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1\16 ESO - Subaru and VLT Observe Virgo Cluster- Neptune Moons- Comet Wirtanen 
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             Information from the European Southern Observatory
 ESO Press Release 02/03
 16 January 2003                                         [ESO Logo]
 For immediate release
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Isolated Star-Forming Cloud Discovered in Intracluster Space
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Subaru and VLT Join Forces in New Study of Virgo Galaxy Cluster [1]

Summary
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At a distance of some 50 million light-years, the Virgo Cluster is 
the nearest galaxy cluster. It is located in the zodiacal 
constellation of the same name (The Virgin) and is a large and dense 
assembly of hundreds of galaxies.

The "intracluster" space between the Virgo galaxies is permeated by 
hot X-ray emitting gas and, as has become clear recently, by a sparse
"intracluster population of stars".

So far, stars have been observed to form in the luminous parts of
galaxies. The most massive young stars are often visible indirectly 
by the strong emission from surrounding cocoons of hot gas, which is 
heated by the intense radiation from the embedded stars. These "HII 
regions" (pronounced "Eitch-Two" and so named because of their 
content of ionized hydrogen) may be very bright and they often trace 
the beautiful spiral arms seen in disk galaxies like our own Milky 
Way.

New observations by the Japanese 8-m Subaru telescope and the ESO 
Very Large Telescope (VLT) have now shown that massive stars can also 
form in isolation, far from the luminous parts of galaxies. During a 
most productive co-operation between astronomers working at these two
world-class telescopes, a compact HII region has been discovered at 
the very boundary between the outer halo of a Virgo cluster galaxy 
and Virgo intracluster space.

This cloud is illuminated and heated by a few hot and massive young 
stars.  The estimated total mass of the stars in the cloud is only a 
few hundred times that of the Sun.

Such an object is rare at the present epoch. However, there may have 
been more in the past, at which time they were perhaps responsible 
for the formation of a fraction of the intracluster stellar 
population in clusters of galaxies. Massive stars in such isolated 
HII regions will explode as supernovae at the end of their short 
lives, and enrich the intracluster medium with heavy elements.

Observations of two other Virgo cluster galaxies, Messier 86 and 
Messier 84, indicate the presence of other isolated HII regions, thus 
suggesting that isolated star formation may occur more generally in 
galaxies. If so, this process may provide a natural explanation to 
the current riddle why some young stars are found high up in the halo 
of our own Milky Way galaxy, far from the star-forming clouds in the 
main plane.

PR Photo 04a/03: Sky field in the Virgo Cluster near Messier 84 (WFI
camera at La Silla).  PR Photo 04b/03: H-alpha image of a field near 
the centre of the Virgo cluster (Subaru).  PR Photo 04c/03: Colour 
composite of NGC 4388 and adjacent sky region (Subaru).  PR Photo 
04d/03: The newly discovered intracluster HII region and some 
planetary nebulae (Subaru).  PR Photo 04e/03: Spectrum of the 
intracluster HII region (VLT).
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The Virgo Cluster
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ESO PR Photo 04a/03                ESO PR Photo 04b/03
[Preview - JPEG: 400 x 428 pix     [Preview - JPEG: 433 x 400 pix
- 74k                              - 60k
[Normal - JPEG: 800 x 855 pix -    [Normal - JPEG: 865 x 800 pix -
408k]                              456k]
[Hi-Res - JPEG: 4252 x 4544 pix -  [Hi-Res - JPEG: 3077 x 2847 pix -
10.3M]                             4.2M]

 Captions: PR Photo 04a/03 displays a sky field near some of the 
 brighter galaxies in the Virgo Cluster. It was obtained in April 2000 
 with the Wide Field Imager (WFI) at the La Silla Observatory 
 (exposure 6 x 5 min; red R-band; seeing 1.3 arcsec). The large 
 elliptical galaxy at the centre is Messier 84; the elongated image of 
 NGC 4388 (an active spiral galaxy, seen from the side) is in the 
 lower left corner. The field measures 16.9 x 15.7 arcmin^2. PR Photo 
 04b/03 shows a larger region of the Virgo cluster, with the galaxies 
 Messier 86 (at the upper edge of the field, to the left of the 
 centre), as well as Messier 84 (upper right) and NGC 4388 (just below 
 the centre) that are also seen in PR Photo 04a/03. It is reproduced 
 from a long-exposure Subaru Suprime-Cam image, obtained in the red 
 light of ionized hydrogen (the H-alpha spectral line at wavelength 
 656.2 nm). In order to show the faintest possible hydrogen emitting 
 objects embedded in the outskirts of bright galaxies, their smooth 
 envelopes have been "subtracted" during the image processing. The 
 field measures 34 x 27 arcmin^2. Part of this sky field is shown in 
 colour in PR Photo 04c/03.

The galaxies in the Universe are rarely isolated - they prefer 
company. Many are found within dense structures, referred to as galaxy 
clusters, cf. e.g., ESO PR Photo 16a/99.

The galaxy cluster nearest to us is seen in the direction of the 
zodiacal constellation Virgo (The Virgin), at a distance of 
approximately 50 million light-years. PR Photo 04a/03 (from the Wide 
Field Imager camera at the ESO La Silla Observatory) shows a small sky 
region near the centre of this cluster with some of the brighter 
cluster galaxies. PR Photo 04b/03 displays an image of a larger field 
(partially overlapping Photo 04a/03) in the light of ionized hydrogen 
- it was obtained by the Japanese 8.2-m Subaru telescope on Mauna Kea 
(Hawaii, USA). The field includes some of the large galaxies in this 
cluster, e.g., Messier 86, Messier 84 and NGC 4388. In order to show 
the faintest possible hydrogen emitting objects embedded in the 
outskirts of bright galaxies, their smooth envelopes have been 
"subtracted" during the image processing. This is why they look quite 
different in the two photos.

Clusters of galaxies are believed to have formed because of the strong
gravitational pull from dark and luminous matter. The Virgo cluster is
considered to be a relatively young cluster, because studies of the
distribution of its member galaxies and X-ray investigations of hot 
cluster gas have revealed small "subclusters of galaxies" around the 
major galaxies Messier 87, Messier 86 and Messier 49. These 
subclusters are yet to merge to form a dense and smooth galaxy 
cluster.

The Virgo cluster is apparently cigar-shaped, with its longest 
dimension of about 10 million light-years near the line-of-sight 
direction - we see it "from the end".

(continued)

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