The
galaxy NGC 4365 in the Virgo supercluster of galaxies, showing its many
globular clusters (most of the more than one hundred dots in this image
are actually globular star clusters). A new study of over seven
thousand globular clusters around ten Virgo galaxies finds that they are
often gathered into distinct groupings whose shapes reflect the
formation of the clusters and the histories of the galaxies. Credit: NASA/HST and ESO VLT
Globular
clusters are gravitationally bound ensembles of stars, as many as a
million stars in some cases, grouped in roughly spherical clusters with
diameters as small as only tens of light-years. Globular clusters are
typically located in the outer regions (the halos) of galaxies; the
Milky Way galaxy has about two hundred globular clusters orbiting it.
Astronomers are interested in globular clusters in part because they are
home to many of the oldest known stars, but also because of their
locations in the halos. Collisions between galaxies are commonplace,
and globular clusters may provide fossil evidence of these encounters
because they are strongly affected by such interactions. During a
collision, a galaxy can grow by absorbing or merging with its neighbor,
and some models predict that clusters form during these interactions.
Moreover, it is possible that in a merger large numbers of globular
clusters originally belonging to a smaller galaxy may be captured by the
larger galaxy. In any case, the distribution of globular clusters
around a galaxy holds clues to their origins and the history of its host
galaxy.
The Virgo Cluster of galaxies, containing between one and two
thousand galaxies, is located about fifty-four million light-years away
in the direction of the constellation of Virgo. The ten brightest
galaxies of the Virgo Cluster alone contain 7053 detected globular
clusters. CfA astronomers Raffaele D'Abrusco, Pepi Fabbiano, and
Andreas Zezas carefully examined this set of globular clusters looking
for information about the history of these galaxies. In a new paper,
they report discovering distinctive structures among the globular
cluster systems, meaning that the globular clusters around these
galaxies are not distributed symmetrically. Their configurations often
take shapes ranging from roughly linear to circular, with some more
complex shapes as well. The scientists found 229 such structures in this
subsample, forty-two of them classified as being medium or large and
stretching over as much as seventy-five thousand light-years. The
elongated structures tend to be aligned with an axis of the host galaxy,
as would be expected if a merger were responsible.
The scientists argue that these structures are indeed the remnants of
galaxies that were accreted in the past, and among other things they
estimate limits on the masses of these parent galaxies. Computer
simulations provide some rough level of agreement. The authors note that
with more detailed computations, these structures offer a powerful new
tool to advance the study of galaxy evolution.
Reference(s):
"Spatial Structures in the Globular Cluster Distribution of the Ten
Brightest Virgo Galaxies," R. D'Abrusco, G. Fabbiano, A. Zezas, ApJ 2015 (in press)
