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1\16 ESO - Subaru and VLT Observe Virgo Cluster- Neptune Moons- Comet Wirtanen 
Part 2 of 3

Stars in intracluster space
---------------------------
Galaxy clusters are dominated by dark matter. The largest fraction of 
the luminous (i.e. "visible") cluster mass is made up of the hot gas 
that permeates all of the cluster. Recent observations of 
"intracluster" stars have confirmed that, in addition to the 
individual galaxies, the Virgo cluster also contains a so-called 
"diffuse stellar component", which is located in the space between the 
cluster galaxies.

The first hint of this dates back to 1951 when Swiss astronomer Fritz 
Zwicky (1898-1974), working at the 5-m telescope at Mount Palomar in 
California (USA), claimed the discovery of diffuse light coming from 
the space between the galaxies in another large cluster of galaxies, 
the Coma cluster. The brightness of this intracluster light is 100 
times fainter than the average night-sky brightness on the ground 
(mostly caused by the glow of atoms in the upper terrestrial 
atmosphere) and its measurement is difficult even with present 
technology. We now know that this intracluster glow comes from 
individual stars in that region.

Planetary nebulae
-----------------
More recently, astronomers have undertaken a new and different 
approach to detect the elusive intracluster stars. They now search for 
Sun-like stars in their final dying phase during which they eject 
their outer layers into surrounding space. At the same time they 
unveil their small and hot stellar core which appears as a "white 
dwarf star".

Such objects are known as "planetary nebulae" because some of those 
nearby, e.g. the "Dumbbell Nebula" (cf. ESO PR Photo 38a/98) resemble 
the disks of the outer solar system planets when viewed in small 
telescopes.

The ejected envelope is illuminated and heated by the very hot star at 
its centre. This nebula emits strongly in characteristic emission 
lines of oxygen (green; at wavelengths 495.9 and 500.7 nm) and 
hydrogen (red; the H-alpha line at 656.2 nm). Planetary nebulae may be 
distinguished from other emission nebulae by the fact that their main 
green oxygen line at 500.7 nm is normally about 3 to 5 times brighter 
than the red H-alpha line.

Search for intracluster planetary nebulae
-----------------------------------------
An international team of astronomers [2] is now carrying out a very
challenging research programme, aimed at finding intracluster 
planetary nebulae. For this, they observe the regions between cluster 
galaxies with specially designed, narrow-band optical filters tuned to 
the wavelength of the green oxygen lines.

The main goal is to study the overall properties of the diffuse 
stellar component in the nearby Virgo cluster. How much diffuse light 
comes from the intracluster space, how is it distributed within the 
cluster, and what is its origin?

Because the stars in this region are apparently predominantly old, the 
most likely explanation of their presence in this region is that they 
formed inside individual galaxies, which were subsequently stripped of 
many of their stars during close encounters with other galaxies during 
the initial stages of cluster formation. These "lost" stars were then 
dispersed into intracluster space where we now find them.

The Subaru observations
-----------------------
ESO PR Photo 04c/03              ESO PR Photo 04d/03
[Preview - JPEG: 471 x 400 pix   [Preview - JPEG: 444 x 400 pix
- 62k                            - 92k
[Normal - JPEG: 941 x 800 pix -  [Normal - JPEG: 888 x 800 pix -
776k]                            600k]
[Hi-Res - JPEG: 3028 x 2573 pix -
4.4M]

 Captions: PR Photo 04c/03 shows the general location of the newly
 discovered compact HII region with respect to a previously published
 Subaru Suprime-Cam image of NGC 4388. The image combines H-alpha
 narrow-band (hydrogen), O[III] narrow-band (oxygen), and broad-band
 optical V-band data. The extended pink filamentary structure in this 
 image is due to gas ionized by the radiation from the nucleus of the 
 galaxy. The vertical lines are caused by detector saturation of 
 bright objects. The field of view is 11.6 x 5.0 arcmin^2. The 
 outlined region indicates the sky field shown in PR Photo 04d/03 
 which is an H-alpha image of a 4 x 3 arcmin^2 region in the Virgo 
 intracluster region. This is part of the area selected for 
 spectroscopic follow-up observations with the FORS2 multimode 
 instrument at the 8.2-m VLT YEPUN telescope. The image shows the 
 confirmed compact HII region (in blue circle to the left) and the 
 confirmed intracluster planetary nebula (in yellow and red circle at 
 the top). The two other objects (in red circles) are additional 
 planetary nebulae candidates, which will soon be observed 
 spectroscopically. North is up, and East is left. The newly 
 discovered HII-region (blue circle) is well visible on PR Photo 
 04c/03 and faintly on the high-resolution versions of PR Photo 04a/03 
 and PR Photo 04b/03.

Japanese and European astronomers used the Suprime-Cam wide-field 
mosaic camera at the 8-m Subaru telescope (Mauna Kea, Hawaii, USA) to 
search for intracluster planetary nebulae in one of the densest 
regions of the Virgo cluster, cf. PR Photo 04b/03. They needed a 
telescope of this large size in order to select such objects and 
securely discriminate them from the thousands of foreground stars in 
the Milky Way and background galaxies.

In particular, by observing in two narrow-band filters sensitive to 
oxygen and hydrogen, respectively, the planetary nebulae visible in 
this field could be "separated" from distant (high-redshift) 
background galaxies, which do not have strong emission in both the 
green and red band. It is very time-consuming to observe the weak 
H-alpha emission and this can only be done with a big telescope.

Some 40 intracluster planetary nebulae candidates were found in this 
field which had the expected oxygen/H-alpha line intensity ratios of 3 
- 5, such as those depicted PR Photo 04d/03. Unexpectedly, however, 
the data also showed a small number of star-like emission objects with 
oxygen/H-alpha line ratios of about 1. This is more typical of a cloud 
of ionized gas around young, massive stars - like the so-called HII 
regions in our own galaxy, the Milky Way.

However, it would be very unusual to find such star formation regions 
in the intracluster region, so follow-up spectroscopic observations 
were clearly needed for confirmation.

The VLT measurements
--------------------
ESO PR Photo 04e/03             Captions: PR Photo 04e/03 displays
                                the emission spectrum (in the
                                visible/near-IR spectral region) of
                                the compact HII region in the Virgo
Preview - JPEG: 506 x 400 pix - intracluster field, as obtained with
- 35k                           the FORS2 multi-mode instrument of
Normal - JPEG: 1011 x 800 pix - the 8.2-m VLT YEPUN telescope on
128k                            Paranal. Emission lines from oxygen
                                ([OIII]) and hydrogen (H-alpha,
                                H-beta, H-gamma) atoms as well as
                                ionized sulphur ([SII], [SIII]) are
                                identified.

(continued)

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