The galaxies in the Universe are
rarely isolated - they prefer company. Many are found
within dense structures, referred to as galaxy
clusters.
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. Image 1
(from the Wide Field Imager camera at the ESO La Silla
Observatory) shows a small sky region near the center
of this cluster with some of the brighter cluster
galaxies. Image 2 displays an image
of a larger field (partially overlapping Image 1)
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, Messier 86, Messier 84,
and NGC4388. 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".
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 contemporary
technology. We now know that this intracluster glow
comes from individual stars in that region.
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, such as the
"Dumbbell Nebula" (see 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 center. This
nebula emits strongly in characteristic emission lines
of oxygen (green; at wavelengths 495.9 and 500.7 nanometers)
and hydrogen (red; the H-alpha line at 656 nanometers).
Planetary nebulae may be distinguished from other
emission nebulae by the fact that their main green
oxygen line at 500.7 nanometers is normally about
3 to 5 times brighter than the red H-alpha line.
An international team of astronomers
is now carrying out a very challenging research program,
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.
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
(see Image 1 and 2).
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
to 5, such as those depicted in Image
5. 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 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 kilometers,
i.e., about 17 times farther than the Moon.
The recorded (long-slit) spectrum
(Image 5) 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. Image
3 shows the general location of this "compact
HII region" with respect to NGC4388.
Image 4 shows its exact location in the H-alpha
image in addition to the location one confirmed and
two candidate planetary nebulae.
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.
This compact star-forming region
is located about 3.4 arcminutes north and 0.9 arcminutes
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 kilometers/s. This is considerably faster than
the mean velocity of the Virgo cluster (about 1200
kilometers/second), but similar to that of NGC 4388
(2520 km/s), 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 proto-galaxies 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.
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.
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).
Ortwin Gerhard
Astronomisches Institut
Universität Basel
Switzerland
Phone: +41-61-2055-419
email: gerhard@astro.unibas.ch
Sadanori Okamura
Dept. of Astronomy
University of Tokyo
Japan
Phone: +81-3-5800-6880
email: okamura@astron.s.u-tokyo.ac.jp
Magda Arnaboldi
INAF
Osservatorio Astronomico di Pino Torinese
Italy
Phone: +39-011-8101902
email: arnaboldi@to.astro.it
