The Virgo Cluster of Galaxies in the Making (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Magda Arnaboldi
INAF, Osservatorio Astronomico di Pino Torinese, Italy
Phone: +39-011-8101953
email:

Ortwin Gerhard
Astronomical Institute, University of Basel, Switzerland
Phone: +41 61 2055419
Email:

For immediate release: 22 October 2004

ESO Press Release 24/04

The Virgo Cluster of Galaxies in the Making

VLT Observations of Planetary Nebulae Confirm the Dynamical
Youth of Virgo [1]

Summary

An international team of astronomers [2] has succeeded in
measuring with high precision the velocities of a large
number of planetary nebulae [3] in the intergalactic space
within the Virgo Cluster of galaxies. For this they used
the highly efficient FLAMES spectrograph [4] on the ESO
Very Large Telescope at the Paranal Observatory (Chile).

These planetary nebulae stars free floating in the otherwise
seemingly empty space between the galaxies of large clusters
can be used as "probes" of the gravitational forces acting
within these clusters. They trace the masses, visible as
well as invisible, within these regions. This, in turn,
allows astronomers to study the formation history of these
large bound structures in the universe.

The accurate velocity measurements of 40 of these stars
confirm the view that Virgo is a highly non-uniform galaxy
cluster, consisting of several subunits that have not yet
had time to come to equilibrium. These new data clearly
show that the Virgo Cluster of galaxies is still in its
making.

They also prove for the first time that one of the bright
galaxies in the region scrutinized, Messier 87, has a very
extended halo of stars, reaching out to at least 65 kpc.
This is more than twice the size of our own galaxy, the
Milky Way.

PR Photo 29a/04: Velocity Measurements of Forty Intracluster
Planetary Nebulae (FLAMES/VLT)
PR Photo 29b/04: Intracluster Planetary Nebulae in the SUC
field in the Virgo Cluster (Digital Sky
Survey)

A young cluster

At a distance of approximately 50 million light-years, the
Virgo Cluster is the nearest galaxy cluster. It is located in
the zodiacal constellation Virgo (The Virgin) and contains
many hundreds of galaxies, ranging from giant and massive
elliptical galaxies and spirals like our own Milky Way, to
dwarf galaxies, hundreds of times smaller than their big
brethren. French astronomer Charles Messier entered 16 members
of the Virgo cluster in his famous catalogue of nebulae. An
image of the core of the cluster obtained with the Wide Field
Imager camera at the ESO La Silla Observatory was published
last year as PR Photo 04a/03.

Clusters of galaxies are believed to have formed over a long
period of time by the assembly of smaller entities, through
the strong gravitational pull from dark and luminous matter.
The Virgo cluster is considered to be a relatively young
cluster because previous studies have revealed small "sub-
clusters of galaxies" around the major galaxies Messier 87,
Messier 86 and Messier 49. These sub-clusters have yet to
merge to form a denser and smoother galaxy cluster.

Recent observations have shown that the so-called
"intracluster" space, the region between galaxies in a
cluster, is permeated by a sparse "intracluster population
of stars", which can be used to study in detail the structure
of the cluster.

Cosmic wanderers

The first discoveries of intracluster stars in the Virgo
cluster were made serendipitously by Italian astronomer,
Magda Arnaboldi (Torino Observatory, Italy) and her
colleagues, in 1996. In order to study the extended halos
of galaxies in the Virgo cluster, with the ESO New Technology
Telescope at La Silla, they searched for objects known as
"planetary nebulae" [3].

Planetary nebulae (PNe) can be detected out to large distances
from their strong emission lines. These narrow emission lines
also allow for a precise measure of their radial velocities.
Planetary Nebulae can thus serve to investigate the motions
of stars in the halo regions of distant galaxies.

In their study, the astronomers found several planetary
nebulae apparently not related to any galaxies but moving
in the gravity field of the whole cluster. These "wanderers"
belonged to a newly discovered intracluster population of
stars.

