Isolated Star-Forming Cloud Discovered in Intracluster Space

Subaru and VLT Join Forces in New Study of Virgo Galaxy Cluster [1]

Summary

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).

The Virgo Cluster

ESO PR Photo 04a/03 ESO PR Photo 04b/03

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”.

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

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
intracluster field, as obtained with the FORS2 multi-mode instrument of
the 8.2-m VLT YEPUN telescope on Paranal. Emission lines from oxygen
([OIII]) and hydrogen (H-alpha, H-beta, H-gamma) atoms as well as
ionized sulphur ([SII], [SIII]) are identified.

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

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

[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.