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

The only way to make sure that these unusual objects are actually 
powered by young stars is by a detailed spectroscopical study, 
analyzing the emitted light over a wide range of wavelengths. One of 
the objects was observed in this way in April 2002 with the FORS2 
multi-mode instrument at the 8.2-m VLT YEPUN telescope at the ESO 
Paranal Observatory (Chile).

This was a most challenging observation, even for this very powerful
facility, requiring several hours of exposure time. The brightness of 
the faint object (the flux of the oxygen [OIII 500.7]-line) was 
comparable to that of a 60-Watt light bulb at a distance of about 6.6 
million km, i.e., about 17 times farther than the Moon.

The recorded (long-slit) spectrum (PR Photo 04e/03) is indeed that of 
an HII region, with characteristic emission lines from hydrogen, 
oxygen and sulphur, and with underlying blue "continuum" emission from 
hot, young stars. This is the first concrete evidence that some of the 
ionized hydrogen gas in the intracluster medium near NGC 4388 is 
heated by massive stars, rather than radiation from the nucleus of the 
galaxy.

Comparing the spectrum with simple starburst models showed that this 
HII region is "powered" by one or two hot and massive (O-type) stars. 
The best-fitting starburst model implies an estimated total mass of 
young stars of some 400 solar masses with an age of about 3 million 
years. The object is obviously very compact - it is indeed unresolved 
in all the images. The inferred radius of the HII region is about 11 
light-years.

Young stars form far from galaxies
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This compact star-forming region is located about 3.4 arcmin north and 
0.9 arcmin west of the galaxy NGC 4388, corresponding to a distance of 
some 82,000 light-years (projected) from the main star-forming regions 
in this galaxy. The small cloud is moving away from us with an 
observed velocity of 2670 km/sec. This is considerably faster than the 
mean velocity of the Virgo cluster (about 1200 km/sec) but similar to 
that of NGC 4388 (2520 km/sec), indicating that it is probably falling 
through the Virgo cluster core together with NGC 4388, but it cannot 
have moved far during the comparatively short lifetime of its massive 
stars.

It is not known whether it once was or still is bound to NGC 4388, or
whether it only belonged to the surroundings that fell into the Virgo
cluster with this galaxy. In any case, the existence of this HII 
region is a clear demonstration that stars can form in the "diffuse" 
outskirts of galaxies, if not in intracluster space.

Because of internal dynamical processes, the stars in this object 
cannot remain forever in a dense cluster. Within a few hundred million 
years they will disperse and mix with the diffuse stellar population 
nearby. This isolated star formation is therefore likely to contribute 
to the intracluster stellar population, either directly, or after 
having moved away from the halo of NGC 4388.

This mode of isolated star formation does not contribute much to the 
total intracluster light emission - at the current rate it can explain 
only a small fraction of the diffuse light now observed in this 
region. However, it may have been more significant in the past, when 
protogalaxies and proto-galaxy groups, rich in neutral gas and with 
gas clouds at large distances from their centers, fell into the 
forming Virgo cluster for the first time.

Prospects
---------
The existence of isolated compact HII regions like this one is 
important as a very different site of star formation than those 
normally seen in galaxies. The massive stars born in such isolated 
clouds will explode as supernovae and enrich the Virgo intracluster 
medium with metals.

Other possible - but not yet spectroscopically verified - compact HII
regions in the halos of both Messier 86 and Messier 84 have been 
detected during this work. This finding thus also calls into question 
the current use of emission-line planetary nebulae luminosities as a 
distance indicator; to obtain the best possible accuracy, it will 
henceforth be necessary to weed out possible HII regions in the 
samples.

If compact HII regions exist generally in galaxies, they may possibly 
be the birthplaces of some of the young stars now observed in the halo 
of our Milky Way galaxy, high above the main plane. Observational 
programmes with both the Subaru and VLT telescopes are now planned to 
discover more of these interesting objects and to explore their 
properties.

More information
----------------
The information in this Press Release is based on a research article 
just published in the Astrophysical Journal ("Isolated Star Formation: 
A Compact HII Region in the Virgo Cluster" by Ortwin Gerhard and 
co-authors; Vol. 580, L121, astro-ph/0211341). The first results of 
the research project have been published jointly with the Suprime-Cam 
team members (Okamura et al., 2002, Publ. Astron. Soc. Japan, 54, 
883-889, astro-ph/0211352) and another article by Arnaboldi et al. 
will soon appear in the Astronomical Journal (astro-ph/0211351).

Notes
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[1]: This press release is issued jointly by Subaru (in English and
Japanese) and ESO.

[2]: The members of the team are Ortwin Gerhard (Astronomisches 
Institut, Universitaet Basel, Switzerland), Magda Arnaboldi 
(Osservatorio di Capodimonte, Napoli, and Osservatorio di Pino 
Torinese, Italy), Kenneth C.  Freeman (Mount Stromlo Observatory, ACT, 
Australia), Sadanori Okamura (Dept.  of Astronomy, University of 
Tokyo, Japan) and the Suprime-Cam team members.

Contacts

Ortwin Gerhard
Astronomisches Institut
Universitaet Basel
Switzerland
Phone: +41-61-2055-419
email: gerhard{at}astro.unibas.ch

Sadanori Okamura
Dept. of Astronomy
University of Tokyo
Japan
Phone: +81-3-5800-6880
email: okamura{at}astron.s.u-tokyo.ac.jp

Magda Arnaboldi
INAF
Osservatorio Astronomico di Pino Torinese
Italy
Phone: +39-011-8101902
email: arnaboldi{at}to.astro.it

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