Hubble's deep view of an anaemic galaxy

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 10 February, 2009

The Hubble Space Telescope has revealed a rare spiral galaxy in the Coma Cluster that has been described by scientists as "anaemic".

This deep Hubble image reveals NGC 4921 in intricate detail. Image: NASA/ESA/K.Cook (Lawerence Livermore National Laboratory, USA).

Using Hubble's Advanced Camera for Surveys, scientists have revealed the fine structure of galaxy NGC 4921, set against a rich background of remote galaxies spawned in the Universe's early years. These young galaxies show a range of shape, size and colour, and sport the ragged appearance of galaxies at a time before the familiar division into spirals and ellipticals had become established.

Galaxy NGC 4921 is located in the centre of the Coma Galaxy Cluster (also known as Abell 1656), located 320 million light years from Earth in the constellation Coma Berenices. The cluster hosts over 1,000 galaxies, and as a result, interactions and merging events are commonplace, transforming gas-rich spirals into elliptical systems without much active star formation.

NGC 4921 is one of the rare spirals in the Coma Cluster, and is an example of an unusual breed of "anaemic spiral" where the normal vigorous star formation that creates a spiral galaxy's familiar bright arms is much less intense. As a result there is just a delicate swirl of dust in a ring around the galaxy, accompanied by some bright young blue stars that are easily discerned in the Hubble image. Also revealed is the galaxy's bright nucleus and central bar. Much of the pale spiral structure in the outer parts of the galaxy is unusually smooth and gives the whole galaxy the ghostly look of a vast translucent jellyfish.

Towards the diffuse edge of the galaxy are dwarf companion galaxies, and further out, unrelated galaxies in both the far Universe and Milky Way. The image was created from a total of eighty separate pictures through yellow and near infrared filters.

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