about half of the stars in each cluster are polluted at the same level that is seen in Milky Way’s globular clusters.
This high proportion of polluted second generation stars means that the formation of the Fornax dSph’s globular clusters should be covered by the same theory as those in the Milky Way.
Based on the number of polluted stars in these clusters they would have to have been up to 10 times more massive in the past, before kicking out huge numbers of their first generation stars and reducing to their current size.
But, unlike the Milky Way, the Fornax dSph doesn’t have enough old stars to account for the huge number that were supposedly banished from the clusters.
“If these kicked-out stars were there, we would see them – but we don’t! Our leading formation theory just can’t be right. There’s nowhere that the Fornax could have hidden these ejected stars, so it appears that the clusters couldn’t have been so much larger in the past,” said team member Dr Frank Grundahl of Aarhus University in Denmark, who is a co-author on a paper published in the Astrophysical Journal (arXiv.org preprint).
This finding means that a leading theory on how these mixed generation globular clusters formed cannot be correct and astronomers will have to think once more about how these mysterious objects came to exist.
_____
Søren S. Larsen et al. 2014. Nitrogen Abundances and Multiple Stellar Populations in the Globular Clusters of the Fornax dSph. ApJ 797, 15; doi: 10.1088/0004-637X/797/1/15