Since these first observations, several hundreds of these
wanderers have been discovered. They must represent the tip
of the iceberg of a huge population of stars swarming among
the galaxies in these enormous clusters. Indeed, as planetary
nebulae are the final stage of common low mass stars -- like our Sun -- they are
representative of the stellar population
in general. And as planetary nebulae are rather short-lived
(a few tens of thousand years -- a blitz on astronomical
timescales), astronomers can estimate that one star in about
8,000 million of solar-type stars is visible as a planetary
nebula at any given moment. There must thus be a comparable
number of stars in between galaxies as in the galaxies
themselves. But because they are diluted in such a huge
volume, they are barely detectable.

Because these stars are predominantly old, the most likely
explanation for their presence in the intracluster space is
that they formed within 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.

Thus planetary nebulae can provide a unique handle on the
number, type of stars and motions in regions that may harbour
a substantial amount of mass. Their motions contain the fossil
record of the history of galaxy interaction and the formation
of the galaxy cluster.

Measuring the speed of dying stars

ESO PR Photo 29a/04

Velocity Measurements of Forty Intracluster Planetary
Nebulae

Caption: ESO PR Photo 29a/04 shows the intracluster
planetary nebulae radial velocity distributions in three
different regions of the sky (identified with the following
labels: FCJ, CORE and SUC) in the Virgo cluster core region.
The central panel shows the image of the VIRGO cluster core
obtained from the Digital Sky Survey. The four brighter
galaxies in the field are on the left Messier 87 near the
FCJ field, and Messier 86, Messier 84 and NGC 4388 in the
SUC field. In the FCJ panel, the blue dashed line shows a
Gaussian curve with a mean velocity, vrad = 1276 km/s, and
a dispersion, σrad = 247 km/s. In CORE, the green dashed
line shows a Gaussian curve with vrad = 1436 km/s and
σrad = 538 km/s for Virgo Cluster dwarf ellipticals and
lenticular galaxies within 2 degrees of Messier 87. In the
SUC panel, the dashed red line shows a Gaussian curve with
vrad = 1079 km/s and σrad = 286 km/s, associated to the
Messier 84 (M84) peak. The overplotted dash-dotted lines
show the SUC-FLAMES spectra of intracluster HII regions,
which have radial velocities in the M84 and NGC 4388
velocity ranges.

The international team of astronomers [2] went on further to
make a detailed study of the motions of the planetary nebulae
in the Virgo cluster in order to determine its dynamical
structure and compare it with numerical simulations. To this
aim, they carried out a challenging research programme, aimed
at confirming intracluster planetary nebula candidates they
found earlier and measuring their radial velocities in three
different regions ("survey fields") in the Virgo cluster core.

This is far from an easy task. The emission in the main Oxygen
emission line from a planetary nebula in Virgo is comparable
to that of a 60-Watt light bulb at a distance of about 6.6
million kilometres, about 17 times the average distance to the
Moon. Furthermore intracluster planetary nebula samples are
sparse, with only a few tens of planetary nebulae in a quarter
of a degree square sky field -- about the size of the Moon.
Spectroscopic observations thus require 8 metre class
telescopes and spectrographs with a large field of view. The
astronomers had therefore to rely on the FLAMES-GIRAFFE
spectrograph on the VLT [4], with its relatively high spectral
resolution, its field of view of 25 arcmin and the possibility
to take up to 130 spectra at a time.

The astronomers studied a total of 107 stars, among which 71
were believed to be genuine intracluster planetary candidates.
They observed between 21 and 49 objects simultaneously for
about 2 hours per field. The three parts of the Virgo core
surveyed contain several bright galaxies (Messier 84, 86, 87,
and NGC 4388) and a large number of smaller galaxies. They
were chosen to represent different entities of the cluster.

The spectroscopic measurements could confirm the intracluster
nature of 40 of the planetary nebulae studied. They also
provided a wealth of knowledge on the structure of this part
of the Virgo cluster.

In The Making

ESO PR Photo 29b/04

Intracluster Planetary Nebulae in the SUC field in the
Virgo Cluster. (Digital Sky Survey)

Caption: ESO PR Photo 29b/04: Zoomed in view of the
pointing relative to the SUC field. The image shows a
30 x 30 arcminute field centred on the Messier 86/
Messier 84 region of the Virgo cluster. The brighter
galaxies in the field are (clockwise from the left) M86,
M84 and NGC 4388. Their systemic velocities are -244,
1060 and 2524 km/s, respectively. Here the envelopes of
bright galaxies are subtracted as much as possible for the
detection of planetary nebulae embedded there. The larges
circle indicates the FLAMES field-of-view. Intracluster
planetary nebula candidates are marked by circles and show
a highly non-uniform distribution in this field. The
numbers near each circle indicate the measured
line-of-sight velocity for that intracluster planetary
nebula. The colour code used is blue for velocities smaller
than the M84 systemic velocity (1060 km/s), red for larger
velocities.

