A portrait of one hundred thousand and one galaxies
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
Posted: August 8, 2002
These images have been used by different groups of astronomers for various kinds of scientific investigations, ranging from individual stars and nebulae in NGC 300, to distant galaxies and other objects in the background.
This material provides an interesting demonstration of the multiple use of astronomical data, now facilitated by the establishment of extensively documented data archives, like the ESO Science Data Archive that now is growing rapidly and already contains over 15 Terabyte.
Based on the concept of Astronomical Virtual Observatories (AVOs), the use of archival data sets is on the rise and provides a large number of scientists with excellent opportunities for front-line investigations without having to wait for precious observing time. In addition to presenting a magnificent astronomical photo, the present account also illustrates this important new tool of the modern science of astronomy and astrophysics.
Located some 7 million light-years away, the spiral galaxy NGC 300 is a beautiful representative of its class, a Milky-Way-like member of the prominent Sculptor group of galaxies in the southern constellation of that name. NGC 300 is a big object in the sky - being so close, it extends over an angle of almost 25 arcmin, only slightly less than the size of the full moon. It is also relative bright, even a small pair of binoculars will unveil this magnificent spiral galaxy as a hazy glowing patch on a dark sky background.
The comparatively small distance of NGC 300 and its face-on orientation provide astronomers with a wonderful opportunity to study in great detail its structure as well as its various stellar populations and interstellar medium. It was exactly for this purpose that some images of NGC 300 were obtained with the Wide-Field Imager (WFI) on the MPG/ESO 2.2-m telescope at the La Silla Observatory. This advanced 67-million pixel digital camera has already produced many impressive pictures, some of which are displayed in the WFI Photo Gallery.
With its large field of view, 34 x 34 arcmin^2, the WFI is optimally suited to show the full extent of the spiral galaxy NGC 300 and its immediate surroundings in the sky.
NGC 300 and "Virtual Astronomy"
The idea to exploit one and the same data set is not new, but thanks to rapid technological developments it has recently developed into a very powerful tool for the astronomers in their continued quest to understand the Universe. This kind of work has now become very efficient with the advent of a fully searchable data archive from which observational data can then - after the expiry of a nominal one-year proprietary period for the observers - be made available to other astronomers.
The ESO Science Data Archive was established some years ago and now encompasses more than 15 Terabyte. Normally, the identification of specific data sets in such a large archive would be a very difficult and time-consuming task. However, effective projects and software "tools" like ASTROVIRTEL and Querator now allow the users quickly to "filter" large amounts of data and extract those of their specific interest. Indeed, "Archival Astronomy" has already led to many important discoveries, cf. the ASTROVIRTEL list of publications.
There is no doubt that "Virtual Astronomical Observatories" will play an increasingly important role in the future. The present wide-field images of NGC 300 provide an impressive demonstration of the enormous potential of this innovative approach. Some of the ways they were used are explained below.
Cepheids in NGC 300 and the cosmic distance scale
When fully studied, these unique observational data will yield a new and very accurate distance to NGC 300, making this galaxy a future cornerstone in the calibration of the cosmic distance scale. Moreover, they will also allow to understand in more detail how the brightness of a Cepheid-type star depends on its chemical composition, currently a major uncertainty in the application of the Cepheid method to the calibration of the extragalactic distance scale. Indeed, the effect of the abundance of different elements on the luminosity of a Cepheid can be especially well measured in NGC 300 due to the existence of large variations of these abundances in the stars located in the disk of this galaxy.
Gieren and his group, in collaboration with astronomers Fabio Bresolin and Rolf Kudritzki (Institute of Astronomy, Hawaii, USA) are currently measuring the variations of these chemical abundances in stars in the disk of NGC 300, by means of spectra of about 60 blue supergiant stars, obtained with the FORS multi-mode instruments at the ESO Very Large Telescope (VLT) on Paranal. These stars, that are among the optically brightest in NGC 300, were first identified in the WFI images of this galaxy obtained in different colours - the same that were used to produce the top image. The nature of those stars was later spectroscopically confirmed at the VLT.
As an important byproduct of these measurements, the luminosities of the blue supergiant stars in NGC 300 will themselves be calibrated (as a new cosmic "standard candle"), taking advantage of their stellar wind properties that can be measured from the VLT spectra. The WFI Cepheid observations in NGC 300, as well as the VLT blue supergiant star observations, form part of a large research project recently initiated by Gieren and his group that is concerned with the improvement of various stellar distance indicators in nearby galaxies (the "ARAUCARIA" project).
Clues on star formation history in NGC 300
The WFI images obtained in several broad and narrow band filters from the ultraviolet to the near-infrared spectral region (U, B, V, R, I and H-alpha) allow a detailed study of groups of heavy, hot stars (known as "OB associations") and a large number of huge clouds of ionized hydrogen ("HII shells") in this galaxy. Corresponding studies have been carried out by Gieren's group, resulting in the discovery of an amazing number of OB associations, including a number of giant associations.
Dark matter and the observed shapes of distant galaxies
However, the scientific interest of the German astronomers was very different from that of their colleagues and they were not at all concerned about the main object in the field, NGC 300. In a very different approach, they instead wanted to study those images to measure the amount of dark matter in the Universe, by means of the weak gravitational lensing effect produced by distant galaxy clusters.
Various observations, ranging from the measurement of internal motions ("rotation curves") in spiral galaxies to the presence of hot X-ray gas in clusters of galaxies and the motion of galaxies in those clusters, indicate that there is about ten times more matter in the Universe than what is observed in the form of stars, gas and galaxies ("luminous matter"). As this additional matter does not emit light at any wavelengths, it is commonly referred to as "dark" matter - its true nature is yet entirely unclear.
Insight into the distribution of dark matter in the Universe can be gained by looking at the shapes of images of very remote galaxies, billions of light-years away. Light from such distant objects travels vast distances through space before arriving here on Earth, and whenever it passes heavy clusters of galaxies, it is bent a little due to the associated gravitational field. Thus, in long-exposure, high-quality images, this "weak lensing" effect can be perceived as a coherent pattern of distortion of the images of background galaxies.
Gravitational lensing in the NGC 300 field
Assuming that the lensed background galaxies lie at a mean redshift of 1.0, i.e. a distance of 8 billion light-years, a mass of about 2 x 10^14 solar masses was obtained for the CL0053-37 cluster.
One hundred thousand galaxies
No less than about 100,000 galaxies of all types are visible in this amazing image. Three known quasars ([ICS96] 005342.1-375947, [ICS96] 005236.1-374352, [ICS96] 005336.9-380354) with redshifts 2.25, 2.35 and 2.75, respectively, happen to lie inside this sky field, together with many interacting galaxies, some of which feature tidal tails. There are also several groups of highly reddened galaxies - probably distant clusters in formation, cf. PR Photo 18f/02. Others are seen right through the outer regions of NGC 300, cf. PR Photo 18g/02.
More detailed investigations of the numerous galaxies in this field are now underway. From the nearby spiral galaxy NGC 300 to objects in the young Universe, it is all there, truly an astronomical treasure trove!
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