Four globular clusters in Fornax — annotated
Globular cluster Fornax 1
Globular cluster Fornax 2
Globular cluster Fornax 3
Globular cluster Fornax 5
Fornax galaxy with four globular clusters marked
Fornax dwarf galaxy
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Hubble observations cast further doubt on how globular clusters formed
Thanks to the NASA/ESA Hubble Space
Telescope, some of the most mysterious cosmic residents have just become
even more puzzling. New observations of globular clusters in a small
galaxy show they are very similar to those found in the Milky Way, and
so must have formed in a similar way. One of the leading theories on how
these clusters form predicts that globular clusters should only be
found nestled in among large quantities of old stars. But these old
stars, though rife in the Milky Way, are not present in this small
galaxy, and so, the mystery deepens.
Globular clusters
— large balls of stars that orbit the centres of galaxies, but can lie
very far from them — remain one of the biggest cosmic mysteries. They
were once thought to consist of a single population of stars that all
formed together. However, research has since shown that many of the
Milky Way's globular clusters had far more complex formation histories
and are made up of at least two distinct populations of stars.
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. In particular, the polluted stars contain up to
50-100 times more nitrogen than the first generation of stars.
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 the Milky Way,
where the ejected stars could easily hide among the many similar, old
stars in the vast halo, but the new observations, which look at this type of cluster in a much smaller galaxy, call this theory into question.
Astronomers used Hubble's Wide Field Camera 3 (WFC3) to observe four globular clusters in a small nearby galaxy known as the Fornax Dwarf Spheroidal galaxy [1].
"We knew that the Milky Way's clusters were more complex than was
originally thought, and there are theories to explain why. But to
really test our theories about how these clusters form we needed to know
what happened in other environments," says Søren Larsen of Radboud University in Nijmegen, the Netherlands, lead author of the new paper. "Before
now we didn’t know whether globular clusters in smaller galaxies had
multiple generations or not, but our observations show clearly that they
do!"
The astronomers' detailed observations of the four Fornax clusters
show that they also contain a second polluted population of stars [2]
and indicate that not only did they form in a similar way to one
another, their formation process is also similar to clusters in the
Milky Way. Specifically, the astronomers used the Hubble observations to
measure 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 Fornax globular clusters' formation 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 ten 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 galaxy that hosts
these clusters 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!" explains Frank Grundahl of Aarhus University in Denmark, co-author on the paper. "Our
leading formation theory just can't be right. There's nowhere that
Fornax could have hidden these ejected stars, so it appears that the
clusters couldn't have been so much larger in the past."
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, in the
Milky Way and further afield, came to exist.
The new work is detailed in a paper published today, 20 November 2014, in The Astrophysical Journal.
Notes
[1] The Milky Way’s gravity keeps Fornax orbiting around us as a satellite galaxy.
[2] The clusters studied were named
Fornax 1, 2, 3, and 5. Fornax 1, 3, and 5 are made up of approximately 40% first generation stars to 60% polluted second generation ones, while
Fornax 2 contains around 60% first generation and 40% second generation.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study consists of S.
Larsen (Radboud University, the Netherlands), J. P Brodie (University of
California, USA), F. Grundahl (Aarhus University, Denmark), and J.
Strader (Michigan State University, USA).
More information
Image credit: NASA, ESA, S. Larsen (Radboud University, the Netherlands)
Links
Contacts
Søren Larsen
Radboud University
Nijmegen, Netherlands
Tel: +31 (0)24 365 2806
Email: s.larsen@astro.ru.nl
Frank Grundahl
Aarhus University
Aarhus, Denmark
Tel: +45 21 31 43 67
Email: fgj@phys.au.dk
Georgia Bladon
ESA/Hubble, Public Information Officer
Garching bei München, Germany
Cell: +44 7816291261
Email: gbladon@partner.eso.org
Source: ESA/Hubble - Space Telescope