In the first field near Messier 87 (M87), the astronomers
measured a mean velocity close to 1250 km/s and a rather
small dispersion around this value. Most stars in this field
are thus physically bound to the bright galaxy M87, in the
same way as the Earth is bound to the Sun. Magda Arnaboldi
explains: "This study has led to the remarkable discovery
that Messier 87 has a stellar halo in approximate dynamical
equilibrium out to at least 65 kpc, or more than 200,000
light-years. This is more than twice the size of our own
galaxy, the Milky Way, and was not known before."

The velocity dispersion observed in the second field, which
is far away from bright galaxies, is larger than in the first
one by a factor four. This very large dispersion, indicating
stars moving in very disparate directions at different
speeds, also tells us that this field most probably contains
many intracluster stars whose motions are barely influenced
by large galaxies. The new data suggest as a tantalizing
possibility that this intracluster population of stars could
be the leftover from the disruption of small galaxies as they
orbit M87.

The velocity distribution in the third field, as deduced
from FLAMES spectra, is again different. The velocities
show substructures related to the large galaxies Messier 86,
Messier 84 and NGC 4388. Most likely, the large majority of
all these planetary nebulae belong to a very extended halo
around Messier 84.

Ortwin Gerhard (University of Basel, Switzerland), member
of the team, is thrilled: "Taken together these velocity
measurements confirm the view that the Virgo Cluster is a
highly non-uniform and unrelaxed galaxy cluster, consisting
of several subunits. With the FLAMES spectrograph, we have
thus been able to watch the motions in the Virgo Cluster, at
a moment when its subunits are still coming together. And it
is certainly a view worth seeing!"

More information

The results presented in this ESO Press Release are based on
a research paper ("The Line-of-Sight Velocity Distributions
of Intracluster Planetary Nebulae in the Virgo Cluster Core"
by M. Arnaboldi et al.) that has just appeared in the
research journal Astrophysical Journal Letters Vol. 614,
p. 33.

Notes

[1]: The University of Basel Press Release on this topic is
available at
http://www.zuv.unibas.ch/uni_media/2...1022virgo.html

[2]: The members of the team are Magda Arnaboldi (INAF,
Osservatorio di Pino Torinese, Italy), Ortwin Gerhard
(Astronomisches Institut, Universität Basel, Switzerland),
Alfonso Aguerri (Instituto de Astrofisica de Canarias,
Spain), Kenneth C. Freeman (Mount Stromlo Observatory,
ACT, Australia), Nicola Napolitano (Kapteyn Astronomical
Institute, The Netherlands), Sadanori Okamura (Dept. of
Astronomy, University of Tokyo, Japan), and Naoki Yasuda
(Institute for Cosmic Ray Research, University of Tokyo,
Japan).

[3]: Planetary nebulae are 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".
The ejected envelope is illuminated and heated by the
stellar core and emits strongly in characteristic emission
lines of several elements, notably oxygen (at wavelengths
495.9 and 500.7 nm). Their name stems from the fact that
some of these nearby objects, such as the "Dumbbell Nebula"
(see ESO PR Photo 38a/98) resemble the discs of the giant
planets in the solar system when viewed with small
telescopes.

[4]: FLAMES, the Fibre Large Array Multi-Element Spectrograph,
is installed at the 8.2-m VLT KUEYEN Unit Telescope. It is
able to observe the spectra of a large number of individual,
faint objects (or small sky areas) simultaneously and covers
a sky field of no less than 25 arcmin in diameter, i.e.,
almost as large as the full Moon. It is the result of a
collaboration between ESO, the Observatoire de Paris-Meudon,
the Observatoire de Genève-Lausanne, and the Anglo Australian
Observatory (AAO).

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