Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004eso..pres...24.&link_type=abstract
ESO Press Release, 10/2004
Astronomy and Astrophysics
Astronomy
Scientific paper
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
ESO PR Photo 29a/04
Velocity Measurements of Forty Intracluster Planetary Nebulae
[Preview - JPEG: 400 x 502 pix - 50k] [Normal - JPEG: 800 x 1004 pix - 330k] [Full Res - JPEG: 2321 x 2912 pix - 1.2M]
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
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