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Just over two months ago, in the 4th November
news, we described the latest findings concerning the unusual celestial object known as "Hanny's Voorwerp", a gigantic gas cloud illuminated by the light
from a quasar that is no longer active in the nucleus of IC 2947, a galaxy near the object.
The picture of this strange system has now been greatly enriched thanks to a new image recently produced by NASA from two images taken in April 2010 by the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3), onboard the Hubble Space Telescope.
The newly reduced images (above) distinctly show star clusters, identifiable as yellowish regions within the green whisps of Hanny's Voorwerp. Until now they had gone unnoticed due to the relatively low resolution of the available images, but the new data makes them clearly visible, and also show that the nearby galaxy has asymmetric spiral arms, large regions of dust and high rates of star formation near the nucleus.
These characteristics suggest that IC 2947 began to collide with another galaxy about 1 billion years ago, the companion now being merged in IC 2947. The encounter first produced the conditions necessary to trigger quasar activity in the galactic nucleus, and later, the conditions necessary to expel the gas that formed Hanny's Voorwerp.
Hanny's Voorwerp, the optically visible cloud extending between 44,000 and 136,000 light years from the galactic nucleus, is, in fact, only part of a larger gas cloud that extends for 300,000 light years around IC 2947, as shown by radio observations. It may be that the gas flow from IC 2947 compressed the gas in Hanny's Voorwerp and triggered the star formation now seen.
The star clusters appear to be confined to an area only a few thousand light years across and seem to consist of stars no younger than 2 million years, as if
the influence of the quasar (the super-massive black hole at the galactic nucleus) was diminished during the last 2 million years until finally ceasing activity 100-200 thousand years ago.
The last trace of its activity is in fact the light re-emitted from the gas (including oxygen that produces the green colour) of Hanny's Voorwerp.
A curiosity: the large, dark, circular area, apparently empty, may be the projected shadow of some object in the vicinity of the quasar that intercepted a portion of its light while the quasar was active.
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